This is Info file gawk.info, produced by Makeinfo version 1.66 from the
input file ./gawk.texi.

INFO-DIR-SECTION Programming Languages
START-INFO-DIR-ENTRY
* Gawk: (gawk.info).           A Text Scanning and Processing Language.
END-INFO-DIR-ENTRY

   This file documents `awk', a program that you can use to select
particular records in a file and perform operations upon them.

   This is Edition 1.0.2 of `The GNU Awk User's Guide', for the
3.0.2 version of the GNU implementation of AWK.

   Copyright (C) 1989, 1991, 92, 93, 96 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.


File: gawk.info,  Node: Top,  Next: Preface,  Prev: (dir),  Up: (dir)

General Introduction
********************

   This file documents `awk', a program that you can use to select
particular records in a file and perform operations upon them.

   This is Edition 1.0.2 of `The GNU Awk User's Guide',
for the 3.0.2 version of the GNU implementation
of AWK.

* Menu:

* Preface::                     What this Info file is about; brief
                                history and acknowledgements.
* What Is Awk::                 What is the `awk' language; using this
                                Info file.
* Getting Started::             A basic introduction to using `awk'. How
                                to run an `awk' program. Command line
                                syntax.
* One-liners::                  Short, sample `awk' programs.
* Regexp::                      All about matching things using regular
                                expressions.
* Reading Files::               How to read files and manipulate fields.
* Printing::                    How to print using `awk'.  Describes the
                                `print' and `printf' statements.
                                Also describes redirection of output.
* Expressions::                 Expressions are the basic building blocks of
                                statements.
* Patterns and Actions::        Overviews of patterns and actions.
* Statements::                  The various control statements are described
                                in detail.
* Built-in Variables::          Built-in Variables
* Arrays::                      The description and use of arrays. Also
                                includes array-oriented control statements.
* Built-in::                    The built-in functions are summarized here.
* User-defined::                User-defined functions are described in
                                detail.
* Invoking Gawk::               How to run `gawk'.
* Library Functions::           A Library of `awk' Functions.
* Sample Programs::             Many `awk' programs with complete
                                explanations.
* Language History::            The evolution of the `awk' language.
* Gawk Summary::                `gawk' Options and Language Summary.
* Installation::                Installing `gawk' under various operating
                                systems.
* Notes::                       Something about the implementation of
                                `gawk'.
* Glossary::                    An explanation of some unfamiliar terms.
* Copying::                     Your right to copy and distribute `gawk'.
* Index::                       Concept and Variable Index.

* History::                     The history of `gawk' and `awk'.
* Manual History::              Brief history of the GNU project and this
                                Info file.
* Acknowledgements::            Acknowledgements.
* This Manual::                 Using this Info file. Includes sample
                                input files that you can use.
* Conventions::                 Typographical Conventions.
* Sample Data Files::           Sample data files for use in the `awk'
                                programs illustrated in this Info file.
* Names::                       What name to use to find `awk'.
* Running gawk::                How to run `gawk' programs; includes
                                command line syntax.
* One-shot::                    Running a short throw-away `awk' program.
* Read Terminal::               Using no input files (input from terminal
                                instead).
* Long::                        Putting permanent `awk' programs in
                                files.
* Executable Scripts::          Making self-contained `awk' programs.
* Comments::                    Adding documentation to `gawk' programs.
* Very Simple::                 A very simple example.
* Two Rules::                   A less simple one-line example with two rules.
* More Complex::                A more complex example.
* Statements/Lines::            Subdividing or combining statements into
                                lines.
* Other Features::              Other Features of `awk'.
* When::                        When to use `gawk' and when to use other
                                things.
* Regexp Usage::                How to Use Regular Expressions.
* Escape Sequences::            How to write non-printing characters.
* Regexp Operators::            Regular Expression Operators.
* GNU Regexp Operators::        Operators specific to GNU software.
* Case-sensitivity::            How to do case-insensitive matching.
* Leftmost Longest::            How much text matches.
* Computed Regexps::            Using Dynamic Regexps.
* Records::                     Controlling how data is split into records.
* Fields::                      An introduction to fields.
* Non-Constant Fields::         Non-constant Field Numbers.
* Changing Fields::             Changing the Contents of a Field.
* Field Separators::            The field separator and how to change it.
* Basic Field Splitting::       How fields are split with single characters or
                                simple strings.
* Regexp Field Splitting::      Using regexps as the field separator.
* Single Character Fields::     Making each character a separate field.
* Command Line Field Separator:: Setting `FS' from the command line.
* Field Splitting Summary::     Some final points and a summary table.
* Constant Size::               Reading constant width data.
* Multiple Line::               Reading multi-line records.
* Getline::                     Reading files under explicit program control
                                using the `getline' function.
* Getline Intro::               Introduction to the `getline' function.
* Plain Getline::               Using `getline' with no arguments.
* Getline/Variable::            Using `getline' into a variable.
* Getline/File::                Using `getline' from a file.
* Getline/Variable/File::       Using `getline' into a variable from a
                                file.
* Getline/Pipe::                Using `getline' from a pipe.
* Getline/Variable/Pipe::       Using `getline' into a variable from a
                                pipe.
* Getline Summary::             Summary Of `getline' Variants.
* Print::                       The `print' statement.
* Print Examples::              Simple examples of `print' statements.
* Output Separators::           The output separators and how to change them.
* OFMT::                        Controlling Numeric Output With `print'.
* Printf::                      The `printf' statement.
* Basic Printf::                Syntax of the `printf' statement.
* Control Letters::             Format-control letters.
* Format Modifiers::            Format-specification modifiers.
* Printf Examples::             Several examples.
* Redirection::                 How to redirect output to multiple files and
                                pipes.
* Special Files::               File name interpretation in `gawk'.
                                `gawk' allows access to inherited file
                                descriptors.
* Close Files And Pipes::       Closing Input and Output Files and Pipes.
* Constants::                   String, numeric, and regexp constants.
* Scalar Constants::            Numeric and string constants.
* Regexp Constants::            Regular Expression constants.
* Using Constant Regexps::      When and how to use a regexp constant.
* Variables::                   Variables give names to values for later use.
* Using Variables::             Using variables in your programs.
* Assignment Options::          Setting variables on the command line and a
                                summary of command line syntax. This is an
                                advanced method of input.
* Conversion::                  The conversion of strings to numbers and vice
                                versa.
* Arithmetic Ops::              Arithmetic operations (`+', `-',
                                etc.)
* Concatenation::               Concatenating strings.
* Assignment Ops::              Changing the value of a variable or a field.
* Increment Ops::               Incrementing the numeric value of a variable.
* Truth Values::                What is "true" and what is "false".
* Typing and Comparison::       How variables acquire types, and how this
                                affects comparison of numbers and strings with
                                `<', etc.
* Boolean Ops::                 Combining comparison expressions using boolean
                                operators `||' ("or"), `&&'
                                ("and") and `!' ("not").
* Conditional Exp::             Conditional expressions select between two
                                subexpressions under control of a third
                                subexpression.
* Function Calls::              A function call is an expression.
* Precedence::                  How various operators nest.
* Pattern Overview::            What goes into a pattern.
* Kinds of Patterns::           A list of all kinds of patterns.
* Regexp Patterns::             Using regexps as patterns.
* Expression Patterns::         Any expression can be used as a pattern.
* Ranges::                      Pairs of patterns specify record ranges.
* BEGIN/END::                   Specifying initialization and cleanup rules.
* Using BEGIN/END::             How and why to use BEGIN/END rules.
* I/O And BEGIN/END::           I/O issues in BEGIN/END rules.
* Empty::                       The empty pattern, which matches every record.
* Action Overview::             What goes into an action.
* If Statement::                Conditionally execute some `awk'
                                statements.
* While Statement::             Loop until some condition is satisfied.
* Do Statement::                Do specified action while looping until some
                                condition is satisfied.
* For Statement::               Another looping statement, that provides
                                initialization and increment clauses.
* Break Statement::             Immediately exit the innermost enclosing loop.
* Continue Statement::          Skip to the end of the innermost enclosing
                                loop.
* Next Statement::              Stop processing the current input record.
* Nextfile Statement::          Stop processing the current file.
* Exit Statement::              Stop execution of `awk'.
* User-modified::               Built-in variables that you change to control
                                `awk'.
* Auto-set::                    Built-in variables where `awk' gives you
                                information.
* ARGC and ARGV::               Ways to use `ARGC' and `ARGV'.
* Array Intro::                 Introduction to Arrays
* Reference to Elements::       How to examine one element of an array.
* Assigning Elements::          How to change an element of an array.
* Array Example::               Basic Example of an Array
* Scanning an Array::           A variation of the `for' statement. It
                                loops through the indices of an array's
                                existing elements.
* Delete::                      The `delete' statement removes an element
                                from an array.
* Numeric Array Subscripts::    How to use numbers as subscripts in
                                `awk'.
* Uninitialized Subscripts::    Using Uninitialized variables as subscripts.
* Multi-dimensional::           Emulating multi-dimensional arrays in
                                `awk'.
* Multi-scanning::              Scanning multi-dimensional arrays.
* Calling Built-in::            How to call built-in functions.
* Numeric Functions::           Functions that work with numbers, including
                                `int', `sin' and `rand'.
* String Functions::            Functions for string manipulation, such as
                                `split', `match', and
                                `sprintf'.
* I/O Functions::               Functions for files and shell commands.
* Time Functions::              Functions for dealing with time stamps.
* Definition Syntax::           How to write definitions and what they mean.
* Function Example::            An example function definition and what it
                                does.
* Function Caveats::            Things to watch out for.
* Return Statement::            Specifying the value a function returns.
* Options::                     Command line options and their meanings.
* Other Arguments::             Input file names and variable assignments.
* AWKPATH Variable::            Searching directories for `awk' programs.
* Obsolete::                    Obsolete Options and/or features.
* Undocumented::                Undocumented Options and Features.
* Known Bugs::                  Known Bugs in `gawk'.
* Portability Notes::           What to do if you don't have `gawk'.
* Nextfile Function::           Two implementations of a `nextfile'
                                function.
* Assert Function::             A function for assertions in `awk'
                                programs.
* Round Function::              A function for rounding if `sprintf' does
                                not do it correctly.
* Ordinal Functions::           Functions for using characters as numbers and
                                vice versa.
* Join Function::               A function to join an array into a string.
* Mktime Function::             A function to turn a date into a timestamp.
* Gettimeofday Function::       A function to get formatted times.
* Filetrans Function::          A function for handling data file transitions.
* Getopt Function::             A function for processing command line
                                arguments.
* Passwd Functions::            Functions for getting user information.
* Group Functions::             Functions for getting group information.
* Library Names::               How to best name private global variables in
                                library functions.
* Clones::                      Clones of common utilities.
* Cut Program::                 The `cut' utility.
* Egrep Program::               The `egrep' utility.
* Id Program::                  The `id' utility.
* Split Program::               The `split' utility.
* Tee Program::                 The `tee' utility.
* Uniq Program::                The `uniq' utility.
* Wc Program::                  The `wc' utility.
* Miscellaneous Programs::      Some interesting `awk' programs.
* Dupword Program::             Finding duplicated words in a document.
* Alarm Program::               An alarm clock.
* Translate Program::           A program similar to the `tr' utility.
* Labels Program::              Printing mailing labels.
* Word Sorting::                A program to produce a word usage count.
* History Sorting::             Eliminating duplicate entries from a history
                                file.
* Extract Program::             Pulling out programs from Texinfo source
                                files.
* Simple Sed::                  A Simple Stream Editor.
* Igawk Program::               A wrapper for `awk' that includes files.
* V7/SVR3.1::                   The major changes between V7 and System V
                                Release 3.1.
* SVR4::                        Minor changes between System V Releases 3.1
                                and 4.
* POSIX::                       New features from the POSIX standard.
* BTL::                         New features from the Bell Laboratories
                                version of `awk'.
* POSIX/GNU::                   The extensions in `gawk' not in POSIX
                                `awk'.
* Command Line Summary::        Recapitulation of the command line.
* Language Summary::            A terse review of the language.
* Variables/Fields::            Variables, fields, and arrays.
* Fields Summary::              Input field splitting.
* Built-in Summary::            `awk''s built-in variables.
* Arrays Summary::              Using arrays.
* Data Type Summary::           Values in `awk' are numbers or strings.
* Rules Summary::               Patterns and Actions, and their component
                                parts.
* Pattern Summary::             Quick overview of patterns.
* Regexp Summary::              Quick overview of regular expressions.
* Actions Summary::             Quick overview of actions.
* Operator Summary::            `awk' operators.
* Control Flow Summary::        The control statements.
* I/O Summary::                 The I/O statements.
* Printf Summary::              A summary of `printf'.
* Special File Summary::        Special file names interpreted internally.
* Built-in Functions Summary::  Built-in numeric and string functions.
* Time Functions Summary::      Built-in time functions.
* String Constants Summary::    Escape sequences in strings.
* Functions Summary::           Defining and calling functions.
* Historical Features::         Some undocumented but supported "features".
* Gawk Distribution::           What is in the `gawk' distribution.
* Getting::                     How to get the distribution.
* Extracting::                  How to extract the distribution.
* Distribution contents::       What is in the distribution.
* Unix Installation::           Installing `gawk' under various versions
                                of Unix.
* Quick Installation::          Compiling `gawk' under Unix.
* Configuration Philosophy::    How it's all supposed to work.
* VMS Installation::            Installing `gawk' on VMS.
* VMS Compilation::             How to compile `gawk' under VMS.
* VMS Installation Details::    How to install `gawk' under VMS.
* VMS Running::                 How to run `gawk' under VMS.
* VMS POSIX::                   Alternate instructions for VMS POSIX.
* PC Installation::             Installing and Compiling `gawk' on MS-DOS
                                and OS/2
* Atari Installation::          Installing `gawk' on the Atari ST.
* Atari Compiling::             Compiling `gawk' on Atari
* Atari Using::                 Running `gawk' on Atari
* Amiga Installation::          Installing `gawk' on an Amiga.
* Bugs::                        Reporting Problems and Bugs.
* Other Versions::              Other freely available `awk'
                                implementations.
* Compatibility Mode::          How to disable certain `gawk' extensions.
* Additions::                   Making Additions To `gawk'.
* Adding Code::                 Adding code to the main body of `gawk'.
* New Ports::                   Porting `gawk' to a new operating system.
* Future Extensions::           New features that may be implemented one day.
* Improvements::                Suggestions for improvements by volunteers.

                  To Miriam, for making me complete.

                  To Chana, for the joy you bring us.

                To Rivka, for the exponential increase.

                  To Nachum, for the added dimension.


File: gawk.info,  Node: Preface,  Next: What Is Awk,  Prev: Top,  Up: Top

Preface
*******

   This Info file teaches you about the `awk' language and how you can
use it effectively.  You should already be familiar with basic system
commands, such as `cat' and `ls',(1) and basic shell facilities, such
as Input/Output (I/O) redirection and pipes.

   Implementations of the `awk' language are available for many
different computing environments.  This Info file, while describing the
`awk' language in general, also describes a particular implementation
of `awk' called `gawk' (which stands for "GNU Awk").  `gawk' runs on a
broad range of Unix systems, ranging from 80386 PC-based computers, up
through large scale systems, such as Crays. `gawk' has also been ported
to MS-DOS and OS/2 PC's, Atari and Amiga micro-computers, and VMS.

* Menu:

* History::                     The history of `gawk' and `awk'.
* Manual History::              Brief history of the GNU project and this
                                Info file.
* Acknowledgements::            Acknowledgements.

   ---------- Footnotes ----------

   (1)  These commands are available on POSIX compliant systems, as
well as on traditional Unix based systems. If you are using some other
operating system, you still need to be familiar with the ideas of I/O
redirection and pipes.


File: gawk.info,  Node: History,  Next: Manual History,  Prev: Preface,  Up: Preface

History of `awk' and `gawk'
===========================

   The name `awk' comes from the initials of its designers: Alfred V.
Aho, Peter J. Weinberger, and Brian W. Kernighan.  The original version
of `awk' was written in 1977 at AT&T Bell Laboratories.  In 1985 a new
version made the programming language more powerful, introducing
user-defined functions, multiple input streams, and computed regular
expressions.  This new version became generally available with Unix
System V Release 3.1.  The version in System V Release 4 added some new
features and also cleaned up the behavior in some of the "dark corners"
of the language.  The specification for `awk' in the POSIX Command
Language and Utilities standard further clarified the language based on
feedback from both the `gawk' designers, and the original Bell Labs
`awk' designers.

   The GNU implementation, `gawk', was written in 1986 by Paul Rubin
and Jay Fenlason, with advice from Richard Stallman.  John Woods
contributed parts of the code as well.  In 1988 and 1989, David
Trueman, with help from Arnold Robbins, thoroughly reworked `gawk' for
compatibility with the newer `awk'.  Current development focuses on bug
fixes, performance improvements, standards compliance, and
occasionally, new features.


File: gawk.info,  Node: Manual History,  Next: Acknowledgements,  Prev: History,  Up: Preface

The GNU Project and This Book
=============================

   The Free Software Foundation (FSF) is a non-profit organization
dedicated to the production and distribution of freely distributable
software.  It was founded by Richard M. Stallman, the author of the
original Emacs editor.  GNU Emacs is the most widely used version of
Emacs today.

   The GNU project is an on-going effort on the part of the Free
Software Foundation to create a complete, freely distributable, POSIX
compliant computing environment.  (GNU stands for "GNU's not Unix".)
The FSF uses the "GNU General Public License" (or GPL) to ensure that
source code for their software is always available to the end user. A
copy of the GPL is included for your reference (*note GNU GENERAL
PUBLIC LICENSE: Copying.).  The GPL applies to the C language source
code for `gawk'.

   As of this writing (1995), the only major component of the GNU
environment still uncompleted is the operating system kernel, and work
proceeds apace on that.  A shell, an editor (Emacs), highly portable
optimizing C, C++, and Objective-C compilers, a symbolic debugger, and
dozens of large and small utilities (such as `gawk'), have all been
completed and are freely available.

   Until the GNU operating system is released, the FSF recommends the
use of Linux, a freely distributable, Unix-like operating system for
80386 and other systems.  There are many books on Linux. One freely
available one is `Linux Installation and Getting Started', by Matt
Welsh.  Many Linux distributions are available, often in computer
stores or bundled on CD-ROM with books about Linux. Also, the FSF
provides a Linux distribution ("Debian"); contact them for more
information.  *Note Getting the `gawk' Distribution: Getting, for the
FSF's contact information.  (There are two other freely available,
Unix-like operating systems for 80386 and other systems, NetBSD and
FreeBSD. Both are based on the 4.4-Lite Berkeley Software Distribution,
and both use recent versions of `gawk' for their versions of `awk'.)

   This Info file itself has gone through several previous, preliminary
editions.  I started working on a preliminary draft of `The GAWK
Manual', by Diane Close, Paul Rubin, and Richard Stallman in the fall
of 1988.  It was around 90 pages long, and barely described the
original, "old" version of `awk'. After substantial revision, the first
version of the `The GAWK Manual' to be released was Edition 0.11 Beta in
October of 1989.  The manual then underwent more substantial revision
for Edition 0.13 of December 1991.  David Trueman, Pat Rankin, and
Michal Jaegermann contributed sections of the manual for Edition 0.13.
That edition was published by the FSF as a bound book early in 1992.
Since then there have been several minor revisions, notably Edition
0.14 of November 1992 that was published by the FSF in January of 1993,
and Edition 0.16 of August 1993.

   Edition 1.0 of `The GNU Awk User's Guide' represents a significant
re-working of `The GAWK Manual', with much additional material.  The
FSF and I agree that I am now the primary author.  I also felt that it
needed a more descriptive title.

   `The GNU Awk User's Guide' will undoubtedly continue to evolve.  An
electronic version comes with the `gawk' distribution from the FSF.  If
you find an error in this Info file, please report it!  *Note Reporting
Problems and Bugs: Bugs, for information on submitting problem reports
electronically, or write to me in care of the FSF.


File: gawk.info,  Node: Acknowledgements,  Prev: Manual History,  Up: Preface

Acknowledgements
================

   I would like to acknowledge Richard M. Stallman, for his vision of a
better world, and for his courage in founding the FSF and starting the
GNU project.

   The initial draft of `The GAWK Manual' had the following
acknowledgements:

     Many people need to be thanked for their assistance in producing
     this manual.  Jay Fenlason contributed many ideas and sample
     programs.  Richard Mlynarik and Robert Chassell gave helpful
     comments on drafts of this manual.  The paper `A Supplemental
     Document for `awk'' by John W.  Pierce of the Chemistry Department
     at UC San Diego, pinpointed several issues relevant both to `awk'
     implementation and to this manual, that would otherwise have
     escaped us.

   The following people provided many helpful comments on Edition 0.13
of `The GAWK Manual': Rick Adams, Michael Brennan, Rich Burridge, Diane
Close, Christopher ("Topher") Eliot, Michael Lijewski, Pat Rankin,
Miriam Robbins, and Michal Jaegermann.

   The following people provided many helpful comments for Edition 1.0
of `The GNU Awk User's Guide': Karl Berry, Michael Brennan, Darrel
Hankerson, Michal Jaegermann, Michael Lijewski, and Miriam Robbins.
Pat Rankin, Michal Jaegermann, Darrel Hankerson and Scott Deifik
updated their respective sections for Edition 1.0.

   Robert J. Chassell provided much valuable advice on the use of
Texinfo.  He also deserves special thanks for convincing me *not* to
title this Info file `How To Gawk Politely'.  Karl Berry helped
significantly with the TeX part of Texinfo.

   David Trueman deserves special credit; he has done a yeoman job of
evolving `gawk' so that it performs well, and without bugs.  Although
he is no longer involved with `gawk', working with him on this project
was a significant pleasure.

   Scott Deifik, Darrel Hankerson, Kai Uwe Rommel, Pat Rankin, and
Michal Jaegermann (in no particular order) are long time members of the
`gawk' "crack portability team."  Without their hard work and help,
`gawk' would not be nearly the fine program it is today.  It has been
and continues to be a pleasure working with this team of fine people.

   Jeffrey Friedl provided invaluable help in tracking down a number of
last minute problems with regular expressions in `gawk' 3.0.

   David and I would like to thank Brian Kernighan of Bell Labs for
invaluable assistance during the testing and debugging of `gawk', and
for help in clarifying numerous points about the language.  We could
not have done nearly as good a job on either `gawk' or its
documentation without his help.

   I would like to thank Marshall and Elaine Hartholz of Seattle, and
Dr.  Bert and Rita Schreiber of Detroit for large amounts of quiet
vacation time in their homes, which allowed me to make significant
progress on this Info file and on `gawk' itself.  Phil Hughes of SSC
contributed in a very important way by loaning me his laptop Linux
system, not once, but twice, allowing me to do a lot of work while away
from home.

   Finally, I must thank my wonderful wife, Miriam, for her patience
through the many versions of this project, for her proof-reading, and
for sharing me with the computer.  I would like to thank my parents for
their love, and for the grace with which they raised and educated me.
I also must acknowledge my gratitude to G-d, for the many opportunities
He has sent my way, as well as for the gifts He has given me with which
to take advantage of those opportunities.

Arnold Robbins
Atlanta, Georgia
January, 1996


File: gawk.info,  Node: What Is Awk,  Next: Getting Started,  Prev: Preface,  Up: Top

Introduction
************

   If you are like many computer users, you would frequently like to
make changes in various text files wherever certain patterns appear, or
extract data from parts of certain lines while discarding the rest.  To
write a program to do this in a language such as C or Pascal is a
time-consuming inconvenience that may take many lines of code.  The job
may be easier with `awk'.

   The `awk' utility interprets a special-purpose programming language
that makes it possible to handle simple data-reformatting jobs with
just a few lines of code.

   The GNU implementation of `awk' is called `gawk'; it is fully upward
compatible with the System V Release 4 version of `awk'.  `gawk' is
also upward compatible with the POSIX specification of the `awk'
language.  This means that all properly written `awk' programs should
work with `gawk'.  Thus, we usually don't distinguish between `gawk'
and other `awk' implementations.

   Using `awk' you can:

   * manage small, personal databases

   * generate reports

   * validate data

   * produce indexes, and perform other document preparation tasks

   * even experiment with algorithms that can be adapted later to other
     computer languages

* Menu:

* This Manual::                 Using this Info file. Includes sample
                                input files that you can use.
* Conventions::                 Typographical Conventions.
* Sample Data Files::           Sample data files for use in the `awk'
                                programs illustrated in this Info file.


File: gawk.info,  Node: This Manual,  Next: Conventions,  Prev: What Is Awk,  Up: What Is Awk

Using This Book
===============

   The term `awk' refers to a particular program, and to the language
you use to tell this program what to do.  When we need to be careful,
we call the program "the `awk' utility" and the language "the `awk'
language."  The term `gawk' refers to a version of `awk' developed as
part the GNU project.  The purpose of this Info file is to explain both
the `awk' language and how to run the `awk' utility.

   The main purpose of the Info file is to explain the features of
`awk', as defined in the POSIX standard.  It does so in the context of
one particular implementation, `gawk'. While doing so, it will also
attempt to describe important differences between `gawk' and other
`awk' implementations.  Finally, any `gawk' features that are not in
the POSIX standard for `awk' will be noted.

   The term "`awk' program" refers to a program written by you in the
`awk' programming language.

   *Note Getting Started with `awk': Getting Started, for the bare
essentials you need to know to start using `awk'.

   Some useful "one-liners" are included to give you a feel for the
`awk' language (*note Useful One Line Programs: One-liners.).

   Many sample `awk' programs have been provided for you (*note A
Library of `awk' Functions: Library Functions.; also *note Practical
`awk' Programs: Sample Programs.).

   The entire `awk' language is summarized for quick reference in *Note
`gawk' Summary: Gawk Summary.  Look there if you just need to refresh
your memory about a particular feature.

   If you find terms that you aren't familiar with, try looking them up
in the glossary (*note Glossary::.).

   Most of the time complete `awk' programs are used as examples, but in
some of the more advanced sections, only the part of the `awk' program
that illustrates the concept being described is shown.

   While this Info file is aimed principally at people who have not been
exposed to `awk', there is a lot of information here that even the `awk'
expert should find useful.  In particular, the description of POSIX
`awk', and the example programs in *Note A Library of `awk' Functions:
Library Functions, and *Note Practical `awk' Programs: Sample Programs,
should be of interest.

Dark Corners
------------

   Until the POSIX standard (and `The Gawk Manual'), many features of
`awk' were either poorly documented, or not documented at all.
Descriptions of such features (often called "dark corners") are noted
in this Info file with "(d.c.)".  They also appear in the index under
the heading "dark corner."


File: gawk.info,  Node: Conventions,  Next: Sample Data Files,  Prev: This Manual,  Up: What Is Awk

Typographical Conventions
=========================

   This Info file is written using Texinfo, the GNU documentation
formatting language.  A single Texinfo source file is used to produce
both the printed and on-line versions of the documentation.  This
section briefly documents the typographical conventions used in Texinfo.

   Examples you would type at the command line are preceded by the
common shell primary and secondary prompts, `$' and `>'.  Output from
the command is preceded by the glyph "-|".  This typically represents
the command's standard output.  Error messages, and other output on the
command's standard error, are preceded by the glyph "error-->".  For
example:

     $ echo hi on stdout
     -| hi on stdout
     $ echo hello on stderr 1>&2
     error--> hello on stderr

   Characters that you type at the keyboard look `like this'.  In
particular, there are special characters called "control characters."
These are characters that you type by holding down both the `CONTROL'
key and another key, at the same time.  For example, a `Control-d' is
typed by first pressing and holding the `CONTROL' key, next pressing
the `d' key, and finally releasing both keys.


File: gawk.info,  Node: Sample Data Files,  Prev: Conventions,  Up: What Is Awk

Data Files for the Examples
===========================

   Many of the examples in this Info file take their input from two
sample data files.  The first, called `BBS-list', represents a list of
computer bulletin board systems together with information about those
systems.  The second data file, called `inventory-shipped', contains
information about shipments on a monthly basis.  In both files, each
line is considered to be one "record".

   In the file `BBS-list', each record contains the name of a computer
bulletin board, its phone number, the board's baud rate(s), and a code
for the number of hours it is operational.  An `A' in the last column
means the board operates 24 hours a day.  A `B' in the last column
means the board operates evening and weekend hours, only.  A `C' means
the board operates only on weekends.

     aardvark     555-5553     1200/300          B
     alpo-net     555-3412     2400/1200/300     A
     barfly       555-7685     1200/300          A
     bites        555-1675     2400/1200/300     A
     camelot      555-0542     300               C
     core         555-2912     1200/300          C
     fooey        555-1234     2400/1200/300     B
     foot         555-6699     1200/300          B
     macfoo       555-6480     1200/300          A
     sdace        555-3430     2400/1200/300     A
     sabafoo      555-2127     1200/300          C

   The second data file, called `inventory-shipped', represents
information about shipments during the year.  Each record contains the
month of the year, the number of green crates shipped, the number of
red boxes shipped, the number of orange bags shipped, and the number of
blue packages shipped, respectively.  There are 16 entries, covering
the 12 months of one year and four months of the next year.

     Jan  13  25  15 115
     Feb  15  32  24 226
     Mar  15  24  34 228
     Apr  31  52  63 420
     May  16  34  29 208
     Jun  31  42  75 492
     Jul  24  34  67 436
     Aug  15  34  47 316
     Sep  13  55  37 277
     Oct  29  54  68 525
     Nov  20  87  82 577
     Dec  17  35  61 401
     
     Jan  21  36  64 620
     Feb  26  58  80 652
     Mar  24  75  70 495
     Apr  21  70  74 514

   If you are reading this in GNU Emacs using Info, you can copy the
regions of text showing these sample files into your own test files.
This way you can try out the examples shown in the remainder of this
document.  You do this by using the command `M-x write-region' to copy
text from the Info file into a file for use with `awk' (*Note
Miscellaneous File Operations: (emacs)Misc File Ops, for more
information).  Using this information, create your own `BBS-list' and
`inventory-shipped' files, and practice what you learn in this Info
file.

   If you are using the stand-alone version of Info, see *Note
Extracting Programs from Texinfo Source Files: Extract Program, for an
`awk' program that will extract these data files from `gawk.texi', the
Texinfo source file for this Info file.


File: gawk.info,  Node: Getting Started,  Next: One-liners,  Prev: What Is Awk,  Up: Top

Getting Started with `awk'
**************************

   The basic function of `awk' is to search files for lines (or other
units of text) that contain certain patterns.  When a line matches one
of the patterns, `awk' performs specified actions on that line.  `awk'
keeps processing input lines in this way until the end of the input
files are reached.

   Programs in `awk' are different from programs in most other
languages, because `awk' programs are "data-driven"; that is, you
describe the data you wish to work with, and then what to do when you
find it.  Most other languages are "procedural"; you have to describe,
in great detail, every step the program is to take.  When working with
procedural languages, it is usually much harder to clearly describe the
data your program will process.  For this reason, `awk' programs are
often refreshingly easy to both write and read.

   When you run `awk', you specify an `awk' "program" that tells `awk'
what to do.  The program consists of a series of "rules".  (It may also
contain "function definitions", an advanced feature which we will
ignore for now.  *Note User-defined Functions: User-defined.)  Each
rule specifies one pattern to search for, and one action to perform
when that pattern is found.

   Syntactically, a rule consists of a pattern followed by an action.
The action is enclosed in curly braces to separate it from the pattern.
Rules are usually separated by newlines.  Therefore, an `awk' program
looks like this:

     PATTERN { ACTION }
     PATTERN { ACTION }
     ...

* Menu:

* Names::                       What name to use to find `awk'.
* Running gawk::                How to run `gawk' programs; includes
                                command line syntax.
* Very Simple::                 A very simple example.
* Two Rules::                   A less simple one-line example with two rules.
* More Complex::                A more complex example.
* Statements/Lines::            Subdividing or combining statements into
                                lines.
* Other Features::              Other Features of `awk'.
* When::                        When to use `gawk' and when to use other
                                things.


File: gawk.info,  Node: Names,  Next: Running gawk,  Prev: Getting Started,  Up: Getting Started

A Rose By Any Other Name
========================

   The `awk' language has evolved over the years. Full details are
provided in *Note The Evolution of the `awk' Language: Language History.
The language described in this Info file is often referred to as "new
`awk'."

   Because of this, many systems have multiple versions of `awk'.  Some
systems have an `awk' utility that implements the original version of
the `awk' language, and a `nawk' utility for the new version.  Others
have an `oawk' for the "old `awk'" language, and plain `awk' for the
new one.  Still others only have one version, usually the new one.(1)

   All in all, this makes it difficult for you to know which version of
`awk' you should run when writing your programs.  The best advice we
can give here is to check your local documentation. Look for `awk',
`oawk', and `nawk', as well as for `gawk'. Chances are, you will have
some version of new `awk' on your system, and that is what you should
use when running your programs.  (Of course, if you're reading this
Info file, chances are good that you have `gawk'!)

   Throughout this Info file, whenever we refer to a language feature
that should be available in any complete implementation of POSIX `awk',
we simply use the term `awk'.  When referring to a feature that is
specific to the GNU implementation, we use the term `gawk'.

   ---------- Footnotes ----------

   (1)  Often, these systems use `gawk' for their `awk' implementation!


File: gawk.info,  Node: Running gawk,  Next: Very Simple,  Prev: Names,  Up: Getting Started

How to Run `awk' Programs
=========================

   There are several ways to run an `awk' program.  If the program is
short, it is easiest to include it in the command that runs `awk', like
this:

     awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...

where PROGRAM consists of a series of patterns and actions, as
described earlier.  (The reason for the single quotes is described
below, in *Note One-shot Throw-away `awk' Programs: One-shot.)

   When the program is long, it is usually more convenient to put it in
a file and run it with a command like this:

     awk -f PROGRAM-FILE INPUT-FILE1 INPUT-FILE2 ...

* Menu:

* One-shot::                    Running a short throw-away `awk' program.
* Read Terminal::               Using no input files (input from terminal
                                instead).
* Long::                        Putting permanent `awk' programs in
                                files.
* Executable Scripts::          Making self-contained `awk' programs.
* Comments::                    Adding documentation to `gawk' programs.


File: gawk.info,  Node: One-shot,  Next: Read Terminal,  Prev: Running gawk,  Up: Running gawk

One-shot Throw-away `awk' Programs
----------------------------------

   Once you are familiar with `awk', you will often type in simple
programs the moment you want to use them.  Then you can write the
program as the first argument of the `awk' command, like this:

     awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...

where PROGRAM consists of a series of PATTERNS and ACTIONS, as
described earlier.

   This command format instructs the "shell", or command interpreter,
to start `awk' and use the PROGRAM to process records in the input
file(s).  There are single quotes around PROGRAM so that the shell
doesn't interpret any `awk' characters as special shell characters.
They also cause the shell to treat all of PROGRAM as a single argument
for `awk' and allow PROGRAM to be more than one line long.

   This format is also useful for running short or medium-sized `awk'
programs from shell scripts, because it avoids the need for a separate
file for the `awk' program.  A self-contained shell script is more
reliable since there are no other files to misplace.

   *Note Useful One Line Programs: One-liners, presents several short,
self-contained programs.

   As an interesting side point, the command

     awk '/foo/' FILES ...

is essentially the same as

     egrep foo FILES ...


File: gawk.info,  Node: Read Terminal,  Next: Long,  Prev: One-shot,  Up: Running gawk

Running `awk' without Input Files
---------------------------------

   You can also run `awk' without any input files.  If you type the
command line:

     awk 'PROGRAM'

then `awk' applies the PROGRAM to the "standard input", which usually
means whatever you type on the terminal.  This continues until you
indicate end-of-file by typing `Control-d'.  (On other operating
systems, the end-of-file character may be different.  For example, on
OS/2 and MS-DOS, it is `Control-z'.)

   For example, the following program prints a friendly piece of advice
(from Douglas Adams' `The Hitchhiker's Guide to the Galaxy'), to keep
you from worrying about the complexities of computer programming
(`BEGIN' is a feature we haven't discussed yet).

     $ awk "BEGIN { print \"Don't Panic!\" }"
     -| Don't Panic!

   This program does not read any input.  The `\' before each of the
inner double quotes is necessary because of the shell's quoting rules,
in particular because it mixes both single quotes and double quotes.

   This next simple `awk' program emulates the `cat' utility; it copies
whatever you type at the keyboard to its standard output. (Why this
works is explained shortly.)

     $ awk '{ print }'
     Now is the time for all good men
     -| Now is the time for all good men
     to come to the aid of their country.
     -| to come to the aid of their country.
     Four score and seven years ago, ...
     -| Four score and seven years ago, ...
     What, me worry?
     -| What, me worry?
     Control-d


File: gawk.info,  Node: Long,  Next: Executable Scripts,  Prev: Read Terminal,  Up: Running gawk

Running Long Programs
---------------------

   Sometimes your `awk' programs can be very long.  In this case it is
more convenient to put the program into a separate file.  To tell `awk'
to use that file for its program, you type:

     awk -f SOURCE-FILE INPUT-FILE1 INPUT-FILE2 ...

   The `-f' instructs the `awk' utility to get the `awk' program from
the file SOURCE-FILE.  Any file name can be used for SOURCE-FILE.  For
example, you could put the program:

     BEGIN { print "Don't Panic!" }

into the file `advice'.  Then this command:

     awk -f advice

does the same thing as this one:

     awk "BEGIN { print \"Don't Panic!\" }"

which was explained earlier (*note Running `awk' without Input Files:
Read Terminal.).  Note that you don't usually need single quotes around
the file name that you specify with `-f', because most file names don't
contain any of the shell's special characters.  Notice that in
`advice', the `awk' program did not have single quotes around it.  The
quotes are only needed for programs that are provided on the `awk'
command line.

   If you want to identify your `awk' program files clearly as such,
you can add the extension `.awk' to the file name.  This doesn't affect
the execution of the `awk' program, but it does make "housekeeping"
easier.


File: gawk.info,  Node: Executable Scripts,  Next: Comments,  Prev: Long,  Up: Running gawk

Executable `awk' Programs
-------------------------

   Once you have learned `awk', you may want to write self-contained
`awk' scripts, using the `#!' script mechanism.  You can do this on
many Unix systems(1) (and someday on the GNU system).

   For example, you could update the file `advice' to look like this:

     #! /bin/awk -f
     
     BEGIN    { print "Don't Panic!" }

After making this file executable (with the `chmod' utility), you can
simply type `advice' at the shell, and the system will arrange to run
`awk'(2) as if you had typed `awk -f advice'.

     $ advice
     -| Don't Panic!

Self-contained `awk' scripts are useful when you want to write a
program which users can invoke without their having to know that the
program is written in `awk'.

   Some older systems do not support the `#!' mechanism. You can get a
similar effect using a regular shell script.  It would look something
like this:

     : The colon ensures execution by the standard shell.
     awk 'PROGRAM' "$@"

   Using this technique, it is *vital* to enclose the PROGRAM in single
quotes to protect it from interpretation by the shell.  If you omit the
quotes, only a shell wizard can predict the results.

   The `"$@"' causes the shell to forward all the command line
arguments to the `awk' program, without interpretation.  The first
line, which starts with a colon, is used so that this shell script will
work even if invoked by a user who uses the C shell.  (Not all older
systems obey this convention, but many do.)

   ---------- Footnotes ----------

   (1)  The `#!' mechanism works on Linux systems, Unix systems derived
from Berkeley Unix, System V Release 4, and some System V Release 3
systems.

   (2)  The line beginning with `#!' lists the full file name of an
interpreter to be run, and an optional initial command line argument to
pass to that interpreter.  The operating system then runs the
interpreter with the given argument and the full argument list of the
executed program.  The first argument in the list is the full file name
of the `awk' program.  The rest of the argument list will either be
options to `awk', or data files, or both.


File: gawk.info,  Node: Comments,  Prev: Executable Scripts,  Up: Running gawk

Comments in `awk' Programs
--------------------------

   A "comment" is some text that is included in a program for the sake
of human readers; it is not really part of the program.  Comments can
explain what the program does, and how it works.  Nearly all
programming languages have provisions for comments, because programs are
typically hard to understand without their extra help.

   In the `awk' language, a comment starts with the sharp sign
character, `#', and continues to the end of the line.  The `#' does not
have to be the first character on the line. The `awk' language ignores
the rest of a line following a sharp sign.  For example, we could have
put the following into `advice':

     # This program prints a nice friendly message.  It helps
     # keep novice users from being afraid of the computer.
     BEGIN    { print "Don't Panic!" }

   You can put comment lines into keyboard-composed throw-away `awk'
programs also, but this usually isn't very useful; the purpose of a
comment is to help you or another person understand the program at a
later time.


File: gawk.info,  Node: Very Simple,  Next: Two Rules,  Prev: Running gawk,  Up: Getting Started

A Very Simple Example
=====================

   The following command runs a simple `awk' program that searches the
input file `BBS-list' for the string of characters: `foo'.  (A string
of characters is usually called a "string".  The term "string" is
perhaps based on similar usage in English, such as "a string of
pearls," or, "a string of cars in a train.")

     awk '/foo/ { print $0 }' BBS-list

When lines containing `foo' are found, they are printed, because
`print $0' means print the current line.  (Just `print' by itself means
the same thing, so we could have written that instead.)

   You will notice that slashes, `/', surround the string `foo' in the
`awk' program.  The slashes indicate that `foo' is a pattern to search
for.  This type of pattern is called a "regular expression", and is
covered in more detail later (*note Regular Expressions: Regexp.).  The
pattern is allowed to match parts of words.  There are single-quotes
around the `awk' program so that the shell won't interpret any of it as
special shell characters.

   Here is what this program prints:

     $ awk '/foo/ { print $0 }' BBS-list
     -| fooey        555-1234     2400/1200/300     B
     -| foot         555-6699     1200/300          B
     -| macfoo       555-6480     1200/300          A
     -| sabafoo      555-2127     1200/300          C

   In an `awk' rule, either the pattern or the action can be omitted,
but not both.  If the pattern is omitted, then the action is performed
for *every* input line.  If the action is omitted, the default action
is to print all lines that match the pattern.

   Thus, we could leave out the action (the `print' statement and the
curly braces) in the above example, and the result would be the same:
all lines matching the pattern `foo' would be printed.  By comparison,
omitting the `print' statement but retaining the curly braces makes an
empty action that does nothing; then no lines would be printed.


File: gawk.info,  Node: Two Rules,  Next: More Complex,  Prev: Very Simple,  Up: Getting Started

An Example with Two Rules
=========================

   The `awk' utility reads the input files one line at a time.  For
each line, `awk' tries the patterns of each of the rules.  If several
patterns match then several actions are run, in the order in which they
appear in the `awk' program.  If no patterns match, then no actions are
run.

   After processing all the rules (perhaps none) that match the line,
`awk' reads the next line (however, *note The `next' Statement: Next
Statement., and also *note The `nextfile' Statement: Nextfile
Statement.).  This continues until the end of the file is reached.

   For example, the `awk' program:

     /12/  { print $0 }
     /21/  { print $0 }

contains two rules.  The first rule has the string `12' as the pattern
and `print $0' as the action.  The second rule has the string `21' as
the pattern and also has `print $0' as the action.  Each rule's action
is enclosed in its own pair of braces.

   This `awk' program prints every line that contains the string `12'
*or* the string `21'.  If a line contains both strings, it is printed
twice, once by each rule.

   This is what happens if we run this program on our two sample data
files, `BBS-list' and `inventory-shipped', as shown here:

     $ awk '/12/ { print $0 }
     >      /21/ { print $0 }' BBS-list inventory-shipped
     -| aardvark     555-5553     1200/300          B
     -| alpo-net     555-3412     2400/1200/300     A
     -| barfly       555-7685     1200/300          A
     -| bites        555-1675     2400/1200/300     A
     -| core         555-2912     1200/300          C
     -| fooey        555-1234     2400/1200/300     B
     -| foot         555-6699     1200/300          B
     -| macfoo       555-6480     1200/300          A
     -| sdace        555-3430     2400/1200/300     A
     -| sabafoo      555-2127     1200/300          C
     -| sabafoo      555-2127     1200/300          C
     -| Jan  21  36  64 620
     -| Apr  21  70  74 514

Note how the line in `BBS-list' beginning with `sabafoo' was printed
twice, once for each rule.


File: gawk.info,  Node: More Complex,  Next: Statements/Lines,  Prev: Two Rules,  Up: Getting Started

A More Complex Example
======================

   Here is an example to give you an idea of what typical `awk'
programs do.  This example shows how `awk' can be used to summarize,
select, and rearrange the output of another utility.  It uses features
that haven't been covered yet, so don't worry if you don't understand
all the details.

     ls -lg | awk '$6 == "Nov" { sum += $5 }
                  END { print sum }'

   This command prints the total number of bytes in all the files in the
current directory that were last modified in November (of any year).
(In the C shell you would need to type a semicolon and then a backslash
at the end of the first line; in a POSIX-compliant shell, such as the
Bourne shell or Bash, the GNU Bourne-Again shell, you can type the
example as shown.)

   The `ls -lg' part of this example is a system command that gives you
a listing of the files in a directory, including file size and the date
the file was last modified. Its output looks like this:

     -rw-r--r--  1 arnold   user   1933 Nov  7 13:05 Makefile
     -rw-r--r--  1 arnold   user  10809 Nov  7 13:03 gawk.h
     -rw-r--r--  1 arnold   user    983 Apr 13 12:14 gawk.tab.h
     -rw-r--r--  1 arnold   user  31869 Jun 15 12:20 gawk.y
     -rw-r--r--  1 arnold   user  22414 Nov  7 13:03 gawk1.c
     -rw-r--r--  1 arnold   user  37455 Nov  7 13:03 gawk2.c
     -rw-r--r--  1 arnold   user  27511 Dec  9 13:07 gawk3.c
     -rw-r--r--  1 arnold   user   7989 Nov  7 13:03 gawk4.c

The first field contains read-write permissions, the second field
contains the number of links to the file, and the third field
identifies the owner of the file. The fourth field identifies the group
of the file.  The fifth field contains the size of the file in bytes.
The sixth, seventh and eighth fields contain the month, day, and time,
respectively, that the file was last modified.  Finally, the ninth field
contains the name of the file.

   The `$6 == "Nov"' in our `awk' program is an expression that tests
whether the sixth field of the output from `ls -lg' matches the string
`Nov'.  Each time a line has the string `Nov' for its sixth field, the
action `sum += $5' is performed.  This adds the fifth field (the file
size) to the variable `sum'.  As a result, when `awk' has finished
reading all the input lines, `sum' is the sum of the sizes of files
whose lines matched the pattern.  (This works because `awk' variables
are automatically initialized to zero.)

   After the last line of output from `ls' has been processed, the
`END' rule is executed, and the value of `sum' is printed.  In this
example, the value of `sum' would be 80600.

   These more advanced `awk' techniques are covered in later sections
(*note Overview of Actions: Action Overview.).  Before you can move on
to more advanced `awk' programming, you have to know how `awk'
interprets your input and displays your output.  By manipulating fields
and using `print' statements, you can produce some very useful and
impressive looking reports.


File: gawk.info,  Node: Statements/Lines,  Next: Other Features,  Prev: More Complex,  Up: Getting Started

`awk' Statements Versus Lines
=============================

   Most often, each line in an `awk' program is a separate statement or
separate rule, like this:

     awk '/12/  { print $0 }
          /21/  { print $0 }' BBS-list inventory-shipped

   However, `gawk' will ignore newlines after any of the following:

     ,    {    ?    :    ||    &&    do    else

A newline at any other point is considered the end of the statement.
(Splitting lines after `?' and `:' is a minor `gawk' extension.  The
`?' and `:' referred to here is the three operand conditional
expression described in *Note Conditional Expressions: Conditional Exp.)

   If you would like to split a single statement into two lines at a
point where a newline would terminate it, you can "continue" it by
ending the first line with a backslash character, `\'.  The backslash
must be the final character on the line to be recognized as a
continuation character.  This is allowed absolutely anywhere in the
statement, even in the middle of a string or regular expression.  For
example:

     awk '/This regular expression is too long, so continue it\
      on the next line/ { print $1 }'

We have generally not used backslash continuation in the sample programs
in this Info file.  Since in `gawk' there is no limit on the length of
a line, it is never strictly necessary; it just makes programs more
readable.  For this same reason, as well as for clarity, we have kept
most statements short in the sample programs presented throughout the
Info file.  Backslash continuation is most useful when your `awk'
program is in a separate source file, instead of typed in on the
command line.  You should also note that many `awk' implementations are
more particular about where you may use backslash continuation. For
example, they may not allow you to split a string constant using
backslash continuation.  Thus, for maximal portability of your `awk'
programs, it is best not to split your lines in the middle of a regular
expression or a string.

   *Caution: backslash continuation does not work as described above
with the C shell.*  Continuation with backslash works for `awk'
programs in files, and also for one-shot programs *provided* you are
using a POSIX-compliant shell, such as the Bourne shell or Bash, the
GNU Bourne-Again shell.  But the C shell (`csh') behaves differently!
There, you must use two backslashes in a row, followed by a newline.
Note also that when using the C shell, *every* newline in your awk
program must be escaped with a backslash. To illustrate:

     % awk 'BEGIN { \
     ?   print \\
     ?       "hello, world" \
     ? }'
     -| hello, world

Here, the `%' and `?' are the C shell's primary and secondary prompts,
analogous to the standard shell's `$' and `>'.

   `awk' is a line-oriented language.  Each rule's action has to begin
on the same line as the pattern.  To have the pattern and action on
separate lines, you *must* use backslash continuation--there is no
other way.

   Note that backslash continuation and comments do not mix. As soon as
`awk' sees the `#' that starts a comment, it ignores *everything* on
the rest of the line. For example:

     $ gawk 'BEGIN { print "dont panic" # a friendly \
     >                                    BEGIN rule
     > }'
     error--> gawk: cmd. line:2:                BEGIN rule
     error--> gawk: cmd. line:2:                ^ parse error

Here, it looks like the backslash would continue the comment onto the
next line. However, the backslash-newline combination is never even
noticed, since it is "hidden" inside the comment. Thus, the `BEGIN' is
noted as a syntax error.

   When `awk' statements within one rule are short, you might want to
put more than one of them on a line.  You do this by separating the
statements with a semicolon, `;'.

   This also applies to the rules themselves.  Thus, the previous
program could have been written:

     /12/ { print $0 } ; /21/ { print $0 }

*Note:* the requirement that rules on the same line must be separated
with a semicolon was not in the original `awk' language; it was added
for consistency with the treatment of statements within an action.


File: gawk.info,  Node: Other Features,  Next: When,  Prev: Statements/Lines,  Up: Getting Started

Other Features of `awk'
=======================

   The `awk' language provides a number of predefined, or built-in
variables, which your programs can use to get information from `awk'.
There are other variables your program can set to control how `awk'
processes your data.

   In addition, `awk' provides a number of built-in functions for doing
common computational and string related operations.

   As we develop our presentation of the `awk' language, we introduce
most of the variables and many of the functions. They are defined
systematically in *Note Built-in Variables::, and *Note Built-in
Functions: Built-in.


File: gawk.info,  Node: When,  Prev: Other Features,  Up: Getting Started

When to Use `awk'
=================

   You might wonder how `awk' might be useful for you.  Using utility
programs, advanced patterns, field separators, arithmetic statements,
and other selection criteria, you can produce much more complex output.
The `awk' language is very useful for producing reports from large
amounts of raw data, such as summarizing information from the output of
other utility programs like `ls'.  (*Note A More Complex Example: More
Complex.)

   Programs written with `awk' are usually much smaller than they would
be in other languages.  This makes `awk' programs easy to compose and
use.  Often, `awk' programs can be quickly composed at your terminal,
used once, and thrown away.  Since `awk' programs are interpreted, you
can avoid the (usually lengthy) compilation part of the typical
edit-compile-test-debug cycle of software development.

   Complex programs have been written in `awk', including a complete
retargetable assembler for eight-bit microprocessors (*note
Glossary::., for more information) and a microcode assembler for a
special purpose Prolog computer.  However, `awk''s capabilities are
strained by tasks of such complexity.

   If you find yourself writing `awk' scripts of more than, say, a few
hundred lines, you might consider using a different programming
language.  Emacs Lisp is a good choice if you need sophisticated string
or pattern matching capabilities.  The shell is also good at string and
pattern matching; in addition, it allows powerful use of the system
utilities.  More conventional languages, such as C, C++, and Lisp, offer
better facilities for system programming and for managing the complexity
of large programs.  Programs in these languages may require more lines
of source code than the equivalent `awk' programs, but they are easier
to maintain and usually run more efficiently.


File: gawk.info,  Node: One-liners,  Next: Regexp,  Prev: Getting Started,  Up: Top

Useful One Line Programs
************************

   Many useful `awk' programs are short, just a line or two.  Here is a
collection of useful, short programs to get you started.  Some of these
programs contain constructs that haven't been covered yet.  The
description of the program will give you a good idea of what is going
on, but please read the rest of the Info file to become an `awk' expert!

   Most of the examples use a data file named `data'.  This is just a
placeholder; if you were to use these programs yourself, you would
substitute your own file names for `data'.

   Since you are reading this in Info, each line of the example code is
enclosed in quotes, to represent text that you would type literally.
The examples themselves represent shell commands that use single quotes
to keep the shell from interpreting the contents of the program.  When
reading the examples, focus on the text between the open and close
quotes.

`awk '{ if (length($0) > max) max = length($0) }'
`     END { print max }' data'
     This program prints the length of the longest input line.

`awk 'length($0) > 80' data'
     This program prints every line that is longer than 80 characters.
     The sole rule has a relational expression as its pattern, and has
     no action (so the default action, printing the record, is used).

`expand data | awk '{ if (x < length()) x = length() }'
`                   END { print "maximum line length is " x }''
     This program prints the length of the longest line in `data'.  The
     input is processed by the `expand' program to change tabs into
     spaces, so the widths compared are actually the right-margin
     columns.

`awk 'NF > 0' data'
     This program prints every line that has at least one field.  This
     is an easy way to delete blank lines from a file (or rather, to
     create a new file similar to the old file but from which the blank
     lines have been deleted).

`awk 'BEGIN { for (i = 1; i <= 7; i++)'
`               print int(101 * rand()) }''
     This program prints seven random numbers from zero to 100,
     inclusive.

`ls -lg FILES | awk '{ x += $5 } ; END { print "total bytes: " x }''
     This program prints the total number of bytes used by FILES.

`ls -lg FILES | awk '{ x += $5 }'
`                 END { print "total K-bytes: " (x + 1023)/1024 }''
     This program prints the total number of kilobytes used by FILES.

`awk -F: '{ print $1 }' /etc/passwd | sort'
     This program prints a sorted list of the login names of all users.

`awk 'END { print NR }' data'
     This program counts lines in a file.

`awk 'NR % 2 == 0' data'
     This program prints the even numbered lines in the data file.  If
     you were to use the expression `NR % 2 == 1' instead, it would
     print the odd numbered lines.


File: gawk.info,  Node: Regexp,  Next: Reading Files,  Prev: One-liners,  Up: Top

Regular Expressions
*******************

   A "regular expression", or "regexp", is a way of describing a set of
strings.  Because regular expressions are such a fundamental part of
`awk' programming, their format and use deserve a separate chapter.

   A regular expression enclosed in slashes (`/') is an `awk' pattern
that matches every input record whose text belongs to that set.

   The simplest regular expression is a sequence of letters, numbers, or
both.  Such a regexp matches any string that contains that sequence.
Thus, the regexp `foo' matches any string containing `foo'.  Therefore,
the pattern `/foo/' matches any input record containing the three
characters `foo', *anywhere* in the record.  Other kinds of regexps let
you specify more complicated classes of strings.

* Menu:

* Regexp Usage::                How to Use Regular Expressions.
* Escape Sequences::            How to write non-printing characters.
* Regexp Operators::            Regular Expression Operators.
* GNU Regexp Operators::        Operators specific to GNU software.
* Case-sensitivity::            How to do case-insensitive matching.
* Leftmost Longest::            How much text matches.
* Computed Regexps::            Using Dynamic Regexps.


File: gawk.info,  Node: Regexp Usage,  Next: Escape Sequences,  Prev: Regexp,  Up: Regexp

How to Use Regular Expressions
==============================

   A regular expression can be used as a pattern by enclosing it in
slashes.  Then the regular expression is tested against the entire text
of each record.  (Normally, it only needs to match some part of the
text in order to succeed.)  For example, this prints the second field
of each record that contains the three characters `foo' anywhere in it:

     $ awk '/foo/ { print $2 }' BBS-list
     -| 555-1234
     -| 555-6699
     -| 555-6480
     -| 555-2127

   Regular expressions can also be used in matching expressions.  These
expressions allow you to specify the string to match against; it need
not be the entire current input record.  The two operators, `~' and
`!~', perform regular expression comparisons.  Expressions using these
operators can be used as patterns or in `if', `while', `for', and `do'
statements.  (*Note Control Statements in Actions: Statements.)

`EXP ~ /REGEXP/'
     This is true if the expression EXP (taken as a string) is matched
     by REGEXP.  The following example matches, or selects, all input
     records with the upper-case letter `J' somewhere in the first
     field:

          $ awk '$1 ~ /J/' inventory-shipped
          -| Jan  13  25  15 115
          -| Jun  31  42  75 492
          -| Jul  24  34  67 436
          -| Jan  21  36  64 620

     So does this:

          awk '{ if ($1 ~ /J/) print }' inventory-shipped

`EXP !~ /REGEXP/'
     This is true if the expression EXP (taken as a character string)
     is *not* matched by REGEXP.  The following example matches, or
     selects, all input records whose first field *does not* contain
     the upper-case letter `J':

          $ awk '$1 !~ /J/' inventory-shipped
          -| Feb  15  32  24 226
          -| Mar  15  24  34 228
          -| Apr  31  52  63 420
          -| May  16  34  29 208
          ...

   When a regexp is written enclosed in slashes, like `/foo/', we call
it a "regexp constant", much like `5.27' is a numeric constant, and
`"foo"' is a string constant.


File: gawk.info,  Node: Escape Sequences,  Next: Regexp Operators,  Prev: Regexp Usage,  Up: Regexp

Escape Sequences
================

   Some characters cannot be included literally in string constants
(`"foo"') or regexp constants (`/foo/').  You represent them instead
with "escape sequences", which are character sequences beginning with a
backslash (`\').

   One use of an escape sequence is to include a double-quote character
in a string constant.  Since a plain double-quote would end the string,
you must use `\"' to represent an actual double-quote character as a
part of the string.  For example:

     $ awk 'BEGIN { print "He said \"hi!\" to her." }'
     -| He said "hi!" to her.

   The  backslash character itself is another character that cannot be
included normally; you write `\\' to put one backslash in the string or
regexp.  Thus, the string whose contents are the two characters `"' and
`\' must be written `"\"\\"'.

   Another use of backslash is to represent unprintable characters such
as tab or newline.  While there is nothing to stop you from entering
most unprintable characters directly in a string constant or regexp
constant, they may look ugly.

   Here is a table of all the escape sequences used in `awk', and what
they represent. Unless noted otherwise, all of these escape sequences
apply to both string constants and regexp constants.

`\\'
     A literal backslash, `\'.

`\a'
     The "alert" character, `Control-g', ASCII code 7 (BEL).

`\b'
     Backspace, `Control-h', ASCII code 8 (BS).

`\f'
     Formfeed, `Control-l', ASCII code 12 (FF).

`\n'
     Newline, `Control-j', ASCII code 10 (LF).

`\r'
     Carriage return, `Control-m', ASCII code 13 (CR).

`\t'
     Horizontal tab, `Control-i', ASCII code 9 (HT).

`\v'
     Vertical tab, `Control-k', ASCII code 11 (VT).

`\NNN'
     The octal value NNN, where NNN are one to three digits between `0'
     and `7'.  For example, the code for the ASCII ESC (escape)
     character is `\033'.

`\xHH...'
     The hexadecimal value HH, where HH are hexadecimal digits (`0'
     through `9' and either `A' through `F' or `a' through `f').  Like
     the same construct in ANSI C, the escape sequence continues until
     the first non-hexadecimal digit is seen.  However, using more than
     two hexadecimal digits produces undefined results. (The `\x'
     escape sequence is not allowed in POSIX `awk'.)

`\/'
     A literal slash (necessary for regexp constants only).  You use
     this when you wish to write a regexp constant that contains a
     slash. Since the regexp is delimited by slashes, you need to
     escape the slash that is part of the pattern, in order to tell
     `awk' to keep processing the rest of the regexp.

`\"'
     A literal double-quote (necessary for string constants only).  You
     use this when you wish to write a string constant that contains a
     double-quote. Since the string is delimited by double-quotes, you
     need to escape the quote that is part of the string, in order to
     tell `awk' to keep processing the rest of the string.

   In `gawk', there are additional two character sequences that begin
with backslash that have special meaning in regexps.  *Note Additional
Regexp Operators Only in `gawk': GNU Regexp Operators.

   In a string constant, what happens if you place a backslash before
something that is not one of the characters listed above?  POSIX `awk'
purposely leaves this case undefined.  There are two choices.

   * Strip the backslash out.  This is what Unix `awk' and `gawk' both
     do.  For example, `"a\qc"' is the same as `"aqc"'.

   * Leave the backslash alone.  Some other `awk' implementations do
     this.  In such implementations, `"a\qc"' is the same as if you had
     typed `"a\\qc"'.

   In a regexp, a backslash before any character that is not in the
above table, and not listed in *Note Additional Regexp Operators Only
in `gawk': GNU Regexp Operators, means that the next character should
be taken literally, even if it would normally be a regexp operator.
E.g., `/a\+b/' matches the three characters `a+b'.

   For complete portability, do not use a backslash before any
character not listed in the table above.

   Another interesting question arises. Suppose you use an octal or
hexadecimal escape to represent a regexp metacharacter (*note Regular
Expression Operators: Regexp Operators.).  Does `awk' treat the
character as literal character, or as a regexp operator?

   It turns out that historically, such characters were taken literally
(d.c.).  However, the POSIX standard indicates that they should be
treated as real metacharacters, and this is what `gawk' does.  However,
in compatibility mode (*note Command Line Options: Options.), `gawk'
treats the characters represented by octal and hexadecimal escape
sequences literally when used in regexp constants. Thus, `/a\52b/' is
equivalent to `/a\*b/'.

   To summarize:

  1. The escape sequences in the table above are always processed first,
     for both string constants and regexp constants. This happens very
     early, as soon as `awk' reads your program.

  2. `gawk' processes both regexp constants and dynamic regexps (*note
     Using Dynamic Regexps: Computed Regexps.), for the special
     operators listed in *Note Additional Regexp Operators Only in
     `gawk': GNU Regexp Operators.

  3. A backslash before any other character means to treat that
     character literally.


File: gawk.info,  Node: Regexp Operators,  Next: GNU Regexp Operators,  Prev: Escape Sequences,  Up: Regexp

Regular Expression Operators
============================

   You can combine regular expressions with the following characters,
called "regular expression operators", or "metacharacters", to increase
the power and versatility of regular expressions.

   The escape sequences described in *Note Escape Sequences::, are
valid inside a regexp.  They are introduced by a `\'.  They are
recognized and converted into the corresponding real characters as the
very first step in processing regexps.

   Here is a table of metacharacters.  All characters that are not
escape sequences and that are not listed in the table stand for
themselves.

`\'
     This is used to suppress the special meaning of a character when
     matching.  For example:

          \$

     matches the character `$'.

`^'
     This matches the beginning of a string.  For example:

          ^@chapter

     matches the `@chapter' at the beginning of a string, and can be
     used to identify chapter beginnings in Texinfo source files.  The
     `^' is known as an "anchor", since it anchors the pattern to
     matching only at the beginning of the string.

     It is important to realize that `^' does not match the beginning of
     a line embedded in a string.  In this example the condition is not
     true:

          if ("line1\nLINE 2" ~ /^L/) ...

`$'
     This is similar to `^', but it matches only at the end of a string.
     For example:

          p$

     matches a record that ends with a `p'.  The `$' is also an anchor,
     and also does not match the end of a line embedded in a string.
     In this example the condition is not true:

          if ("line1\nLINE 2" ~ /1$/) ...

`.'
     The period, or dot, matches any single character, *including* the
     newline character.  For example:

          .P

     matches any single character followed by a `P' in a string.  Using
     concatenation we can make a regular expression like `U.A', which
     matches any three-character sequence that begins with `U' and ends
     with `A'.

     In strict POSIX mode (*note Command Line Options: Options.), `.'
     does not match the NUL character, which is a character with all
     bits equal to zero.  Otherwise, NUL is just another character.
     Other versions of `awk' may not be able to match the NUL character.

`[...]'
     This is called a "character list".  It matches any *one* of the
     characters that are enclosed in the square brackets.  For example:

          [MVX]

     matches any one of the characters `M', `V', or `X' in a string.

     Ranges of characters are indicated by using a hyphen between the
     beginning and ending characters, and enclosing the whole thing in
     brackets.  For example:

          [0-9]

     matches any digit.  Multiple ranges are allowed. E.g., the list
     `[A-Za-z0-9]' is a common way to express the idea of "all
     alphanumeric characters."

     To include one of the characters `\', `]', `-' or `^' in a
     character list, put a `\' in front of it.  For example:

          [d\]]

     matches either `d', or `]'.

     This treatment of `\' in character lists is compatible with other
     `awk' implementations, and is also mandated by POSIX.  The regular
     expressions in `awk' are a superset of the POSIX specification for
     Extended Regular Expressions (EREs).  POSIX EREs are based on the
     regular expressions accepted by the traditional `egrep' utility.

     "Character classes" are a new feature introduced in the POSIX
     standard.  A character class is a special notation for describing
     lists of characters that have a specific attribute, but where the
     actual characters themselves can vary from country to country
     and/or from character set to character set.  For example, the
     notion of what is an alphabetic character differs in the USA and
     in France.

     A character class is only valid in a regexp *inside* the brackets
     of a character list.  Character classes consist of `[:', a keyword
     denoting the class, and `:]'.  Here are the character classes
     defined by the POSIX standard.

    `[:alnum:]'
          Alphanumeric characters.

    `[:alpha:]'
          Alphabetic characters.

    `[:blank:]'
          Space and tab characters.

    `[:cntrl:]'
          Control characters.

    `[:digit:]'
          Numeric characters.

    `[:graph:]'
          Characters that are printable and are also visible.  (A space
          is printable, but not visible, while an `a' is both.)

    `[:lower:]'
          Lower-case alphabetic characters.

    `[:print:]'
          Printable characters (characters that are not control
          characters.)

    `[:punct:]'
          Punctuation characters (characters that are not letter,
          digits, control characters, or space characters).

    `[:space:]'
          Space characters (such as space, tab, and formfeed, to name a
          few).

    `[:upper:]'
          Upper-case alphabetic characters.

    `[:xdigit:]'
          Characters that are hexadecimal digits.

     For example, before the POSIX standard, to match alphanumeric
     characters, you had to write `/[A-Za-z0-9]/'.  If your character
     set had other alphabetic characters in it, this would not match
     them.  With the POSIX character classes, you can write
     `/[[:alnum:]]/', and this will match *all* the alphabetic and
     numeric characters in your character set.

     Two additional special sequences can appear in character lists.
     These apply to non-ASCII character sets, which can have single
     symbols (called "collating elements") that are represented with
     more than one character, as well as several characters that are
     equivalent for "collating", or sorting, purposes.  (E.g., in
     French, a plain "e" and a grave-accented "`e" are equivalent.)

    Collating Symbols
          A "collating symbol" is a multi-character collating element
          enclosed in `[.' and `.]'.  For example, if `ch' is a
          collating element, then `[[.ch.]]' is a regexp that matches
          this collating element, while `[ch]' is a regexp that matches
          either `c' or `h'.

    Equivalence Classes
          An "equivalence class" is a locale-specific name for a list of
          characters that are equivalent. The name is enclosed in `[='
          and `=]'.  For example, the name `e' might be used to
          represent all of "e," "`e," and "'e." In this case, `[[=e]]'
          is a regexp that matches any of `e', `'e',  or ``e'.

     These features are very valuable in non-English speaking locales.

     *Caution:* The library functions that `gawk' uses for regular
     expression matching currently only recognize POSIX character
     classes; they do not recognize collating symbols or equivalence
     classes.

`[^ ...]'
     This is a "complemented character list".  The first character after
     the `[' *must* be a `^'.  It matches any characters *except* those
     in the square brackets.  For example:

          [^0-9]

     matches any character that is not a digit.

`|'
     This is the "alternation operator", and it is used to specify
     alternatives.  For example:

          ^P|[0-9]

     matches any string that matches either `^P' or `[0-9]'.  This
     means it matches any string that starts with `P' or contains a
     digit.

     The alternation applies to the largest possible regexps on either
     side.  In other words, `|' has the lowest precedence of all the
     regular expression operators.

`(...)'
     Parentheses are used for grouping in regular expressions as in
     arithmetic.  They can be used to concatenate regular expressions
     containing the alternation operator, `|'.  For example,
     `@(samp|code)\{[^}]+\}' matches both `@code{foo}' and
     `@samp{bar}'. (These are Texinfo formatting control sequences.)

`*'
     This symbol means that the preceding regular expression is to be
     repeated as many times as necessary to find a match.  For example:

          ph*

     applies the `*' symbol to the preceding `h' and looks for matches
     of one `p' followed by any number of `h's.  This will also match
     just `p' if no `h's are present.

     The `*' repeats the *smallest* possible preceding expression.
     (Use parentheses if you wish to repeat a larger expression.)  It
     finds as many repetitions as possible.  For example:

          awk '/\(c[ad][ad]*r x\)/ { print }' sample

     prints every record in `sample' containing a string of the form
     `(car x)', `(cdr x)', `(cadr x)', and so on.  Notice the escaping
     of the parentheses by preceding them with backslashes.

`+'
     This symbol is similar to `*', but the preceding expression must be
     matched at least once.  This means that:

          wh+y

     would match `why' and `whhy' but not `wy', whereas `wh*y' would
     match all three of these strings.  This is a simpler way of
     writing the last `*' example:

          awk '/\(c[ad]+r x\)/ { print }' sample

`?'
     This symbol is similar to `*', but the preceding expression can be
     matched either once or not at all.  For example:

          fe?d

     will match `fed' and `fd', but nothing else.

`{N}'
`{N,}'
`{N,M}'
     One or two numbers inside braces denote an "interval expression".
     If there is one number in the braces, the preceding regexp is
     repeated N times.  If there are two numbers separated by a comma,
     the preceding regexp is repeated N to M times.  If there is one
     number followed by a comma, then the preceding regexp is repeated
     at least N times.

    `wh{3}y'
          matches `whhhy' but not `why' or `whhhhy'.

    `wh{3,5}y'
          matches `whhhy' or `whhhhy' or `whhhhhy', only.

    `wh{2,}y'
          matches `whhy' or `whhhy', and so on.

     Interval expressions were not traditionally available in `awk'.
     As part of the POSIX standard they were added, to make `awk' and
     `egrep' consistent with each other.

     However, since old programs may use `{' and `}' in regexp
     constants, by default `gawk' does *not* match interval expressions
     in regexps.  If either `--posix' or `--re-interval' are specified
     (*note Command Line Options: Options.), then interval expressions
     are allowed in regexps.

   In regular expressions, the `*', `+', and `?' operators, as well as
the braces `{' and `}', have the highest precedence, followed by
concatenation, and finally by `|'.  As in arithmetic, parentheses can
change how operators are grouped.

   If `gawk' is in compatibility mode (*note Command Line Options:
Options.), character classes and interval expressions are not available
in regular expressions.

   The next node discusses the GNU-specific regexp operators, and
provides more detail concerning how command line options affect the way
`gawk' interprets the characters in regular expressions.


File: gawk.info,  Node: GNU Regexp Operators,  Next: Case-sensitivity,  Prev: Regexp Operators,  Up: Regexp

Additional Regexp Operators Only in `gawk'
==========================================

   GNU software that deals with regular expressions provides a number of
additional regexp operators.  These operators are described in this
section, and are specific to `gawk'; they are not available in other
`awk' implementations.

   Most of the additional operators are for dealing with word matching.
For our purposes, a "word" is a sequence of one or more letters, digits,
or underscores (`_').

`\w'
     This operator matches any word-constituent character, i.e. any
     letter, digit, or underscore. Think of it as a short-hand for
     `[[:alnum:]_]'.

`\W'
     This operator matches any character that is not word-constituent.
     Think of it as a short-hand for `[^[:alnum:]_]'.

`\<'
     This operator matches the empty string at the beginning of a word.
     For example, `/\<away/' matches `away', but not `stowaway'.

`\>'
     This operator matches the empty string at the end of a word.  For
     example, `/stow\>/' matches `stow', but not `stowaway'.

`\y'
     This operator matches the empty string at either the beginning or
     the end of a word (the word boundar*y*).  For example, `\yballs?\y'
     matches either `ball' or `balls' as a separate word.

`\B'
     This operator matches the empty string within a word. In other
     words, `\B' matches the empty string that occurs between two
     word-constituent characters. For example, `/\Brat\B/' matches
     `crate', but it does not match `dirty rat'.  `\B' is essentially
     the opposite of `\y'.

   There are two other operators that work on buffers.  In Emacs, a
"buffer" is, naturally, an Emacs buffer.  For other programs, the
regexp library routines that `gawk' uses consider the entire string to
be matched as the buffer.

   For `awk', since `^' and `$' always work in terms of the beginning
and end of strings, these operators don't add any new capabilities.
They are provided for compatibility with other GNU software.

`\`'
     This operator matches the empty string at the beginning of the
     buffer.

`\''
     This operator matches the empty string at the end of the buffer.

   In other GNU software, the word boundary operator is `\b'. However,
that conflicts with the `awk' language's definition of `\b' as
backspace, so `gawk' uses a different letter.

   An alternative method would have been to require two backslashes in
the GNU operators, but this was deemed to be too confusing, and the
current method of using `\y' for the GNU `\b' appears to be the lesser
of two evils.

   The various command line options (*note Command Line Options:
Options.) control how `gawk' interprets characters in regexps.

No options
     In the default case, `gawk' provide all the facilities of POSIX
     regexps and the GNU regexp operators described in *Note Regular
     Expression Operators: Regexp Operators.  However, interval
     expressions are not supported.

`--posix'
     Only POSIX regexps are supported, the GNU operators are not special
     (e.g., `\w' matches a literal `w').  Interval expressions are
     allowed.

`--traditional'
     Traditional Unix `awk' regexps are matched. The GNU operators are
     not special, interval expressions are not available, and neither
     are the POSIX character classes (`[[:alnum:]]' and so on).
     Characters described by octal and hexadecimal escape sequences are
     treated literally, even if they represent regexp metacharacters.

`--re-interval'
     Allow interval expressions in regexps, even if `--traditional' has
     been provided.


File: gawk.info,  Node: Case-sensitivity,  Next: Leftmost Longest,  Prev: GNU Regexp Operators,  Up: Regexp

Case-sensitivity in Matching
============================

   Case is normally significant in regular expressions, both when
matching ordinary characters (i.e. not metacharacters), and inside
character sets.  Thus a `w' in a regular expression matches only a
lower-case `w' and not an upper-case `W'.

   The simplest way to do a case-independent match is to use a character
list: `[Ww]'.  However, this can be cumbersome if you need to use it
often; and it can make the regular expressions harder to read.  There
are two alternatives that you might prefer.

   One way to do a case-insensitive match at a particular point in the
program is to convert the data to a single case, using the `tolower' or
`toupper' built-in string functions (which we haven't discussed yet;
*note Built-in Functions for String Manipulation: String Functions.).
For example:

     tolower($1) ~ /foo/  { ... }

converts the first field to lower-case before matching against it.
This will work in any POSIX-compliant implementation of `awk'.

   Another method, specific to `gawk', is to set the variable
`IGNORECASE' to a non-zero value (*note Built-in Variables::.).  When
`IGNORECASE' is not zero, *all* regexp and string operations ignore
case.  Changing the value of `IGNORECASE' dynamically controls the case
sensitivity of your program as it runs.  Case is significant by default
because `IGNORECASE' (like most variables) is initialized to zero.

     x = "aB"
     if (x ~ /ab/) ...   # this test will fail
     
     IGNORECASE = 1
     if (x ~ /ab/) ...   # now it will succeed

   In general, you cannot use `IGNORECASE' to make certain rules
case-insensitive and other rules case-sensitive, because there is no way
to set `IGNORECASE' just for the pattern of a particular rule.  To do
this, you must use character lists or `tolower'.  However, one thing
you can do only with `IGNORECASE' is turn case-sensitivity on or off
dynamically for all the rules at once.

   `IGNORECASE' can be set on the command line, or in a `BEGIN' rule
(*note Other Command Line Arguments: Other Arguments.; also *note
Startup and Cleanup Actions: Using BEGIN/END.).  Setting `IGNORECASE'
from the command line is a way to make a program case-insensitive
without having to edit it.

   Prior to version 3.0 of `gawk', the value of `IGNORECASE' only
affected regexp operations. It did not affect string comparison with
`==', `!=', and so on.  Beginning with version 3.0, both regexp and
string comparison operations are affected by `IGNORECASE'.

   Beginning with version 3.0 of `gawk', the equivalences between
upper-case and lower-case characters are based on the ISO-8859-1 (ISO
Latin-1) character set. This character set is a superset of the
traditional 128 ASCII characters, that also provides a number of
characters suitable for use with European languages.

   The value of `IGNORECASE' has no effect if `gawk' is in
compatibility mode (*note Command Line Options: Options.).  Case is
always significant in compatibility mode.


File: gawk.info,  Node: Leftmost Longest,  Next: Computed Regexps,  Prev: Case-sensitivity,  Up: Regexp

How Much Text Matches?
======================

   Consider the following example:

     echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'

   This example uses the `sub' function (which we haven't discussed yet,
*note Built-in Functions for String Manipulation: String Functions.) to
make a change to the input record. Here, the regexp `/a+/' indicates
"one or more `a' characters," and the replacement text is `<A>'.

   The input contains four `a' characters.  What will the output be?
In other words, how many is "one or more"--will `awk' match two, three,
or all four `a' characters?

   The answer is, `awk' (and POSIX) regular expressions always match
the leftmost, *longest* sequence of input characters that can match.
Thus, in this example, all four `a' characters are replaced with `<A>'.

     $ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
     -| <A>bcd

   For simple match/no-match tests, this is not so important. But when
doing regexp-based field and record splitting, and text matching and
substitutions with the `match', `sub', `gsub', and `gensub' functions,
it is very important.  *Note Built-in Functions for String
Manipulation: String Functions, for more information on these functions.
Understanding this principle is also important for regexp-based record
and field splitting (*note How Input is Split into Records: Records.,
and also *note Specifying How Fields are Separated: Field Separators.).


File: gawk.info,  Node: Computed Regexps,  Prev: Leftmost Longest,  Up: Regexp

Using Dynamic Regexps
=====================

   The right hand side of a `~' or `!~' operator need not be a regexp
constant (i.e. a string of characters between slashes).  It may be any
expression.  The expression is evaluated, and converted if necessary to
a string; the contents of the string are used as the regexp.  A regexp
that is computed in this way is called a "dynamic regexp".  For example:

     BEGIN { identifier_regexp = "[A-Za-z_][A-Za-z_0-9]+" }
     $0 ~ identifier_regexp    { print }

sets `identifier_regexp' to a regexp that describes `awk' variable
names, and tests if the input record matches this regexp.

   *Caution:* When using the `~' and `!~' operators, there is a
difference between a regexp constant enclosed in slashes, and a string
constant enclosed in double quotes.  If you are going to use a string
constant, you have to understand that the string is in essence scanned
*twice*; the first time when `awk' reads your program, and the second
time when it goes to match the string on the left-hand side of the
operator with the pattern on the right.  This is true of any string
valued expression (such as `identifier_regexp' above), not just string
constants.

   What difference does it make if the string is scanned twice? The
answer has to do with escape sequences, and particularly with
backslashes.  To get a backslash into a regular expression inside a
string, you have to type two backslashes.

   For example, `/\*/' is a regexp constant for a literal `*'.  Only
one backslash is needed.  To do the same thing with a string, you would
have to type `"\\*"'.  The first backslash escapes the second one, so
that the string actually contains the two characters `\' and `*'.

   Given that you can use both regexp and string constants to describe
regular expressions, which should you use?  The answer is "regexp
constants," for several reasons.

  1. String constants are more complicated to write, and more difficult
     to read. Using regexp constants makes your programs less
     error-prone.  Not understanding the difference between the two
     kinds of constants is a common source of errors.

  2. It is also more efficient to use regexp constants: `awk' can note
     that you have supplied a regexp and store it internally in a form
     that makes pattern matching more efficient.  When using a string
     constant, `awk' must first convert the string into this internal
     form, and then perform the pattern matching.

  3. Using regexp constants is better style; it shows clearly that you
     intend a regexp match.


File: gawk.info,  Node: Reading Files,  Next: Printing,  Prev: Regexp,  Up: Top

Reading Input Files
*******************

   In the typical `awk' program, all input is read either from the
standard input (by default the keyboard, but often a pipe from another
command) or from files whose names you specify on the `awk' command
line.  If you specify input files, `awk' reads them in order, reading
all the data from one before going on to the next.  The name of the
current input file can be found in the built-in variable `FILENAME'
(*note Built-in Variables::.).

   The input is read in units called "records", and processed by the
rules of your program one record at a time.  By default, each record is
one line.  Each record is automatically split into chunks called
"fields".  This makes it more convenient for programs to work on the
parts of a record.

   On rare occasions you will need to use the `getline' command.  The
`getline' command is valuable, both because it can do explicit input
from any number of files, and because the files used with it do not
have to be named on the `awk' command line (*note Explicit Input with
`getline': Getline.).

* Menu:

* Records::                     Controlling how data is split into records.
* Fields::                      An introduction to fields.
* Non-Constant Fields::         Non-constant Field Numbers.
* Changing Fields::             Changing the Contents of a Field.
* Field Separators::            The field separator and how to change it.
* Constant Size::               Reading constant width data.
* Multiple Line::               Reading multi-line records.
* Getline::                     Reading files under explicit program control
                                using the `getline' function.


File: gawk.info,  Node: Records,  Next: Fields,  Prev: Reading Files,  Up: Reading Files

How Input is Split into Records
===============================

   The `awk' utility divides the input for your `awk' program into
records and fields.  Records are separated by a character called the
"record separator".  By default, the record separator is the newline
character.  This is why records are, by default, single lines.  You can
use a different character for the record separator by assigning the
character to the built-in variable `RS'.

   You can change the value of `RS' in the `awk' program, like any
other variable, with the assignment operator, `=' (*note Assignment
Expressions: Assignment Ops.).  The new record-separator character
should be enclosed in quotation marks, which indicate a string
constant.  Often the right time to do this is at the beginning of
execution, before any input has been processed, so that the very first
record will be read with the proper separator.  To do this, use the
special `BEGIN' pattern (*note The `BEGIN' and `END' Special Patterns:
BEGIN/END.).  For example:

     awk 'BEGIN { RS = "/" } ; { print $0 }' BBS-list

changes the value of `RS' to `"/"', before reading any input.  This is
a string whose first character is a slash; as a result, records are
separated by slashes.  Then the input file is read, and the second rule
in the `awk' program (the action with no pattern) prints each record.
Since each `print' statement adds a newline at the end of its output,
the effect of this `awk' program is to copy the input with each slash
changed to a newline.  Here are the results of running the program on
`BBS-list':

     $ awk 'BEGIN { RS = "/" } ; { print $0 }' BBS-list
     -| aardvark     555-5553     1200
     -| 300          B
     -| alpo-net     555-3412     2400
     -| 1200
     -| 300     A
     -| barfly       555-7685     1200
     -| 300          A
     -| bites        555-1675     2400
     -| 1200
     -| 300     A
     -| camelot      555-0542     300               C
     -| core         555-2912     1200
     -| 300          C
     -| fooey        555-1234     2400
     -| 1200
     -| 300     B
     -| foot         555-6699     1200
     -| 300          B
     -| macfoo       555-6480     1200
     -| 300          A
     -| sdace        555-3430     2400
     -| 1200
     -| 300     A
     -| sabafoo      555-2127     1200
     -| 300          C
     -|

Note that the entry for the `camelot' BBS is not split.  In the
original data file (*note Data Files for the Examples: Sample Data
Files.), the line looks like this:

     camelot      555-0542     300               C

It only has one baud rate; there are no slashes in the record.

   Another way to change the record separator is on the command line,
using the variable-assignment feature (*note Other Command Line
Arguments: Other Arguments.).

     awk '{ print $0 }' RS="/" BBS-list

This sets `RS' to `/' before processing `BBS-list'.

   Using an unusual character such as `/' for the record separator
produces correct behavior in the vast majority of cases.  However, the
following (extreme) pipeline prints a surprising `1'.  There is one
field, consisting of a newline.  The value of the built-in variable
`NF' is the number of fields in the current record.

     $ echo | awk 'BEGIN { RS = "a" } ; { print NF }'
     -| 1

Reaching the end of an input file terminates the current input record,
even if the last character in the file is not the character in `RS'
(d.c.).

   The empty string, `""' (a string of no characters), has a special
meaning as the value of `RS': it means that records are separated by
one or more blank lines, and nothing else.  *Note Multiple-Line
Records: Multiple Line, for more details.

   If you change the value of `RS' in the middle of an `awk' run, the
new value is used to delimit subsequent records, but the record
currently being processed (and records already processed) are not
affected.

   After the end of the record has been determined, `gawk' sets the
variable `RT' to the text in the input that matched `RS'.

   The value of `RS' is in fact not limited to a one-character string.
It can be any regular expression (*note Regular Expressions: Regexp.).
In general, each record ends at the next string that matches the
regular expression; the next record starts at the end of the matching
string.  This general rule is actually at work in the usual case, where
`RS' contains just a newline: a record ends at the beginning of the
next matching string (the next newline in the input) and the following
record starts just after the end of this string (at the first character
of the following line).  The newline, since it matches `RS', is not
part of either record.

   When `RS' is a single character, `RT' will contain the same single
character. However, when `RS' is a regular expression, then `RT'
becomes more useful; it contains the actual input text that matched the
regular expression.

   The following example illustrates both of these features.  It sets
`RS' equal to a regular expression that matches either a newline, or a
series of one or more upper-case letters with optional leading and/or
trailing white space (*note Regular Expressions: Regexp.).

     $ echo record 1 AAAA record 2 BBBB record 3 |
     > gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" }
     >             { print "Record =", $0, "and RT =", RT }'
     -| Record = record 1 and RT =  AAAA
     -| Record = record 2 and RT =  BBBB
     -| Record = record 3 and RT =
     -|

The final line of output has an extra blank line. This is because the
value of `RT' is a newline, and then the `print' statement supplies its
own terminating newline.

   *Note A Simple Stream Editor: Simple Sed, for a more useful example
of `RS' as a regexp and `RT'.

   The use of `RS' as a regular expression and the `RT' variable are
`gawk' extensions; they are not available in compatibility mode (*note
Command Line Options: Options.).  In compatibility mode, only the first
character of the value of `RS' is used to determine the end of the
record.

   The `awk' utility keeps track of the number of records that have
been read so far from the current input file.  This value is stored in a
built-in variable called `FNR'.  It is reset to zero when a new file is
started.  Another built-in variable, `NR', is the total number of input
records read so far from all data files.  It starts at zero but is
never automatically reset to zero.


File: gawk.info,  Node: Fields,  Next: Non-Constant Fields,  Prev: Records,  Up: Reading Files

Examining Fields
================

   When `awk' reads an input record, the record is automatically
separated or "parsed" by the interpreter into chunks called "fields".
By default, fields are separated by whitespace, like words in a line.
Whitespace in `awk' means any string of one or more spaces, tabs or
newlines;(1) other characters such as formfeed, and so on, that are
considered whitespace by other languages are *not* considered
whitespace by `awk'.

   The purpose of fields is to make it more convenient for you to refer
to these pieces of the record.  You don't have to use them--you can
operate on the whole record if you wish--but fields are what make
simple `awk' programs so powerful.

   To refer to a field in an `awk' program, you use a dollar-sign, `$',
followed by the number of the field you want.  Thus, `$1' refers to the
first field, `$2' to the second, and so on.  For example, suppose the
following is a line of input:

     This seems like a pretty nice example.

Here the first field, or `$1', is `This'; the second field, or `$2', is
`seems'; and so on.  Note that the last field, `$7', is `example.'.
Because there is no space between the `e' and the `.', the period is
considered part of the seventh field.

   `NF' is a built-in variable whose value is the number of fields in
the current record.  `awk' updates the value of `NF' automatically,
each time a record is read.

   No matter how many fields there are, the last field in a record can
be represented by `$NF'.  So, in the example above, `$NF' would be the
same as `$7', which is `example.'.  Why this works is explained below
(*note Non-constant Field Numbers: Non-Constant Fields.).  If you try
to reference a field beyond the last one, such as `$8' when the record
has only seven fields, you get the empty string.

   `$0', which looks like a reference to the "zeroth" field, is a
special case: it represents the whole input record.  `$0' is used when
you are not interested in fields.

   Here are some more examples:

     $ awk '$1 ~ /foo/ { print $0 }' BBS-list
     -| fooey        555-1234     2400/1200/300     B
     -| foot         555-6699     1200/300          B
     -| macfoo       555-6480     1200/300          A
     -| sabafoo      555-2127     1200/300          C

This example prints each record in the file `BBS-list' whose first
field contains the string `foo'.  The operator `~' is called a
"matching operator" (*note How to Use Regular Expressions: Regexp
Usage.); it tests whether a string (here, the field `$1') matches a
given regular expression.

   By contrast, the following example looks for `foo' in *the entire
record* and prints the first field and the last field for each input
record containing a match.

     $ awk '/foo/ { print $1, $NF }' BBS-list
     -| fooey B
     -| foot B
     -| macfoo A
     -| sabafoo C

   ---------- Footnotes ----------

   (1)  In POSIX `awk', newlines are not considered whitespace for
separating fields.


File: gawk.info,  Node: Non-Constant Fields,  Next: Changing Fields,  Prev: Fields,  Up: Reading Files

Non-constant Field Numbers
==========================

   The number of a field does not need to be a constant.  Any
expression in the `awk' language can be used after a `$' to refer to a
field.  The value of the expression specifies the field number.  If the
value is a string, rather than a number, it is converted to a number.
Consider this example:

     awk '{ print $NR }'

Recall that `NR' is the number of records read so far: one in the first
record, two in the second, etc.  So this example prints the first field
of the first record, the second field of the second record, and so on.
For the twentieth record, field number 20 is printed; most likely, the
record has fewer than 20 fields, so this prints a blank line.

   Here is another example of using expressions as field numbers:

     awk '{ print $(2*2) }' BBS-list

   `awk' must evaluate the expression `(2*2)' and use its value as the
number of the field to print.  The `*' sign represents multiplication,
so the expression `2*2' evaluates to four.  The parentheses are used so
that the multiplication is done before the `$' operation; they are
necessary whenever there is a binary operator in the field-number
expression.  This example, then, prints the hours of operation (the
fourth field) for every line of the file `BBS-list'.  (All of the `awk'
operators are listed, in order of decreasing precedence, in *Note
Operator Precedence (How Operators Nest): Precedence.)

   If the field number you compute is zero, you get the entire record.
Thus, `$(2-2)' has the same value as `$0'.  Negative field numbers are
not allowed; trying to reference one will usually terminate your
running `awk' program.  (The POSIX standard does not define what
happens when you reference a negative field number.  `gawk' will notice
this and terminate your program.  Other `awk' implementations may
behave differently.)

   As mentioned in *Note Examining Fields: Fields, the number of fields
in the current record is stored in the built-in variable `NF' (also
*note Built-in Variables::.).  The expression `$NF' is not a special
feature: it is the direct consequence of evaluating `NF' and using its
value as a field number.


File: gawk.info,  Node: Changing Fields,  Next: Field Separators,  Prev: Non-Constant Fields,  Up: Reading Files

Changing the Contents of a Field
================================

   You can change the contents of a field as seen by `awk' within an
`awk' program; this changes what `awk' perceives as the current input
record.  (The actual input is untouched; `awk' *never* modifies the
input file.)

   Consider this example and its output:

     $ awk '{ $3 = $2 - 10; print $2, $3 }' inventory-shipped
     -| 13 3
     -| 15 5
     -| 15 5
     ...

The `-' sign represents subtraction, so this program reassigns field
three, `$3', to be the value of field two minus ten, `$2 - 10'.  (*Note
Arithmetic Operators: Arithmetic Ops.) Then field two, and the new
value for field three, are printed.

   In order for this to work, the text in field `$2' must make sense as
a number; the string of characters must be converted to a number in
order for the computer to do arithmetic on it.  The number resulting
from the subtraction is converted back to a string of characters which
then becomes field three.  *Note Conversion of Strings and Numbers:
Conversion.

   When you change the value of a field (as perceived by `awk'), the
text of the input record is recalculated to contain the new field where
the old one was.  Therefore, `$0' changes to reflect the altered field.
Thus, this program prints a copy of the input file, with 10 subtracted
from the second field of each line.

     $ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped
     -| Jan 3 25 15 115
     -| Feb 5 32 24 226
     -| Mar 5 24 34 228
     ...

   You can also assign contents to fields that are out of range.  For
example:

     $ awk '{ $6 = ($5 + $4 + $3 + $2)
     >        print $6 }' inventory-shipped
     -| 168
     -| 297
     -| 301
     ...

We've just created `$6', whose value is the sum of fields `$2', `$3',
`$4', and `$5'.  The `+' sign represents addition.  For the file
`inventory-shipped', `$6' represents the total number of parcels
shipped for a particular month.

   Creating a new field changes `awk''s internal copy of the current
input record--the value of `$0'.  Thus, if you do `print $0' after
adding a field, the record printed includes the new field, with the
appropriate number of field separators between it and the previously
existing fields.

   This recomputation affects and is affected by `NF' (the number of
fields; *note Examining Fields: Fields.), and by a feature that has not
been discussed yet, the "output field separator", `OFS', which is used
to separate the fields (*note Output Separators::.).  For example, the
value of `NF' is set to the number of the highest field you create.

   Note, however, that merely *referencing* an out-of-range field does
*not* change the value of either `$0' or `NF'.  Referencing an
out-of-range field only produces an empty string.  For example:

     if ($(NF+1) != "")
         print "can't happen"
     else
         print "everything is normal"

should print `everything is normal', because `NF+1' is certain to be
out of range.  (*Note The `if'-`else' Statement: If Statement, for more
information about `awk''s `if-else' statements.  *Note Variable Typing
and Comparison Expressions: Typing and Comparison, for more information
about the `!=' operator.)

   It is important to note that making an assignment to an existing
field will change the value of `$0', but will not change the value of
`NF', even when you assign the empty string to a field.  For example:

     $ echo a b c d | awk '{ OFS = ":"; $2 = ""
     >                       print $0; print NF }'
     -| a::c:d
     -| 4

The field is still there; it just has an empty value.  You can tell
because there are two colons in a row.

   This example shows what happens if you create a new field.

     $ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new"
     >                       print $0; print NF }'
     -| a::c:d::new
     -| 6

The intervening field, `$5' is created with an empty value (indicated
by the second pair of adjacent colons), and `NF' is updated with the
value six.

   Finally, decrementing `NF' will lose the values of the fields after
the new value of `NF', and `$0' will be recomputed.  Here is an example:

     $ echo a b c d e f | ../gawk '{ print "NF =", NF;
     >                               NF = 3; print $0 }'
     -| NF = 6
     -| a b c


File: gawk.info,  Node: Field Separators,  Next: Constant Size,  Prev: Changing Fields,  Up: Reading Files

Specifying How Fields are Separated
===================================

   This section is rather long; it describes one of the most fundamental
operations in `awk'.

* Menu:

* Basic Field Splitting::        How fields are split with single characters
                                 or simple strings.
* Regexp Field Splitting::       Using regexps as the field separator.
* Single Character Fields::      Making each character a separate field.
* Command Line Field Separator:: Setting `FS' from the command line.
* Field Splitting Summary::      Some final points and a summary table.


File: gawk.info,  Node: Basic Field Splitting,  Next: Regexp Field Splitting,  Prev: Field Separators,  Up: Field Separators

The Basics of Field Separating
------------------------------

   The "field separator", which is either a single character or a
regular expression, controls the way `awk' splits an input record into
fields.  `awk' scans the input record for character sequences that
match the separator; the fields themselves are the text between the
matches.

   In the examples below, we use the bullet symbol "*" to represent
spaces in the output.

   If the field separator is `oo', then the following line:

     moo goo gai pan

would be split into three fields: `m', `*g' and `*gai*pan'.  Note the
leading spaces in the values of the second and third fields.

   The field separator is represented by the built-in variable `FS'.
Shell programmers take note!  `awk' does *not* use the name `IFS' which
is used by the POSIX compatible shells (such as the Bourne shell, `sh',
or the GNU Bourne-Again Shell, Bash).

   You can change the value of `FS' in the `awk' program with the
assignment operator, `=' (*note Assignment Expressions: Assignment
Ops.).  Often the right time to do this is at the beginning of
execution, before any input has been processed, so that the very first
record will be read with the proper separator.  To do this, use the
special `BEGIN' pattern (*note The `BEGIN' and `END' Special Patterns:
BEGIN/END.).  For example, here we set the value of `FS' to the string
`","':

     awk 'BEGIN { FS = "," } ; { print $2 }'

Given the input line,

     John Q. Smith, 29 Oak St., Walamazoo, MI 42139

this `awk' program extracts and prints the string `*29*Oak*St.'.

   Sometimes your input data will contain separator characters that
don't separate fields the way you thought they would.  For instance, the
person's name in the example we just used might have a title or suffix
attached, such as `John Q. Smith, LXIX'.  From input containing such a
name:

     John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139

the above program would extract `*LXIX', instead of `*29*Oak*St.'.  If
you were expecting the program to print the address, you would be
surprised.  The moral is: choose your data layout and separator
characters carefully to prevent such problems.

   Normally, fields are separated by whitespace sequences (spaces, tabs
and newlines), not by single spaces: two spaces in a row do not delimit
an empty field.  The default value of the field separator `FS' is a
string containing a single space, `" "'.  If this value were
interpreted in the usual way, each space character would separate
fields, so two spaces in a row would make an empty field between them.
The reason this does not happen is that a single space as the value of
`FS' is a special case: it is taken to specify the default manner of
delimiting fields.

   If `FS' is any other single character, such as `","', then each
occurrence of that character separates two fields.  Two consecutive
occurrences delimit an empty field.  If the character occurs at the
beginning or the end of the line, that too delimits an empty field.  The
space character is the only single character which does not follow these
rules.


File: gawk.info,  Node: Regexp Field Splitting,  Next: Single Character Fields,  Prev: Basic Field Splitting,  Up: Field Separators

Using Regular Expressions to Separate Fields
--------------------------------------------

   The previous node discussed the use of single characters or simple
strings as the value of `FS'.  More generally, the value of `FS' may be
a string containing any regular expression.  In this case, each match
in the record for the regular expression separates fields.  For
example, the assignment:

     FS = ", \t"

makes every area of an input line that consists of a comma followed by a
space and a tab, into a field separator.  (`\t' is an "escape sequence"
that stands for a tab; *note Escape Sequences::., for the complete list
of similar escape sequences.)

   For a less trivial example of a regular expression, suppose you want
single spaces to separate fields the way single commas were used above.
You can set `FS' to `"[ ]"' (left bracket, space, right bracket).  This
regular expression matches a single space and nothing else (*note
Regular Expressions: Regexp.).

   There is an important difference between the two cases of `FS = " "'
(a single space) and `FS = "[ \t\n]+"' (left bracket, space, backslash,
"t", backslash, "n", right bracket, which is a regular expression
matching one or more spaces, tabs, or newlines).  For both values of
`FS', fields are separated by runs of spaces, tabs and/or newlines.
However, when the value of `FS' is `" "', `awk' will first strip
leading and trailing whitespace from the record, and then decide where
the fields are.

   For example, the following pipeline prints `b':

     $ echo ' a b c d ' | awk '{ print $2 }'
     -| b

However, this pipeline prints `a' (note the extra spaces around each
letter):

     $ echo ' a  b  c  d ' | awk 'BEGIN { FS = "[ \t]+" }
     >                                  { print $2 }'
     -| a

In this case, the first field is "null", or empty.

   The stripping of leading and trailing whitespace also comes into
play whenever `$0' is recomputed.  For instance, study this pipeline:

     $ echo '   a b c d' | awk '{ print; $2 = $2; print }'
     -|    a b c d
     -| a b c d

The first `print' statement prints the record as it was read, with
leading whitespace intact.  The assignment to `$2' rebuilds `$0' by
concatenating `$1' through `$NF' together, separated by the value of
`OFS'.  Since the leading whitespace was ignored when finding `$1', it
is not part of the new `$0'.  Finally, the last `print' statement
prints the new `$0'.


File: gawk.info,  Node: Single Character Fields,  Next: Command Line Field Separator,  Prev: Regexp Field Splitting,  Up: Field Separators

Making Each Character a Separate Field
--------------------------------------

   There are times when you may want to examine each character of a
record separately.  In `gawk', this is easy to do, you simply assign
the null string (`""') to `FS'. In this case, each individual character
in the record will become a separate field.  Here is an example:

     $ echo a b | gawk 'BEGIN { FS = "" }
     >                  {
     >                      for (i = 1; i <= NF; i = i + 1)
     >                          print "Field", i, "is", $i
     >                  }'
     -| Field 1 is a
     -| Field 2 is
     -| Field 3 is b

   Traditionally, the behavior for `FS' equal to `""' was not defined.
In this case, Unix `awk' would simply treat the entire record as only
having one field (d.c.).  In compatibility mode (*note Command Line
Options: Options.), if `FS' is the null string, then `gawk' will also
behave this way.


File: gawk.info,  Node: Command Line Field Separator,  Next: Field Splitting Summary,  Prev: Single Character Fields,  Up: Field Separators

Setting `FS' from the Command Line
----------------------------------

   `FS' can be set on the command line.  You use the `-F' option to do
so.  For example:

     awk -F, 'PROGRAM' INPUT-FILES

sets `FS' to be the `,' character.  Notice that the option uses a
capital `F'.  Contrast this with `-f', which specifies a file
containing an `awk' program.  Case is significant in command line
options: the `-F' and `-f' options have nothing to do with each other.
You can use both options at the same time to set the `FS' variable
*and* get an `awk' program from a file.

   The value used for the argument to `-F' is processed in exactly the
same way as assignments to the built-in variable `FS'.  This means that
if the field separator contains special characters, they must be escaped
appropriately.  For example, to use a `\' as the field separator, you
would have to type:

     # same as FS = "\\"
     awk -F\\\\ '...' files ...

Since `\' is used for quoting in the shell, `awk' will see `-F\\'.
Then `awk' processes the `\\' for escape characters (*note Escape
Sequences::.), finally yielding a single `\' to be used for the field
separator.

   As a special case, in compatibility mode (*note Command Line
Options: Options.), if the argument to `-F' is `t', then `FS' is set to
the tab character.  This is because if you type `-F\t' at the shell,
without any quotes, the `\' gets deleted, so `awk' figures that you
really want your fields to be separated with tabs, and not `t's.  Use
`-v FS="t"' on the command line if you really do want to separate your
fields with `t's (*note Command Line Options: Options.).

   For example, let's use an `awk' program file called `baud.awk' that
contains the pattern `/300/', and the action `print $1'.  Here is the
program:

     /300/   { print $1 }

   Let's also set `FS' to be the `-' character, and run the program on
the file `BBS-list'.  The following command prints a list of the names
of the bulletin boards that operate at 300 baud and the first three
digits of their phone numbers:

     $ awk -F- -f baud.awk BBS-list
     -| aardvark     555
     -| alpo
     -| barfly       555
     ...

Note the second line of output.  In the original file (*note Data Files
for the Examples: Sample Data Files.), the second line looked like this:

     alpo-net     555-3412     2400/1200/300     A

   The `-' as part of the system's name was used as the field
separator, instead of the `-' in the phone number that was originally
intended.  This demonstrates why you have to be careful in choosing
your field and record separators.

   On many Unix systems, each user has a separate entry in the system
password file, one line per user.  The information in these lines is
separated by colons.  The first field is the user's logon name, and the
second is the user's encrypted password.  A password file entry might
look like this:

     arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh

   The following program searches the system password file, and prints
the entries for users who have no password:

     awk -F: '$2 == ""' /etc/passwd


File: gawk.info,  Node: Field Splitting Summary,  Prev: Command Line Field Separator,  Up: Field Separators

Field Splitting Summary
-----------------------

   According to the POSIX standard, `awk' is supposed to behave as if
each record is split into fields at the time that it is read.  In
particular, this means that you can change the value of `FS' after a
record is read, and the value of the fields (i.e. how they were split)
should reflect the old value of `FS', not the new one.

   However, many implementations of `awk' do not work this way.
Instead, they defer splitting the fields until a field is actually
referenced.  The fields will be split using the *current* value of
`FS'! (d.c.) This behavior can be difficult to diagnose. The following
example illustrates the difference between the two methods.  (The
`sed'(1) command prints just the first line of `/etc/passwd'.)

     sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }'

will usually print

     root

on an incorrect implementation of `awk', while `gawk' will print
something like

     root:nSijPlPhZZwgE:0:0:Root:/:

   The following table summarizes how fields are split, based on the
value of `FS'. (`==' means "is equal to.")

`FS == " "'
     Fields are separated by runs of whitespace.  Leading and trailing
     whitespace are ignored.  This is the default.

`FS == ANY OTHER SINGLE CHARACTER'
     Fields are separated by each occurrence of the character.  Multiple
     successive occurrences delimit empty fields, as do leading and
     trailing occurrences.  The character can even be a regexp
     metacharacter; it does not need to be escaped.

`FS == REGEXP'
     Fields are separated by occurrences of characters that match
     REGEXP.  Leading and trailing matches of REGEXP delimit empty
     fields.

`FS == ""'
     Each individual character in the record becomes a separate field.

   ---------- Footnotes ----------

   (1)  The `sed' utility is a "stream editor." Its behavior is also
defined by the POSIX standard.


File: gawk.info,  Node: Constant Size,  Next: Multiple Line,  Prev: Field Separators,  Up: Reading Files

Reading Fixed-width Data
========================

   (This section discusses an advanced, experimental feature.  If you
are a novice `awk' user, you may wish to skip it on the first reading.)

   `gawk' version 2.13 introduced a new facility for dealing with
fixed-width fields with no distinctive field separator.  Data of this
nature arises, for example, in  the input for old FORTRAN programs where
numbers are run together; or in the output of programs that did not
anticipate the use of their output as input for other programs.

   An example of the latter is a table where all the columns are lined
up by the use of a variable number of spaces and *empty fields are just
spaces*.  Clearly, `awk''s normal field splitting based on `FS' will
not work well in this case.  Although a portable `awk' program can use
a series of `substr' calls on `$0' (*note Built-in Functions for String
Manipulation: String Functions.), this is awkward and inefficient for a
large number of fields.

   The splitting of an input record into fixed-width fields is
specified by assigning a string containing space-separated numbers to
the built-in variable `FIELDWIDTHS'.  Each number specifies the width
of the field *including* columns between fields.  If you want to ignore
the columns between fields, you can specify the width as a separate
field that is subsequently ignored.

   The following data is the output of the Unix `w' utility.  It is
useful to illustrate the use of `FIELDWIDTHS'.

      10:06pm  up 21 days, 14:04,  23 users
     User     tty       login  idle   JCPU   PCPU  what
     hzuo     ttyV0     8:58pm            9      5  vi p24.tex
     hzang    ttyV3     6:37pm    50                -csh
     eklye    ttyV5     9:53pm            7      1  em thes.tex
     dportein ttyV6     8:17pm  1:47                -csh
     gierd    ttyD3    10:00pm     1                elm
     dave     ttyD4     9:47pm            4      4  w
     brent    ttyp0    26Jun91  4:46  26:46   4:41  bash
     dave     ttyq4    26Jun9115days     46     46  wnewmail

   The following program takes the above input, converts the idle time
to number of seconds and prints out the first two fields and the
calculated idle time.  (This program uses a number of `awk' features
that haven't been introduced yet.)

     BEGIN  { FIELDWIDTHS = "9 6 10 6 7 7 35" }
     NR > 2 {
         idle = $4
         sub(/^  */, "", idle)   # strip leading spaces
         if (idle == "")
             idle = 0
         if (idle ~ /:/) {
             split(idle, t, ":")
             idle = t[1] * 60 + t[2]
         }
         if (idle ~ /days/)
             idle *= 24 * 60 * 60
     
         print $1, $2, idle
     }

   Here is the result of running the program on the data:

     hzuo      ttyV0  0
     hzang     ttyV3  50
     eklye     ttyV5  0
     dportein  ttyV6  107
     gierd     ttyD3  1
     dave      ttyD4  0
     brent     ttyp0  286
     dave      ttyq4  1296000

   Another (possibly more practical) example of fixed-width input data
would be the input from a deck of balloting cards.  In some parts of
the United States, voters mark their choices by punching holes in
computer cards.  These cards are then processed to count the votes for
any particular candidate or on any particular issue.  Since a voter may
choose not to vote on some issue, any column on the card may be empty.
An `awk' program for processing such data could use the `FIELDWIDTHS'
feature to simplify reading the data.  (Of course, getting `gawk' to
run on a system with card readers is another story!)

   Assigning a value to `FS' causes `gawk' to return to using `FS' for
field splitting.  Use `FS = FS' to make this happen, without having to
know the current value of `FS'.

   This feature is still experimental, and may evolve over time.  Note
that in particular, `gawk' does not attempt to verify the sanity of the
values used in the value of `FIELDWIDTHS'.


File: gawk.info,  Node: Multiple Line,  Next: Getline,  Prev: Constant Size,  Up: Reading Files

Multiple-Line Records
=====================

   In some data bases, a single line cannot conveniently hold all the
information in one entry.  In such cases, you can use multi-line
records.

   The first step in doing this is to choose your data format: when
records are not defined as single lines, how do you want to define them?
What should separate records?

   One technique is to use an unusual character or string to separate
records.  For example, you could use the formfeed character (written
`\f' in `awk', as in C) to separate them, making each record a page of
the file.  To do this, just set the variable `RS' to `"\f"' (a string
containing the formfeed character).  Any other character could equally
well be used, as long as it won't be part of the data in a record.

   Another technique is to have blank lines separate records.  By a
special dispensation, an empty string as the value of `RS' indicates
that records are separated by one or more blank lines.  If you set `RS'
to the empty string, a record always ends at the first blank line
encountered.  And the next record doesn't start until the first
non-blank line that follows--no matter how many blank lines appear in a
row, they are considered one record-separator.

   You can achieve the same effect as `RS = ""' by assigning the string
`"\n\n+"' to `RS'. This regexp matches the newline at the end of the
record, and one or more blank lines after the record.  In addition, a
regular expression always matches the longest possible sequence when
there is a choice (*note How Much Text Matches?: Leftmost Longest.) So
the next record doesn't start until the first non-blank line that
follows--no matter how many blank lines appear in a row, they are
considered one record-separator.

   There is an important difference between `RS = ""' and `RS =
"\n\n+"'. In the first case, leading newlines in the input data file
are ignored, and if a file ends without extra blank lines after the
last record, the final newline is removed from the record.  In the
second case, this special processing is not done (d.c.).

   Now that the input is separated into records, the second step is to
separate the fields in the record.  One way to do this is to divide each
of the lines into fields in the normal manner.  This happens by default
as the result of a special feature: when `RS' is set to the empty
string, the newline character *always* acts as a field separator.  This
is in addition to whatever field separations result from `FS'.

   The original motivation for this special exception was probably to
provide useful behavior in the default case (i.e. `FS' is equal to
`" "').  This feature can be a problem if you really don't want the
newline character to separate fields, since there is no way to prevent
it.  However, you can work around this by using the `split' function to
break up the record manually (*note Built-in Functions for String
Manipulation: String Functions.).

   Another way to separate fields is to put each field on a separate
line: to do this, just set the variable `FS' to the string `"\n"'.
(This simple regular expression matches a single newline.)

   A practical example of a data file organized this way might be a
mailing list, where each entry is separated by blank lines.  If we have
a mailing list in a file named `addresses', that looks like this:

     Jane Doe
     123 Main Street
     Anywhere, SE 12345-6789
     
     John Smith
     456 Tree-lined Avenue
     Smallville, MW 98765-4321
     
     ...

A simple program to process this file would look like this:

     # addrs.awk --- simple mailing list program
     
     # Records are separated by blank lines.
     # Each line is one field.
     BEGIN { RS = "" ; FS = "\n" }
     
     {
           print "Name is:", $1
           print "Address is:", $2
           print "City and State are:", $3
           print ""
     }

   Running the program produces the following output:

     $ awk -f addrs.awk addresses
     -| Name is: Jane Doe
     -| Address is: 123 Main Street
     -| City and State are: Anywhere, SE 12345-6789
     -|
     -| Name is: John Smith
     -| Address is: 456 Tree-lined Avenue
     -| City and State are: Smallville, MW 98765-4321
     -|
     ...

   *Note Printing Mailing Labels: Labels Program, for a more realistic
program that deals with address lists.

   The following table summarizes how records are split, based on the
value of `RS'. (`==' means "is equal to.")

`RS == "\n"'
     Records are separated by the newline character (`\n').  In effect,
     every line in the data file is a separate record, including blank
     lines.  This is the default.

`RS == ANY SINGLE CHARACTER'
     Records are separated by each occurrence of the character.
     Multiple successive occurrences delimit empty records.

`RS == ""'
     Records are separated by runs of blank lines.  The newline
     character always serves as a field separator, in addition to
     whatever value `FS' may have. Leading and trailing newlines in a
     file are ignored.

`RS == REGEXP'
     Records are separated by occurrences of characters that match
     REGEXP.  Leading and trailing matches of REGEXP delimit empty
     records.

   In all cases, `gawk' sets `RT' to the input text that matched the
value specified by `RS'.


File: gawk.info,  Node: Getline,  Prev: Multiple Line,  Up: Reading Files

Explicit Input with `getline'
=============================

   So far we have been getting our input data from `awk''s main input
stream--either the standard input (usually your terminal, sometimes the
output from another program) or from the files specified on the command
line.  The `awk' language has a special built-in command called
`getline' that can be used to read input under your explicit control.

* Menu:

* Getline Intro::            Introduction to the `getline' function.
* Plain Getline::            Using `getline' with no arguments.
* Getline/Variable::         Using `getline' into a variable.
* Getline/File::             Using `getline' from a file.
* Getline/Variable/File::    Using `getline' into a variable from a
                             file.
* Getline/Pipe::             Using `getline' from a pipe.
* Getline/Variable/Pipe::    Using `getline' into a variable from a
                             pipe.
* Getline Summary::          Summary Of `getline' Variants.


File: gawk.info,  Node: Getline Intro,  Next: Plain Getline,  Prev: Getline,  Up: Getline

Introduction to `getline'
-------------------------

   This command is used in several different ways, and should *not* be
used by beginners.  It is covered here because this is the chapter on
input.  The examples that follow the explanation of the `getline'
command include material that has not been covered yet.  Therefore,
come back and study the `getline' command *after* you have reviewed the
rest of this Info file and have a good knowledge of how `awk' works.

   `getline' returns one if it finds a record, and zero if the end of
the file is encountered.  If there is some error in getting a record,
such as a file that cannot be opened, then `getline' returns -1.  In
this case, `gawk' sets the variable `ERRNO' to a string describing the
error that occurred.

   In the following examples, COMMAND stands for a string value that
represents a shell command.


File: gawk.info,  Node: Plain Getline,  Next: Getline/Variable,  Prev: Getline Intro,  Up: Getline

Using `getline' with No Arguments
---------------------------------

   The `getline' command can be used without arguments to read input
from the current input file.  All it does in this case is read the next
input record and split it up into fields.  This is useful if you've
finished processing the current record, but you want to do some special
processing *right now* on the next record.  Here's an example:

     awk '{
          if ((t = index($0, "/*")) != 0) {
               # value will be "" if t is 1
               tmp = substr($0, 1, t - 1)
               u = index(substr($0, t + 2), "*/")
               while (u == 0) {
                    if (getline <= 0) {
                         m = "unexpected EOF or error"
                         m = (m ": " ERRNO)
                         print m > "/dev/stderr"
                         exit
                    }
                    t = -1
                    u = index($0, "*/")
               }
               # substr expression will be "" if */
               # occurred at end of line
               $0 = tmp substr($0, t + u + 3)
          }
          print $0
     }'

   This `awk' program deletes all C-style comments, `/* ...  */', from
the input.  By replacing the `print $0' with other statements, you
could perform more complicated processing on the decommented input,
like searching for matches of a regular expression.  This program has a
subtle problem--it does not work if one comment ends and another begins
on the same line.

   This form of the `getline' command sets `NF' (the number of fields;
*note Examining Fields: Fields.), `NR' (the number of records read so
far; *note How Input is Split into Records: Records.), `FNR' (the
number of records read from this input file), and the value of `$0'.

   *Note:* the new value of `$0' is used in testing the patterns of any
subsequent rules.  The original value of `$0' that triggered the rule
which executed `getline' is lost (d.c.).  By contrast, the `next'
statement reads a new record but immediately begins processing it
normally, starting with the first rule in the program.  *Note The
`next' Statement: Next Statement.


File: gawk.info,  Node: Getline/Variable,  Next: Getline/File,  Prev: Plain Getline,  Up: Getline

Using `getline' Into a Variable
-------------------------------

   You can use `getline VAR' to read the next record from `awk''s input
into the variable VAR.  No other processing is done.

   For example, suppose the next line is a comment, or a special string,
and you want to read it, without triggering any rules.  This form of
`getline' allows you to read that line and store it in a variable so
that the main read-a-line-and-check-each-rule loop of `awk' never sees
it.

   The following example swaps every two lines of input.  For example,
given:

     wan
     tew
     free
     phore

it outputs:

     tew
     wan
     phore
     free

Here's the program:

     awk '{
          if ((getline tmp) > 0) {
               print tmp
               print $0
          } else
               print $0
     }'

   The `getline' command used in this way sets only the variables `NR'
and `FNR' (and of course, VAR).  The record is not split into fields,
so the values of the fields (including `$0') and the value of `NF' do
not change.


File: gawk.info,  Node: Getline/File,  Next: Getline/Variable/File,  Prev: Getline/Variable,  Up: Getline

Using `getline' from a File
---------------------------

   Use `getline < FILE' to read the next record from the file FILE.
Here FILE is a string-valued expression that specifies the file name.
`< FILE' is called a "redirection" since it directs input to come from
a different place.

   For example, the following program reads its input record from the
file `secondary.input' when it encounters a first field with a value
equal to 10 in the current input file.

     awk '{
         if ($1 == 10) {
              getline < "secondary.input"
              print
         } else
              print
     }'

   Since the main input stream is not used, the values of `NR' and
`FNR' are not changed.  But the record read is split into fields in the
normal manner, so the values of `$0' and other fields are changed.  So
is the value of `NF'.

   According to POSIX, `getline < EXPRESSION' is ambiguous if
EXPRESSION contains unparenthesized operators other than `$'; for
example, `getline < dir "/" file' is ambiguous because the
concatenation operator is not parenthesized, and you should write it as
`getline < (dir "/" file)' if you want your program to be portable to
other `awk' implementations.


File: gawk.info,  Node: Getline/Variable/File,  Next: Getline/Pipe,  Prev: Getline/File,  Up: Getline

Using `getline' Into a Variable from a File
-------------------------------------------

   Use `getline VAR < FILE' to read input the file FILE and put it in
the variable VAR.  As above, FILE is a string-valued expression that
specifies the file from which to read.

   In this version of `getline', none of the built-in variables are
changed, and the record is not split into fields.  The only variable
changed is VAR.

   According to POSIX, `getline VAR < EXPRESSION' is ambiguous if
EXPRESSION contains unparenthesized operators other than `$'; for
example, `getline < dir "/" file' is ambiguous because the
concatenation operator is not parenthesized, and you should write it as
`getline < (dir "/" file)' if you want your program to be portable to
other `awk' implementations.

   For example, the following program copies all the input files to the
output, except for records that say `@include FILENAME'.  Such a record
is replaced by the contents of the file FILENAME.

     awk '{
          if (NF == 2 && $1 == "@include") {
               while ((getline line < $2) > 0)
                    print line
               close($2)
          } else
               print
     }'

   Note here how the name of the extra input file is not built into the
program; it is taken directly from the data, from the second field on
the `@include' line.

   The `close' function is called to ensure that if two identical
`@include' lines appear in the input, the entire specified file is
included twice.  *Note Closing Input and Output Files and Pipes: Close
Files And Pipes.

   One deficiency of this program is that it does not process nested
`@include' statements (`@include' statements in included files) the way
a true macro preprocessor would.  *Note An Easy Way to Use Library
Functions: Igawk Program, for a program that does handle nested
`@include' statements.


File: gawk.info,  Node: Getline/Pipe,  Next: Getline/Variable/Pipe,  Prev: Getline/Variable/File,  Up: Getline

Using `getline' from a Pipe
---------------------------

   You can pipe the output of a command into `getline', using `COMMAND
| getline'.  In this case, the string COMMAND is run as a shell command
and its output is piped into `awk' to be used as input.  This form of
`getline' reads one record at a time from the pipe.

   For example, the following program copies its input to its output,
except for lines that begin with `@execute', which are replaced by the
output produced by running the rest of the line as a shell command:

     awk '{
          if ($1 == "@execute") {
               tmp = substr($0, 10)
               while ((tmp | getline) > 0)
                    print
               close(tmp)
          } else
               print
     }'

The `close' function is called to ensure that if two identical
`@execute' lines appear in the input, the command is run for each one.
*Note Closing Input and Output Files and Pipes: Close Files And Pipes.

   Given the input:

     foo
     bar
     baz
     @execute who
     bletch

the program might produce:

     foo
     bar
     baz
     arnold     ttyv0   Jul 13 14:22
     miriam     ttyp0   Jul 13 14:23     (murphy:0)
     bill       ttyp1   Jul 13 14:23     (murphy:0)
     bletch

Notice that this program ran the command `who' and printed the result.
(If you try this program yourself, you will of course get different
results, showing you who is logged in on your system.)

   This variation of `getline' splits the record into fields, sets the
value of `NF' and recomputes the value of `$0'.  The values of `NR' and
`FNR' are not changed.

   According to POSIX, `EXPRESSION | getline' is ambiguous if
EXPRESSION contains unparenthesized operators other than `$'; for
example, `"echo " "date" | getline' is ambiguous because the
concatenation operator is not parenthesized, and you should write it as
`("echo " "date") | getline' if you want your program to be portable to
other `awk' implementations.


File: gawk.info,  Node: Getline/Variable/Pipe,  Next: Getline Summary,  Prev: Getline/Pipe,  Up: Getline

Using `getline' Into a Variable from a Pipe
-------------------------------------------

   When you use `COMMAND | getline VAR', the output of the command
COMMAND is sent through a pipe to `getline' and into the variable VAR.
For example, the following program reads the current date and time into
the variable `current_time', using the `date' utility, and then prints
it.

     awk 'BEGIN {
          "date" | getline current_time
          close("date")
          print "Report printed on " current_time
     }'

   In this version of `getline', none of the built-in variables are
changed, and the record is not split into fields.

   According to POSIX, `EXPRESSION | getline VAR' is ambiguous if
EXPRESSION contains unparenthesized operators other than `$'; for
example, `"echo " "date" | getline VAR' is ambiguous because the
concatenation operator is not parenthesized, and you should write it as
`("echo " "date") | getline VAR' if you want your program to be
portable to other `awk' implementations.


File: gawk.info,  Node: Getline Summary,  Prev: Getline/Variable/Pipe,  Up: Getline

Summary of `getline' Variants
-----------------------------

   With all the forms of `getline', even though `$0' and `NF', may be
updated, the record will not be tested against all the patterns in the
`awk' program, in the way that would happen if the record were read
normally by the main processing loop of `awk'.  However the new record
is tested against any subsequent rules.

   Many `awk' implementations limit the number of pipelines an `awk'
program may have open to just one!  In `gawk', there is no such limit.
You can open as many pipelines as the underlying operating system will
permit.

   An interesting side-effect occurs if you use `getline' (without a
redirection) inside a `BEGIN' rule. Since an unredirected `getline'
reads from the command line data files, the first `getline' command
causes `awk' to set the value of `FILENAME'. Normally, `FILENAME' does
not have a value inside `BEGIN' rules, since you have not yet started
to process the command line data files (d.c.).  (*Note The `BEGIN' and
`END' Special Patterns: BEGIN/END, also *note Built-in Variables that
Convey Information: Auto-set..)

   The following table summarizes the six variants of `getline',
listing which built-in variables are set by each one.

`getline'
     sets `$0', `NF', `FNR', and `NR'.

`getline VAR'
     sets VAR, `FNR', and `NR'.

`getline < FILE'
     sets `$0', and `NF'.

`getline VAR < FILE'
     sets VAR.

`COMMAND | getline'
     sets `$0', and `NF'.

`COMMAND | getline VAR'
     sets VAR.


File: gawk.info,  Node: Printing,  Next: Expressions,  Prev: Reading Files,  Up: Top

Printing Output
***************

   One of the most common actions is to "print", or output, some or all
of the input.  You use the `print' statement for simple output.  You
use the `printf' statement for fancier formatting.  Both are described
in this chapter.

* Menu:

* Print::                       The `print' statement.
* Print Examples::              Simple examples of `print' statements.
* Output Separators::           The output separators and how to change them.
* OFMT::                        Controlling Numeric Output With `print'.
* Printf::                      The `printf' statement.
* Redirection::                 How to redirect output to multiple files and
                                pipes.
* Special Files::               File name interpretation in `gawk'.
                                `gawk' allows access to inherited file
                                descriptors.
* Close Files And Pipes::       Closing Input and Output Files and Pipes.


File: gawk.info,  Node: Print,  Next: Print Examples,  Prev: Printing,  Up: Printing

The `print' Statement
=====================

   The `print' statement does output with simple, standardized
formatting.  You specify only the strings or numbers to be printed, in a
list separated by commas.  They are output, separated by single spaces,
followed by a newline.  The statement looks like this:

     print ITEM1, ITEM2, ...

The entire list of items may optionally be enclosed in parentheses.  The
parentheses are necessary if any of the item expressions uses the `>'
relational operator; otherwise it could be confused with a redirection
(*note Redirecting Output of `print' and `printf': Redirection.).

   The items to be printed can be constant strings or numbers, fields
of the current record (such as `$1'), variables, or any `awk'
expressions.  Numeric values are converted to strings, and then printed.

   The `print' statement is completely general for computing *what*
values to print. However, with two exceptions, you cannot specify *how*
to print them--how many columns, whether to use exponential notation or
not, and so on.  (For the exceptions, *note Output Separators::., and
*Note Controlling Numeric Output with `print': OFMT.) For that, you
need the `printf' statement (*note Using `printf' Statements for
Fancier Printing: Printf.).

   The simple statement `print' with no items is equivalent to `print
$0': it prints the entire current record.  To print a blank line, use
`print ""', where `""' is the empty string.

   To print a fixed piece of text, use a string constant such as
`"Don't Panic"' as one item.  If you forget to use the double-quote
characters, your text will be taken as an `awk' expression, and you
will probably get an error.  Keep in mind that a space is printed
between any two items.

   Each `print' statement makes at least one line of output.  But it
isn't limited to one line.  If an item value is a string that contains a
newline, the newline is output along with the rest of the string.  A
single `print' can make any number of lines this way.


File: gawk.info,  Node: Print Examples,  Next: Output Separators,  Prev: Print,  Up: Printing

Examples of `print' Statements
==============================

   Here is an example of printing a string that contains embedded
newlines (the `\n' is an escape sequence, used to represent the newline
character; see *Note Escape Sequences::):

     $ awk 'BEGIN { print "line one\nline two\nline three" }'
     -| line one
     -| line two
     -| line three

   Here is an example that prints the first two fields of each input
record, with a space between them:

     $ awk '{ print $1, $2 }' inventory-shipped
     -| Jan 13
     -| Feb 15
     -| Mar 15
     ...

   A common mistake in using the `print' statement is to omit the comma
between two items.  This often has the effect of making the items run
together in the output, with no space.  The reason for this is that
juxtaposing two string expressions in `awk' means to concatenate them.
Here is the same program, without the comma:

     $ awk '{ print $1 $2 }' inventory-shipped
     -| Jan13
     -| Feb15
     -| Mar15
     ...

   To someone unfamiliar with the file `inventory-shipped', neither
example's output makes much sense.  A heading line at the beginning
would make it clearer.  Let's add some headings to our table of months
(`$1') and green crates shipped (`$2').  We do this using the `BEGIN'
pattern (*note The `BEGIN' and `END' Special Patterns: BEGIN/END.) to
force the headings to be printed only once:

     awk 'BEGIN {  print "Month Crates"
                   print "----- ------" }
                {  print $1, $2 }' inventory-shipped

Did you already guess what happens? When run, the program prints the
following:

     Month Crates
     ----- ------
     Jan 13
     Feb 15
     Mar 15
     ...

The headings and the table data don't line up!  We can fix this by
printing some spaces between the two fields:

     awk 'BEGIN { print "Month Crates"
                  print "----- ------" }
                { print $1, "     ", $2 }' inventory-shipped

   You can imagine that this way of lining up columns can get pretty
complicated when you have many columns to fix.  Counting spaces for two
or three columns can be simple, but more than this and you can get lost
quite easily.  This is why the `printf' statement was created (*note
Using `printf' Statements for Fancier Printing: Printf.); one of its
specialties is lining up columns of data.

   As a side point, you can continue either a `print' or `printf'
statement simply by putting a newline after any comma (*note `awk'
Statements Versus Lines: Statements/Lines.).


File: gawk.info,  Node: Output Separators,  Next: OFMT,  Prev: Print Examples,  Up: Printing

Output Separators
=================

   As mentioned previously, a `print' statement contains a list of
items, separated by commas.  In the output, the items are normally
separated by single spaces.  This need not be the case; a single space
is only the default.  You can specify any string of characters to use
as the "output field separator" by setting the built-in variable `OFS'.
The initial value of this variable is the string `" "', that is, a
single space.

   The output from an entire `print' statement is called an "output
record".  Each `print' statement outputs one output record and then
outputs a string called the "output record separator".  The built-in
variable `ORS' specifies this string.  The initial value of `ORS' is
the string `"\n"', i.e. a newline character; thus, normally each
`print' statement makes a separate line.

   You can change how output fields and records are separated by
assigning new values to the variables `OFS' and/or `ORS'.  The usual
place to do this is in the `BEGIN' rule (*note The `BEGIN' and `END'
Special Patterns: BEGIN/END.), so that it happens before any input is
processed.  You may also do this with assignments on the command line,
before the names of your input files, or using the `-v' command line
option (*note Command Line Options: Options.).

   The following example prints the first and second fields of each
input record separated by a semicolon, with a blank line added after
each line:

     $ awk 'BEGIN { OFS = ";"; ORS = "\n\n" }
     >            { print $1, $2 }' BBS-list
     -| aardvark;555-5553
     -|
     -| alpo-net;555-3412
     -|
     -| barfly;555-7685
     ...

   If the value of `ORS' does not contain a newline, all your output
will be run together on a single line, unless you output newlines some
other way.


File: gawk.info,  Node: OFMT,  Next: Printf,  Prev: Output Separators,  Up: Printing

Controlling Numeric Output with `print'
=======================================

   When you use the `print' statement to print numeric values, `awk'
internally converts the number to a string of characters, and prints
that string.  `awk' uses the `sprintf' function to do this conversion
(*note Built-in Functions for String Manipulation: String Functions.).
For now, it suffices to say that the `sprintf' function accepts a
"format specification" that tells it how to format numbers (or
strings), and that there are a number of different ways in which
numbers can be formatted.  The different format specifications are
discussed more fully in *Note Format-Control Letters: Control Letters.

   The built-in variable `OFMT' contains the default format
specification that `print' uses with `sprintf' when it wants to convert
a number to a string for printing.  The default value of `OFMT' is
`"%.6g"'.  By supplying different format specifications as the value of
`OFMT', you can change how `print' will print your numbers.  As a brief
example:

     $ awk 'BEGIN {
     >   OFMT = "%.0f"  # print numbers as integers (rounds)
     >   print 17.23 }'
     -| 17

According to the POSIX standard, `awk''s behavior will be undefined if
`OFMT' contains anything but a floating point conversion specification
(d.c.).


File: gawk.info,  Node: Printf,  Next: Redirection,  Prev: OFMT,  Up: Printing

Using `printf' Statements for Fancier Printing
==============================================

   If you want more precise control over the output format than `print'
gives you, use `printf'.  With `printf' you can specify the width to
use for each item, and you can specify various formatting choices for
numbers (such as what radix to use, whether to print an exponent,
whether to print a sign, and how many digits to print after the decimal
point).  You do this by supplying a string, called the "format string",
which controls how and where to print the other arguments.

* Menu:

* Basic Printf::                Syntax of the `printf' statement.
* Control Letters::             Format-control letters.
* Format Modifiers::            Format-specification modifiers.
* Printf Examples::             Several examples.


File: gawk.info,  Node: Basic Printf,  Next: Control Letters,  Prev: Printf,  Up: Printf

Introduction to the `printf' Statement
--------------------------------------

   The `printf' statement looks like this:

     printf FORMAT, ITEM1, ITEM2, ...

The entire list of arguments may optionally be enclosed in parentheses.
The parentheses are necessary if any of the item expressions use the
`>' relational operator; otherwise it could be confused with a
redirection (*note Redirecting Output of `print' and `printf':
Redirection.).

   The difference between `printf' and `print' is the FORMAT argument.
This is an expression whose value is taken as a string; it specifies
how to output each of the other arguments.  It is called the "format
string".

   The format string is very similar to that in the ANSI C library
function `printf'.  Most of FORMAT is text to be output verbatim.
Scattered among this text are "format specifiers", one per item.  Each
format specifier says to output the next item in the argument list at
that place in the format.

   The `printf' statement does not automatically append a newline to its
output.  It outputs only what the format string specifies.  So if you
want a newline, you must include one in the format string.  The output
separator variables `OFS' and `ORS' have no effect on `printf'
statements. For example:

     BEGIN {
        ORS = "\nOUCH!\n"; OFS = "!"
        msg = "Don't Panic!"; printf "%s\n", msg
     }

   This program still prints the familiar `Don't Panic!' message.


File: gawk.info,  Node: Control Letters,  Next: Format Modifiers,  Prev: Basic Printf,  Up: Printf

Format-Control Letters
----------------------

   A format specifier starts with the character `%' and ends with a
"format-control letter"; it tells the `printf' statement how to output
one item.  (If you actually want to output a `%', write `%%'.)  The
format-control letter specifies what kind of value to print.  The rest
of the format specifier is made up of optional "modifiers" which are
parameters to use, such as the field width.

   Here is a list of the format-control letters:

`c'
     This prints a number as an ASCII character.  Thus, `printf "%c",
     65' outputs the letter `A'.  The output for a string value is the
     first character of the string.

`d'
`i'
     These are equivalent. They both print a decimal integer.  The `%i'
     specification is for compatibility with ANSI C.

`e'
`E'
     This prints a number in scientific (exponential) notation.  For
     example,

          printf "%4.3e\n", 1950

     prints `1.950e+03', with a total of four significant figures of
     which three follow the decimal point.  The `4.3' are modifiers,
     discussed below. `%E' uses `E' instead of `e' in the output.

`f'
     This prints a number in floating point notation.  For example,

          printf "%4.3f", 1950

     prints `1950.000', with a total of four significant figures of
     which three follow the decimal point.  The `4.3' are modifiers,
     discussed below.

`g'
`G'
     This prints a number in either scientific notation or floating
     point notation, whichever uses fewer characters. If the result is
     printed in scientific notation, `%G' uses `E' instead of `e'.

`o'
     This prints an unsigned octal integer.  (In octal, or base-eight
     notation, the digits run from `0' to `7'; the decimal number eight
     is represented as `10' in octal.)

`s'
     This prints a string.

`x'
`X'
     This prints an unsigned hexadecimal integer.  (In hexadecimal, or
     base-16 notation, the digits are `0' through `9' and `a' through
     `f'.  The hexadecimal digit `f' represents the decimal number 15.)
     `%X' uses the letters `A' through `F' instead of `a' through `f'.

`%'
     This isn't really a format-control letter, but it does have a
     meaning when used after a `%': the sequence `%%' outputs one `%'.
     It does not consume an argument, and it ignores any modifiers.

   When using the integer format-control letters for values that are
outside the range of a C `long' integer, `gawk' will switch to the `%g'
format specifier. Other versions of `awk' may print invalid values, or
do something else entirely (d.c.).


File: gawk.info,  Node: Format Modifiers,  Next: Printf Examples,  Prev: Control Letters,  Up: Printf

Modifiers for `printf' Formats
------------------------------

   A format specification can also include "modifiers" that can control
how much of the item's value is printed and how much space it gets.  The
modifiers come between the `%' and the format-control letter.  In the
examples below, we use the bullet symbol "*" to represent spaces in the
output. Here are the possible modifiers, in the order in which they may
appear:

`-'
     The minus sign, used before the width modifier (see below), says
     to left-justify the argument within its specified width.  Normally
     the argument is printed right-justified in the specified width.
     Thus,

          printf "%-4s", "foo"

     prints `foo*'.

`SPACE'
     For numeric conversions, prefix positive values with a space, and
     negative values with a minus sign.

`+'
     The plus sign, used before the width modifier (see below), says to
     always supply a sign for numeric conversions, even if the data to
     be formatted is positive. The `+' overrides the space modifier.

`#'
     Use an "alternate form" for certain control letters.  For `%o',
     supply a leading zero.  For `%x', and `%X', supply a leading `0x'
     or `0X' for a non-zero result.  For `%e', `%E', and `%f', the
     result will always contain a decimal point.  For `%g', and `%G',
     trailing zeros are not removed from the result.

`0'
     A leading `0' (zero) acts as a flag, that indicates output should
     be padded with zeros instead of spaces.  This applies even to
     non-numeric output formats (d.c.).  This flag only has an effect
     when the field width is wider than the value to be printed.

`WIDTH'
     This is a number specifying the desired minimum width of a field.
     Inserting any number between the `%' sign and the format control
     character forces the field to be expanded to this width.  The
     default way to do this is to pad with spaces on the left.  For
     example,

          printf "%4s", "foo"

     prints `*foo'.

     The value of WIDTH is a minimum width, not a maximum.  If the item
     value requires more than WIDTH characters, it can be as wide as
     necessary.  Thus,

          printf "%4s", "foobar"

     prints `foobar'.

     Preceding the WIDTH with a minus sign causes the output to be
     padded with spaces on the right, instead of on the left.

`.PREC'
     This is a number that specifies the precision to use when printing.
     For the `e', `E', and `f' formats, this specifies the number of
     digits you want printed to the right of the decimal point.  For
     the `g', and `G' formats, it specifies the maximum number of
     significant digits.  For the `d', `o', `i', `u', `x', and `X'
     formats, it specifies the minimum number of digits to print.  For
     a string, it specifies the maximum number of characters from the
     string that should be printed.  Thus,

          printf "%.4s", "foobar"

     prints `foob'.

   The C library `printf''s dynamic WIDTH and PREC capability (for
example, `"%*.*s"') is supported.  Instead of supplying explicit WIDTH
and/or PREC values in the format string, you pass them in the argument
list.  For example:

     w = 5
     p = 3
     s = "abcdefg"
     printf "%*.*s\n", w, p, s

is exactly equivalent to

     s = "abcdefg"
     printf "%5.3s\n", s

Both programs output `**abc'.

   Earlier versions of `awk' did not support this capability.  If you
must use such a version, you may simulate this feature by using
concatenation to build up the format string, like so:

     w = 5
     p = 3
     s = "abcdefg"
     printf "%" w "." p "s\n", s

This is not particularly easy to read, but it does work.

   C programmers may be used to supplying additional `l' and `h' flags
in `printf' format strings. These are not valid in `awk'.  Most `awk'
implementations silently ignore these flags.  If `--lint' is provided
on the command line (*note Command Line Options: Options.), `gawk' will
warn about their use. If `--posix' is supplied, their use is a fatal
error.


File: gawk.info,  Node: Printf Examples,  Prev: Format Modifiers,  Up: Printf

Examples Using `printf'
-----------------------

   Here is how to use `printf' to make an aligned table:

     awk '{ printf "%-10s %s\n", $1, $2 }' BBS-list

prints the names of bulletin boards (`$1') of the file `BBS-list' as a
string of 10 characters, left justified.  It also prints the phone
numbers (`$2') afterward on the line.  This produces an aligned
two-column table of names and phone numbers:

     $ awk '{ printf "%-10s %s\n", $1, $2 }' BBS-list
     -| aardvark   555-5553
     -| alpo-net   555-3412
     -| barfly     555-7685
     -| bites      555-1675
     -| camelot    555-0542
     -| core       555-2912
     -| fooey      555-1234
     -| foot       555-6699
     -| macfoo     555-6480
     -| sdace      555-3430
     -| sabafoo    555-2127

   Did you notice that we did not specify that the phone numbers be
printed as numbers?  They had to be printed as strings because the
numbers are separated by a dash.  If we had tried to print the phone
numbers as numbers, all we would have gotten would have been the first
three digits, `555'.  This would have been pretty confusing.

   We did not specify a width for the phone numbers because they are the
last things on their lines.  We don't need to put spaces after them.

   We could make our table look even nicer by adding headings to the
tops of the columns.  To do this, we use the `BEGIN' pattern (*note The
`BEGIN' and `END' Special Patterns: BEGIN/END.) to force the header to
be printed only once, at the beginning of the `awk' program:

     awk 'BEGIN { print "Name      Number"
                  print "----      ------" }
          { printf "%-10s %s\n", $1, $2 }' BBS-list

   Did you notice that we mixed `print' and `printf' statements in the
above example?  We could have used just `printf' statements to get the
same results:

     awk 'BEGIN { printf "%-10s %s\n", "Name", "Number"
                  printf "%-10s %s\n", "----", "------" }
          { printf "%-10s %s\n", $1, $2 }' BBS-list

By printing each column heading with the same format specification used
for the elements of the column, we have made sure that the headings are
aligned just like the columns.

   The fact that the same format specification is used three times can
be emphasized by storing it in a variable, like this:

     awk 'BEGIN { format = "%-10s %s\n"
                  printf format, "Name", "Number"
                  printf format, "----", "------" }
          { printf format, $1, $2 }' BBS-list

   See if you can use the `printf' statement to line up the headings and
table data for our `inventory-shipped' example covered earlier in the
section on the `print' statement (*note The `print' Statement: Print.).


File: gawk.info,  Node: Redirection,  Next: Special Files,  Prev: Printf,  Up: Printing

Redirecting Output of `print' and `printf'
==========================================

   So far we have been dealing only with output that prints to the
standard output, usually your terminal.  Both `print' and `printf' can
also send their output to other places.  This is called "redirection".

   A redirection appears after the `print' or `printf' statement.
Redirections in `awk' are written just like redirections in shell
commands, except that they are written inside the `awk' program.

   There are three forms of output redirection: output to a file,
output appended to a file, and output through a pipe to another command.
They are all shown for the `print' statement, but they work identically
for `printf' also.

`print ITEMS > OUTPUT-FILE'
     This type of redirection prints the items into the output file
     OUTPUT-FILE.  The file name OUTPUT-FILE can be any expression.
     Its value is changed to a string and then used as a file name
     (*note Expressions::.).

     When this type of redirection is used, the OUTPUT-FILE is erased
     before the first output is written to it.  Subsequent writes to
     the same OUTPUT-FILE do not erase OUTPUT-FILE, but append to it.
     If OUTPUT-FILE does not exist, then it is created.

     For example, here is how an `awk' program can write a list of BBS
     names to a file `name-list' and a list of phone numbers to a file
     `phone-list'.  Each output file contains one name or number per
     line.

          $ awk '{ print $2 > "phone-list"
          >        print $1 > "name-list" }' BBS-list
          $ cat phone-list
          -| 555-5553
          -| 555-3412
          ...
          $ cat name-list
          -| aardvark
          -| alpo-net
          ...

`print ITEMS >> OUTPUT-FILE'
     This type of redirection prints the items into the pre-existing
     output file OUTPUT-FILE.  The difference between this and the
     single-`>' redirection is that the old contents (if any) of
     OUTPUT-FILE are not erased.  Instead, the `awk' output is appended
     to the file.  If OUTPUT-FILE does not exist, then it is created.

`print ITEMS | COMMAND'
     It is also possible to send output to another program through a
     pipe instead of into a file.   This type of redirection opens a
     pipe to COMMAND and writes the values of ITEMS through this pipe,
     to another process created to execute COMMAND.

     The redirection argument COMMAND is actually an `awk' expression.
     Its value is converted to a string, whose contents give the shell
     command to be run.

     For example, this produces two files, one unsorted list of BBS
     names and one list sorted in reverse alphabetical order:

          awk '{ print $1 > "names.unsorted"
                 command = "sort -r > names.sorted"
                 print $1 | command }' BBS-list

     Here the unsorted list is written with an ordinary redirection
     while the sorted list is written by piping through the `sort'
     utility.

     This example uses redirection to mail a message to a mailing list
     `bug-system'.  This might be useful when trouble is encountered in
     an `awk' script run periodically for system maintenance.

          report = "mail bug-system"
          print "Awk script failed:", $0 | report
          m = ("at record number " FNR " of " FILENAME)
          print m | report
          close(report)

     The message is built using string concatenation and saved in the
     variable `m'.  It is then sent down the pipeline to the `mail'
     program.

     We call the `close' function here because it's a good idea to close
     the pipe as soon as all the intended output has been sent to it.
     *Note Closing Input and Output Files and Pipes: Close Files And
     Pipes, for more information on this.  This example also
     illustrates the use of a variable to represent a FILE or COMMAND:
     it is not necessary to always use a string constant.  Using a
     variable is generally a good idea, since `awk' requires you to
     spell the string value identically every time.

   Redirecting output using `>', `>>', or `|' asks the system to open a
file or pipe only if the particular FILE or COMMAND you've specified
has not already been written to by your program, or if it has been
closed since it was last written to.

   Many `awk' implementations limit the number of pipelines an `awk'
program may have open to just one!  In `gawk', there is no such limit.
You can open as many pipelines as the underlying operating system will
permit.


File: gawk.info,  Node: Special Files,  Next: Close Files And Pipes,  Prev: Redirection,  Up: Printing

Special File Names in `gawk'
============================

   Running programs conventionally have three input and output streams
already available to them for reading and writing.  These are known as
the "standard input", "standard output", and "standard error output".
These streams are, by default, connected to your terminal, but they are
often redirected with the shell, via the `<', `<<', `>', `>>', `>&' and
`|' operators.  Standard error is typically used for writing error
messages; the reason we have two separate streams, standard output and
standard error, is so that they can be redirected separately.

   In other implementations of `awk', the only way to write an error
message to standard error in an `awk' program is as follows:

     print "Serious error detected!" | "cat 1>&2"

This works by opening a pipeline to a shell command which can access the
standard error stream which it inherits from the `awk' process.  This
is far from elegant, and is also inefficient, since it requires a
separate process.  So people writing `awk' programs often neglect to do
this.  Instead, they send the error messages to the terminal, like this:

     print "Serious error detected!" > "/dev/tty"

This usually has the same effect, but not always: although the standard
error stream is usually the terminal, it can be redirected, and when
that happens, writing to the terminal is not correct.  In fact, if
`awk' is run from a background job, it may not have a terminal at all.
Then opening `/dev/tty' will fail.

   `gawk' provides special file names for accessing the three standard
streams.  When you redirect input or output in `gawk', if the file name
matches one of these special names, then `gawk' directly uses the
stream it stands for.

`/dev/stdin'
     The standard input (file descriptor 0).

`/dev/stdout'
     The standard output (file descriptor 1).

`/dev/stderr'
     The standard error output (file descriptor 2).

`/dev/fd/N'
     The file associated with file descriptor N.  Such a file must have
     been opened by the program initiating the `awk' execution
     (typically the shell).  Unless you take special pains in the shell
     from which you invoke `gawk', only descriptors 0, 1 and 2 are
     available.

   The file names `/dev/stdin', `/dev/stdout', and `/dev/stderr' are
aliases for `/dev/fd/0', `/dev/fd/1', and `/dev/fd/2', respectively,
but they are more self-explanatory.

   The proper way to write an error message in a `gawk' program is to
use `/dev/stderr', like this:

     print "Serious error detected!" > "/dev/stderr"

   `gawk' also provides special file names that give access to
information about the running `gawk' process.  Each of these "files"
provides a single record of information.  To read them more than once,
you must first close them with the `close' function (*note Closing
Input and Output Files and Pipes: Close Files And Pipes.).  The
filenames are:

`/dev/pid'
     Reading this file returns the process ID of the current process,
     in decimal, terminated with a newline.

`/dev/ppid'
     Reading this file returns the parent process ID of the current
     process, in decimal, terminated with a newline.

`/dev/pgrpid'
     Reading this file returns the process group ID of the current
     process, in decimal, terminated with a newline.

`/dev/user'
     Reading this file returns a single record terminated with a
     newline.  The fields are separated with spaces.  The fields
     represent the following information:

    `$1'
          The return value of the `getuid' system call (the real user
          ID number).

    `$2'
          The return value of the `geteuid' system call (the effective
          user ID number).

    `$3'
          The return value of the `getgid' system call (the real group
          ID number).

    `$4'
          The return value of the `getegid' system call (the effective
          group ID number).

     If there are any additional fields, they are the group IDs
     returned by `getgroups' system call.  (Multiple groups may not be
     supported on all systems.)

   These special file names may be used on the command line as data
files, as well as for I/O redirections within an `awk' program.  They
may not be used as source files with the `-f' option.

   Recognition of these special file names is disabled if `gawk' is in
compatibility mode (*note Command Line Options: Options.).

   *Caution*:  Unless your system actually has a `/dev/fd' directory
(or any of the other above listed special files), the interpretation of
these file names is done by `gawk' itself.  For example, using
`/dev/fd/4' for output will actually write on file descriptor 4, and
not on a new file descriptor that was `dup''ed from file descriptor 4.
Most of the time this does not matter; however, it is important to
*not* close any of the files related to file descriptors 0, 1, and 2.
If you do close one of these files, unpredictable behavior will result.

   The special files that provide process-related information may
disappear in a future version of `gawk'.  *Note Probable Future
Extensions: Future Extensions.


File: gawk.info,  Node: Close Files And Pipes,  Prev: Special Files,  Up: Printing

Closing Input and Output Files and Pipes
========================================

   If the same file name or the same shell command is used with
`getline' (*note Explicit Input with `getline': Getline.) more than
once during the execution of an `awk' program, the file is opened (or
the command is executed) only the first time.  At that time, the first
record of input is read from that file or command.  The next time the
same file or command is used in `getline', another record is read from
it, and so on.

   Similarly, when a file or pipe is opened for output, the file name
or command associated with it is remembered by `awk' and subsequent
writes to the same file or command are appended to the previous writes.
The file or pipe stays open until `awk' exits.

   This implies that if you want to start reading the same file again
from the beginning, or if you want to rerun a shell command (rather than
reading more output from the command), you must take special steps.
What you must do is use the `close' function, as follows:

     close(FILENAME)

or

     close(COMMAND)

   The argument FILENAME or COMMAND can be any expression.  Its value
must *exactly* match the string that was used to open the file or start
the command (spaces and other "irrelevant" characters included). For
example, if you open a pipe with this:

     "sort -r names" | getline foo

then you must close it with this:

     close("sort -r names")

   Once this function call is executed, the next `getline' from that
file or command, or the next `print' or `printf' to that file or
command, will reopen the file or rerun the command.

   Because the expression that you use to close a file or pipeline must
exactly match the expression used to open the file or run the command,
it is good practice to use a variable to store the file name or command.
The previous example would become

     sortcom = "sort -r names"
     sortcom | getline foo
     ...
     close(sortcom)

This helps avoid hard-to-find typographical errors in your `awk'
programs.

   Here are some reasons why you might need to close an output file:

   * To write a file and read it back later on in the same `awk'
     program.  Close the file when you are finished writing it; then
     you can start reading it with `getline'.

   * To write numerous files, successively, in the same `awk' program.
     If you don't close the files, eventually you may exceed a system
     limit on the number of open files in one process.  So close each
     one when you are finished writing it.

   * To make a command finish.  When you redirect output through a pipe,
     the command reading the pipe normally continues to try to read
     input as long as the pipe is open.  Often this means the command
     cannot really do its work until the pipe is closed.  For example,
     if you redirect output to the `mail' program, the message is not
     actually sent until the pipe is closed.

   * To run the same program a second time, with the same arguments.
     This is not the same thing as giving more input to the first run!

     For example, suppose you pipe output to the `mail' program.  If you
     output several lines redirected to this pipe without closing it,
     they make a single message of several lines.  By contrast, if you
     close the pipe after each line of output, then each line makes a
     separate message.

   `close' returns a value of zero if the close succeeded.  Otherwise,
the value will be non-zero.  In this case, `gawk' sets the variable
`ERRNO' to a string describing the error that occurred.

   If you use more files than the system allows you to have open,
`gawk' will attempt to multiplex the available open files among your
data files.  `gawk''s ability to do this depends upon the facilities of
your operating system: it may not always work.  It is therefore both
good practice and good portability advice to always use `close' on your
files when you are done with them.


File: gawk.info,  Node: Expressions,  Next: Patterns and Actions,  Prev: Printing,  Up: Top

Expressions
***********

   Expressions are the basic building blocks of `awk' patterns and
actions.  An expression evaluates to a value, which you can print, test,
store in a variable or pass to a function.  Additionally, an expression
can assign a new value to a variable or a field, with an assignment
operator.

   An expression can serve as a pattern or action statement on its own.
Most other kinds of statements contain one or more expressions which
specify data on which to operate.  As in other languages, expressions
in `awk' include variables, array references, constants, and function
calls, as well as combinations of these with various operators.

* Menu:

* Constants::                   String, numeric, and regexp constants.
* Using Constant Regexps::      When and how to use a regexp constant.
* Variables::                   Variables give names to values for later use.
* Conversion::                  The conversion of strings to numbers and vice
                                versa.
* Arithmetic Ops::              Arithmetic operations (`+', `-',
                                etc.)
* Concatenation::               Concatenating strings.
* Assignment Ops::              Changing the value of a variable or a field.
* Increment Ops::               Incrementing the numeric value of a variable.
* Truth Values::                What is "true" and what is "false".
* Typing and Comparison::       How variables acquire types, and how this
                                affects comparison of numbers and strings with
                                `<', etc.
* Boolean Ops::                 Combining comparison expressions using boolean
                                operators `||' ("or"), `&&'
                                ("and") and `!' ("not").
* Conditional Exp::             Conditional expressions select between two
                                subexpressions under control of a third
                                subexpression.
* Function Calls::              A function call is an expression.
* Precedence::                  How various operators nest.


File: gawk.info,  Node: Constants,  Next: Using Constant Regexps,  Prev: Expressions,  Up: Expressions

Constant Expressions
====================

   The simplest type of expression is the "constant", which always has
the same value.  There are three types of constants: numeric constants,
string constants, and regular expression constants.

* Menu:

* Scalar Constants::            Numeric and string constants.
* Regexp Constants::            Regular Expression constants.


File: gawk.info,  Node: Scalar Constants,  Next: Regexp Constants,  Prev: Constants,  Up: Constants

Numeric and String Constants
----------------------------

   A "numeric constant" stands for a number.  This number can be an
integer, a decimal fraction, or a number in scientific (exponential)
notation.(1) Here are some examples of numeric constants, which all
have the same value:

     105
     1.05e+2
     1050e-1

   A string constant consists of a sequence of characters enclosed in
double-quote marks.  For example:

     "parrot"

represents the string whose contents are `parrot'.  Strings in `gawk'
can be of any length and they can contain any of the possible eight-bit
ASCII characters including ASCII NUL (character code zero).  Other `awk'
implementations may have difficulty with some character codes.

   ---------- Footnotes ----------

   (1)  The internal representation uses double-precision floating
point numbers. If you don't know what that means, then don't worry
about it.


File: gawk.info,  Node: Regexp Constants,  Prev: Scalar Constants,  Up: Constants

Regular Expression Constants
----------------------------

   A regexp constant is a regular expression description enclosed in
slashes, such as `/^beginning and end$/'.  Most regexps used in `awk'
programs are constant, but the `~' and `!~' matching operators can also
match computed or "dynamic" regexps (which are just ordinary strings or
variables that contain a regexp).


File: gawk.info,  Node: Using Constant Regexps,  Next: Variables,  Prev: Constants,  Up: Expressions

Using Regular Expression Constants
==================================

   When used on the right hand side of the `~' or `!~' operators, a
regexp constant merely stands for the regexp that is to be matched.

   Regexp constants (such as `/foo/') may be used like simple
expressions.  When a regexp constant appears by itself, it has the same
meaning as if it appeared in a pattern, i.e. `($0 ~ /foo/)' (d.c.)
(*note Expressions as Patterns: Expression Patterns.).  This means that
the two code segments,

     if ($0 ~ /barfly/ || $0 ~ /camelot/)
         print "found"

and

     if (/barfly/ || /camelot/)
         print "found"

are exactly equivalent.

   One rather bizarre consequence of this rule is that the following
boolean expression is valid, but does not do what the user probably
intended:

     # note that /foo/ is on the left of the ~
     if (/foo/ ~ $1) print "found foo"

This code is "obviously" testing `$1' for a match against the regexp
`/foo/'.  But in fact, the expression `/foo/ ~ $1' actually means `($0
~ /foo/) ~ $1'.  In other words, first match the input record against
the regexp `/foo/'.  The result will be either zero or one, depending
upon the success or failure of the match.  Then match that result
against the first field in the record.

   Since it is unlikely that you would ever really wish to make this
kind of test, `gawk' will issue a warning when it sees this construct in
a program.

   Another consequence of this rule is that the assignment statement

     matches = /foo/

will assign either zero or one to the variable `matches', depending
upon the contents of the current input record.

   This feature of the language was never well documented until the
POSIX specification.

   Constant regular expressions are also used as the first argument for
the `gensub', `sub' and `gsub' functions, and as the second argument of
the `match' function (*note Built-in Functions for String Manipulation:
String Functions.).  Modern implementations of `awk', including `gawk',
allow the third argument of `split' to be a regexp constant, while some
older implementations do not (d.c.).

   This can lead to confusion when attempting to use regexp constants
as arguments to user defined functions (*note User-defined Functions:
User-defined.).  For example:

     function mysub(pat, repl, str, global)
     {
         if (global)
             gsub(pat, repl, str)
         else
             sub(pat, repl, str)
         return str
     }
     
     {
         ...
         text = "hi! hi yourself!"
         mysub(/hi/, "howdy", text, 1)
         ...
     }

   In this example, the programmer wishes to pass a regexp constant to
the user-defined function `mysub', which will in turn pass it on to
either `sub' or `gsub'.  However, what really happens is that the `pat'
parameter will be either one or zero, depending upon whether or not
`$0' matches `/hi/'.

   As it is unlikely that you would ever really wish to pass a truth
value in this way, `gawk' will issue a warning when it sees a regexp
constant used as a parameter to a user-defined function.


File: gawk.info,  Node: Variables,  Next: Conversion,  Prev: Using Constant Regexps,  Up: Expressions

Variables
=========

   Variables are ways of storing values at one point in your program for
use later in another part of your program.  You can manipulate them
entirely within your program text, and you can also assign values to
them on the `awk' command line.

* Menu:

* Using Variables::             Using variables in your programs.
* Assignment Options::          Setting variables on the command line and a
                                summary of command line syntax. This is an
                                advanced method of input.


File: gawk.info,  Node: Using Variables,  Next: Assignment Options,  Prev: Variables,  Up: Variables

Using Variables in a Program
----------------------------

   Variables let you give names to values and refer to them later.  You
have already seen variables in many of the examples.  The name of a
variable must be a sequence of letters, digits and underscores, but it
may not begin with a digit.  Case is significant in variable names; `a'
and `A' are distinct variables.

   A variable name is a valid expression by itself; it represents the
variable's current value.  Variables are given new values with
"assignment operators", "increment operators" and "decrement operators".
*Note Assignment Expressions: Assignment Ops.

   A few variables have special built-in meanings, such as `FS', the
field separator, and `NF', the number of fields in the current input
record.  *Note Built-in Variables::, for a list of them.  These
built-in variables can be used and assigned just like all other
variables, but their values are also used or changed automatically by
`awk'.  All built-in variables names are entirely upper-case.

   Variables in `awk' can be assigned either numeric or string values.
By default, variables are initialized to the empty string, which is
zero if converted to a number.  There is no need to "initialize" each
variable explicitly in `awk', the way you would in C and in most other
traditional languages.


File: gawk.info,  Node: Assignment Options,  Prev: Using Variables,  Up: Variables

Assigning Variables on the Command Line
---------------------------------------

   You can set any `awk' variable by including a "variable assignment"
among the arguments on the command line when you invoke `awk' (*note
Other Command Line Arguments: Other Arguments.).  Such an assignment has
this form:

     VARIABLE=TEXT

With it, you can set a variable either at the beginning of the `awk'
run or in between input files.

   If you precede the assignment with the `-v' option, like this:

     -v VARIABLE=TEXT

then the variable is set at the very beginning, before even the `BEGIN'
rules are run.  The `-v' option and its assignment must precede all the
file name arguments, as well as the program text.  (*Note Command Line
Options: Options, for more information about the `-v' option.)

   Otherwise, the variable assignment is performed at a time determined
by its position among the input file arguments: after the processing of
the preceding input file argument.  For example:

     awk '{ print $n }' n=4 inventory-shipped n=2 BBS-list

prints the value of field number `n' for all input records.  Before the
first file is read, the command line sets the variable `n' equal to
four.  This causes the fourth field to be printed in lines from the
file `inventory-shipped'.  After the first file has finished, but
before the second file is started, `n' is set to two, so that the
second field is printed in lines from `BBS-list'.

     $ awk '{ print $n }' n=4 inventory-shipped n=2 BBS-list
     -| 15
     -| 24
     ...
     -| 555-5553
     -| 555-3412
     ...

   Command line arguments are made available for explicit examination by
the `awk' program in an array named `ARGV' (*note Using `ARGC' and
`ARGV': ARGC and ARGV.).

   `awk' processes the values of command line assignments for escape
sequences (d.c.) (*note Escape Sequences::.).


File: gawk.info,  Node: Conversion,  Next: Arithmetic Ops,  Prev: Variables,  Up: Expressions

Conversion of Strings and Numbers
=================================

   Strings are converted to numbers, and numbers to strings, if the
context of the `awk' program demands it.  For example, if the value of
either `foo' or `bar' in the expression `foo + bar' happens to be a
string, it is converted to a number before the addition is performed.
If numeric values appear in string concatenation, they are converted to
strings.  Consider this:

     two = 2; three = 3
     print (two three) + 4

This prints the (numeric) value 27.  The numeric values of the
variables `two' and `three' are converted to strings and concatenated
together, and the resulting string is converted back to the number 23,
to which four is then added.

   If, for some reason, you need to force a number to be converted to a
string, concatenate the empty string, `""', with that number.  To force
a string to be converted to a number, add zero to that string.

   A string is converted to a number by interpreting any numeric prefix
of the string as numerals: `"2.5"' converts to 2.5, `"1e3"' converts to
1000, and `"25fix"' has a numeric value of 25.  Strings that can't be
interpreted as valid numbers are converted to zero.

   The exact manner in which numbers are converted into strings is
controlled by the `awk' built-in variable `CONVFMT' (*note Built-in
Variables::.).  Numbers are converted using the `sprintf' function
(*note Built-in Functions for String Manipulation: String Functions.)
with `CONVFMT' as the format specifier.

   `CONVFMT''s default value is `"%.6g"', which prints a value with at
least six significant digits.  For some applications you will want to
change it to specify more precision.  Double precision on most modern
machines gives you 16 or 17 decimal digits of precision.

   Strange results can happen if you set `CONVFMT' to a string that
doesn't tell `sprintf' how to format floating point numbers in a useful
way.  For example, if you forget the `%' in the format, all numbers
will be converted to the same constant string.

   As a special case, if a number is an integer, then the result of
converting it to a string is *always* an integer, no matter what the
value of `CONVFMT' may be.  Given the following code fragment:

     CONVFMT = "%2.2f"
     a = 12
     b = a ""

`b' has the value `"12"', not `"12.00"' (d.c.).

   Prior to the POSIX standard, `awk' specified that the value of
`OFMT' was used for converting numbers to strings.  `OFMT' specifies
the output format to use when printing numbers with `print'.  `CONVFMT'
was introduced in order to separate the semantics of conversion from
the semantics of printing.  Both `CONVFMT' and `OFMT' have the same
default value: `"%.6g"'.  In the vast majority of cases, old `awk'
programs will not change their behavior.  However, this use of `OFMT'
is something to keep in mind if you must port your program to other
implementations of `awk'; we recommend that instead of changing your
programs, you just port `gawk' itself!  *Note The `print' Statement:
Print, for more information on the `print' statement.


File: gawk.info,  Node: Arithmetic Ops,  Next: Concatenation,  Prev: Conversion,  Up: Expressions

Arithmetic Operators
====================

   The `awk' language uses the common arithmetic operators when
evaluating expressions.  All of these arithmetic operators follow normal
precedence rules, and work as you would expect them to.

   Here is a file `grades' containing a list of student names and three
test scores per student (it's a small class):

     Pat   100 97 58
     Sandy  84 72 93
     Chris  72 92 89

This programs takes the file `grades', and prints the average of the
scores.

     $ awk '{ sum = $2 + $3 + $4 ; avg = sum / 3
     >        print $1, avg }' grades
     -| Pat 85
     -| Sandy 83
     -| Chris 84.3333

   This table lists the arithmetic operators in `awk', in order from
highest precedence to lowest:

`- X'
     Negation.

`+ X'
     Unary plus.  The expression is converted to a number.

`X ^ Y'
`X ** Y'
     Exponentiation: X raised to the Y power.  `2 ^ 3' has the value
     eight.  The character sequence `**' is equivalent to `^'.  (The
     POSIX standard only specifies the use of `^' for exponentiation.)

`X * Y'
     Multiplication.

`X / Y'
     Division.  Since all numbers in `awk' are real numbers, the result
     is not rounded to an integer: `3 / 4' has the value 0.75.

`X % Y'
     Remainder.  The quotient is rounded toward zero to an integer,
     multiplied by Y and this result is subtracted from X.  This
     operation is sometimes known as "trunc-mod."  The following
     relation always holds:

          b * int(a / b) + (a % b) == a

     One possibly undesirable effect of this definition of remainder is
     that `X % Y' is negative if X is negative.  Thus,

          -17 % 8 = -1

     In other `awk' implementations, the signedness of the remainder
     may be machine dependent.

`X + Y'
     Addition.

`X - Y'
     Subtraction.

   For maximum portability, do not use the `**' operator.

   Unary plus and minus have the same precedence, the multiplication
operators all have the same precedence, and addition and subtraction
have the same precedence.


File: gawk.info,  Node: Concatenation,  Next: Assignment Ops,  Prev: Arithmetic Ops,  Up: Expressions

String Concatenation
====================

   There is only one string operation: concatenation.  It does not have
a specific operator to represent it.  Instead, concatenation is
performed by writing expressions next to one another, with no operator.
For example:

     $ awk '{ print "Field number one: " $1 }' BBS-list
     -| Field number one: aardvark
     -| Field number one: alpo-net
     ...

   Without the space in the string constant after the `:', the line
would run together.  For example:

     $ awk '{ print "Field number one:" $1 }' BBS-list
     -| Field number one:aardvark
     -| Field number one:alpo-net
     ...

   Since string concatenation does not have an explicit operator, it is
often necessary to insure that it happens where you want it to by using
parentheses to enclose the items to be concatenated.  For example, the
following code fragment does not concatenate `file' and `name' as you
might expect:

     file = "file"
     name = "name"
     print "something meaningful" > file name

It is necessary to use the following:

     print "something meaningful" > (file name)

   We recommend that you use parentheses around concatenation in all
but the most common contexts (such as on the right-hand side of `=').


File: gawk.info,  Node: Assignment Ops,  Next: Increment Ops,  Prev: Concatenation,  Up: Expressions

Assignment Expressions
======================

   An "assignment" is an expression that stores a new value into a
variable.  For example, let's assign the value one to the variable `z':

     z = 1

   After this expression is executed, the variable `z' has the value
one.  Whatever old value `z' had before the assignment is forgotten.

   Assignments can store string values also.  For example, this would
store the value `"this food is good"' in the variable `message':

     thing = "food"
     predicate = "good"
     message = "this " thing " is " predicate

(This also illustrates string concatenation.)

   The `=' sign is called an "assignment operator".  It is the simplest
assignment operator because the value of the right-hand operand is
stored unchanged.

   Most operators (addition, concatenation, and so on) have no effect
except to compute a value.  If you ignore the value, you might as well
not use the operator.  An assignment operator is different; it does
produce a value, but even if you ignore the value, the assignment still
makes itself felt through the alteration of the variable.  We call this
a "side effect".

   The left-hand operand of an assignment need not be a variable (*note
Variables::.); it can also be a field (*note Changing the Contents of a
Field: Changing Fields.) or an array element (*note Arrays in `awk':
Arrays.).  These are all called "lvalues", which means they can appear
on the left-hand side of an assignment operator.  The right-hand
operand may be any expression; it produces the new value which the
assignment stores in the specified variable, field or array element.
(Such values are called "rvalues").

   It is important to note that variables do *not* have permanent types.
The type of a variable is simply the type of whatever value it happens
to hold at the moment.  In the following program fragment, the variable
`foo' has a numeric value at first, and a string value later on:

     foo = 1
     print foo
     foo = "bar"
     print foo

When the second assignment gives `foo' a string value, the fact that it
previously had a numeric value is forgotten.

   String values that do not begin with a digit have a numeric value of
zero. After executing this code, the value of `foo' is five:

     foo = "a string"
     foo = foo + 5

(Note that using a variable as a number and then later as a string can
be confusing and is poor programming style.  The above examples
illustrate how `awk' works, *not* how you should write your own
programs!)

   An assignment is an expression, so it has a value: the same value
that is assigned.  Thus, `z = 1' as an expression has the value one.
One consequence of this is that you can write multiple assignments
together:

     x = y = z = 0

stores the value zero in all three variables.  It does this because the
value of `z = 0', which is zero, is stored into `y', and then the value
of `y = z = 0', which is zero, is stored into `x'.

   You can use an assignment anywhere an expression is called for.  For
example, it is valid to write `x != (y = 1)' to set `y' to one and then
test whether `x' equals one.  But this style tends to make programs
hard to read; except in a one-shot program, you should not use such
nesting of assignments.

   Aside from `=', there are several other assignment operators that do
arithmetic with the old value of the variable.  For example, the
operator `+=' computes a new value by adding the right-hand value to
the old value of the variable.  Thus, the following assignment adds
five to the value of `foo':

     foo += 5

This is equivalent to the following:

     foo = foo + 5

Use whichever one makes the meaning of your program clearer.

   There are situations where using `+=' (or any assignment operator)
is *not* the same as simply repeating the left-hand operand in the
right-hand expression.  For example:

     # Thanks to Pat Rankin for this example
     BEGIN  {
         foo[rand()] += 5
         for (x in foo)
            print x, foo[x]
     
         bar[rand()] = bar[rand()] + 5
         for (x in bar)
            print x, bar[x]
     }

The indices of `bar' are guaranteed to be different, because `rand'
will return different values each time it is called.  (Arrays and the
`rand' function haven't been covered yet.  *Note Arrays in `awk':
Arrays, and see *Note Numeric Built-in Functions: Numeric Functions,
for more information).  This example illustrates an important fact
about the assignment operators: the left-hand expression is only
evaluated *once*.

   It is also up to the implementation as to which expression is
evaluated first, the left-hand one or the right-hand one.  Consider
this example:

     i = 1
     a[i += 2] = i + 1

The value of `a[3]' could be either two or four.

   Here is a table of the arithmetic assignment operators.  In each
case, the right-hand operand is an expression whose value is converted
to a number.

`LVALUE += INCREMENT'
     Adds INCREMENT to the value of LVALUE to make the new value of
     LVALUE.

`LVALUE -= DECREMENT'
     Subtracts DECREMENT from the value of LVALUE.

`LVALUE *= COEFFICIENT'
     Multiplies the value of LVALUE by COEFFICIENT.

`LVALUE /= DIVISOR'
     Divides the value of LVALUE by DIVISOR.

`LVALUE %= MODULUS'
     Sets LVALUE to its remainder by MODULUS.

`LVALUE ^= POWER'
`LVALUE **= POWER'
     Raises LVALUE to the power POWER.  (Only the `^=' operator is
     specified by POSIX.)

   For maximum portability, do not use the `**=' operator.


File: gawk.info,  Node: Increment Ops,  Next: Truth Values,  Prev: Assignment Ops,  Up: Expressions

Increment and Decrement Operators
=================================

   "Increment" and "decrement operators" increase or decrease the value
of a variable by one.  You could do the same thing with an assignment
operator, so the increment operators add no power to the `awk'
language; but they are convenient abbreviations for very common
operations.

   The operator to add one is written `++'.  It can be used to increment
a variable either before or after taking its value.

   To pre-increment a variable V, write `++V'.  This adds one to the
value of V and that new value is also the value of this expression.
The assignment expression `V += 1' is completely equivalent.

   Writing the `++' after the variable specifies post-increment.  This
increments the variable value just the same; the difference is that the
value of the increment expression itself is the variable's *old* value.
Thus, if `foo' has the value four, then the expression `foo++' has the
value four, but it changes the value of `foo' to five.

   The post-increment `foo++' is nearly equivalent to writing `(foo +=
1) - 1'.  It is not perfectly equivalent because all numbers in `awk'
are floating point: in floating point, `foo + 1 - 1' does not
necessarily equal `foo'.  But the difference is minute as long as you
stick to numbers that are fairly small (less than 10e12).

   Any lvalue can be incremented.  Fields and array elements are
incremented just like variables.  (Use `$(i++)' when you wish to do a
field reference and a variable increment at the same time.  The
parentheses are necessary because of the precedence of the field
reference operator, `$'.)

   The decrement operator `--' works just like `++' except that it
subtracts one instead of adding.  Like `++', it can be used before the
lvalue to pre-decrement or after it to post-decrement.

   Here is a summary of increment and decrement expressions.

`++LVALUE'
     This expression increments LVALUE and the new value becomes the
     value of the expression.

`LVALUE++'
     This expression increments LVALUE, but the value of the expression
     is the *old* value of LVALUE.

`--LVALUE'
     Like `++LVALUE', but instead of adding, it subtracts.  It
     decrements LVALUE and delivers the value that results.

`LVALUE--'
     Like `LVALUE++', but instead of adding, it subtracts.  It
     decrements LVALUE.  The value of the expression is the *old* value
     of LVALUE.


File: gawk.info,  Node: Truth Values,  Next: Typing and Comparison,  Prev: Increment Ops,  Up: Expressions

True and False in `awk'
=======================

   Many programming languages have a special representation for the
concepts of "true" and "false."  Such languages usually use the special
constants `true' and `false', or perhaps their upper-case equivalents.

   `awk' is different.  It borrows a very simple concept of true and
false from C.  In `awk', any non-zero numeric value, *or* any non-empty
string value is true.  Any other value (zero or the null string, `""')
is false.  The following program will print `A strange truth value'
three times:

     BEGIN {
        if (3.1415927)
            print "A strange truth value"
        if ("Four Score And Seven Years Ago")
            print "A strange truth value"
        if (j = 57)
            print "A strange truth value"
     }

   There is a surprising consequence of the "non-zero or non-null" rule:
The string constant `"0"' is actually true, since it is non-null (d.c.).


File: gawk.info,  Node: Typing and Comparison,  Next: Boolean Ops,  Prev: Truth Values,  Up: Expressions

Variable Typing and Comparison Expressions
==========================================

   Unlike other programming languages, `awk' variables do not have a
fixed type. Instead, they can be either a number or a string, depending
upon the value that is assigned to them.

   The 1992 POSIX standard introduced the concept of a "numeric
string", which is simply a string that looks like a number, for
example, `" +2"'.  This concept is used for determining the type of a
variable.

   The type of the variable is important, since the types of two
variables determine how they are compared.

   In `gawk', variable typing follows these rules.

  1. A numeric literal or the result of a numeric operation has the
     NUMERIC attribute.

  2. A string literal or the result of a string operation has the STRING
     attribute.

  3. Fields, `getline' input, `FILENAME', `ARGV' elements, `ENVIRON'
     elements and the elements of an array created by `split' that are
     numeric strings have the STRNUM attribute.  Otherwise, they have
     the STRING attribute.  Uninitialized variables also have the
     STRNUM attribute.

  4. Attributes propagate across assignments, but are not changed by
     any use.

   The last rule is particularly important. In the following program,
`a' has numeric type, even though it is later used in a string
operation.

     BEGIN {
              a = 12.345
              b = a " is a cute number"
              print b
     }

   When two operands are compared, either string comparison or numeric
comparison may be used, depending on the attributes of the operands,
according to the following, symmetric, matrix:

     	+----------------------------------------------
     	|	STRING		NUMERIC		STRNUM
     --------+----------------------------------------------
     	|
     STRING	|	string		string		string
     	|
     NUMERIC	|	string		numeric		numeric
     	|
     STRNUM	|	string		numeric		numeric
     --------+----------------------------------------------

   The basic idea is that user input that looks numeric, and *only*
user input, should be treated as numeric, even though it is actually
made of characters, and is therefore also a string.

   "Comparison expressions" compare strings or numbers for
relationships such as equality.  They are written using "relational
operators", which are a superset of those in C.  Here is a table of
them:

`X < Y'
     True if X is less than Y.

`X <= Y'
     True if X is less than or equal to Y.

`X > Y'
     True if X is greater than Y.

`X >= Y'
     True if X is greater than or equal to Y.

`X == Y'
     True if X is equal to Y.

`X != Y'
     True if X is not equal to Y.

`X ~ Y'
     True if the string X matches the regexp denoted by Y.

`X !~ Y'
     True if the string X does not match the regexp denoted by Y.

`SUBSCRIPT in ARRAY'
     True if the array ARRAY has an element with the subscript
     SUBSCRIPT.

   Comparison expressions have the value one if true and zero if false.

   When comparing operands of mixed types, numeric operands are
converted to strings using the value of `CONVFMT' (*note Conversion of
Strings and Numbers: Conversion.).

   Strings are compared by comparing the first character of each, then
the second character of each, and so on.  Thus `"10"' is less than
`"9"'.  If there are two strings where one is a prefix of the other,
the shorter string is less than the longer one.  Thus `"abc"' is less
than `"abcd"'.

   It is very easy to accidentally mistype the `==' operator, and leave
off one of the `='s.  The result is still valid `awk' code, but the
program will not do what you mean:

     if (a = b)   # oops! should be a == b
        ...
     else
        ...

Unless `b' happens to be zero or the null string, the `if' part of the
test will always succeed.  Because the operators are so similar, this
kind of error is very difficult to spot when scanning the source code.

   Here are some sample expressions, how `gawk' compares them, and what
the result of the comparison is.

`1.5 <= 2.0'
     numeric comparison (true)

`"abc" >= "xyz"'
     string comparison (false)

`1.5 != " +2"'
     string comparison (true)

`"1e2" < "3"'
     string comparison (true)

`a = 2; b = "2"'
`a == b'
     string comparison (true)

`a = 2; b = " +2"'
`a == b'
     string comparison (false)

   In this example,

     $ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'
     -| false

the result is `false' since both `$1' and `$2' are numeric strings and
thus both have the STRNUM attribute, dictating a numeric comparison.

   The purpose of the comparison rules and the use of numeric strings is
to attempt to produce the behavior that is "least surprising," while
still "doing the right thing."

   String comparisons and regular expression comparisons are very
different.  For example,

     x == "foo"

has the value of one, or is true, if the variable `x' is precisely
`foo'.  By contrast,

     x ~ /foo/

has the value one if `x' contains `foo', such as `"Oh, what a fool am
I!"'.

   The right hand operand of the `~' and `!~' operators may be either a
regexp constant (`/.../'), or an ordinary expression, in which case the
value of the expression as a string is used as a dynamic regexp (*note
How to Use Regular Expressions: Regexp Usage.; also *note Using Dynamic
Regexps: Computed Regexps.).

   In recent implementations of `awk', a constant regular expression in
slashes by itself is also an expression.  The regexp `/REGEXP/' is an
abbreviation for this comparison expression:

     $0 ~ /REGEXP/

   One special place where `/foo/' is *not* an abbreviation for `$0 ~
/foo/' is when it is the right-hand operand of `~' or `!~'!  *Note
Using Regular Expression Constants: Using Constant Regexps, where this
is discussed in more detail.


File: gawk.info,  Node: Boolean Ops,  Next: Conditional Exp,  Prev: Typing and Comparison,  Up: Expressions

Boolean Expressions
===================

   A "boolean expression" is a combination of comparison expressions or
matching expressions, using the boolean operators "or" (`||'), "and"
(`&&'), and "not" (`!'), along with parentheses to control nesting.
The truth value of the boolean expression is computed by combining the
truth values of the component expressions.  Boolean expressions are
also referred to as "logical expressions".  The terms are equivalent.

   Boolean expressions can be used wherever comparison and matching
expressions can be used.  They can be used in `if', `while', `do' and
`for' statements (*note Control Statements in Actions: Statements.).
They have numeric values (one if true, zero if false), which come into
play if the result of the boolean expression is stored in a variable, or
used in arithmetic.

   In addition, every boolean expression is also a valid pattern, so
you can use one as a pattern to control the execution of rules.

   Here are descriptions of the three boolean operators, with examples.

`BOOLEAN1 && BOOLEAN2'
     True if both BOOLEAN1 and BOOLEAN2 are true.  For example, the
     following statement prints the current input record if it contains
     both `2400' and `foo'.

          if ($0 ~ /2400/ && $0 ~ /foo/) print

     The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is true.
     This can make a difference when BOOLEAN2 contains expressions that
     have side effects: in the case of `$0 ~ /foo/ && ($2 == bar++)',
     the variable `bar' is not incremented if there is no `foo' in the
     record.

`BOOLEAN1 || BOOLEAN2'
     True if at least one of BOOLEAN1 or BOOLEAN2 is true.  For
     example, the following statement prints all records in the input
     that contain *either* `2400' or `foo', or both.

          if ($0 ~ /2400/ || $0 ~ /foo/) print

     The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is false.
     This can make a difference when BOOLEAN2 contains expressions
     that have side effects.

`! BOOLEAN'
     True if BOOLEAN is false.  For example, the following program
     prints all records in the input file `BBS-list' that do *not*
     contain the string `foo'.

          awk '{ if (! ($0 ~ /foo/)) print }' BBS-list

   The `&&' and `||' operators are called "short-circuit" operators
because of the way they work.  Evaluation of the full expression is
"short-circuited" if the result can be determined part way through its
evaluation.

   You can continue a statement that uses `&&' or `||' simply by
putting a newline after them.  But you cannot put a newline in front of
either of these operators without using backslash continuation (*note
`awk' Statements Versus Lines: Statements/Lines.).

   The actual value of an expression using the `!' operator will be
either one or zero, depending upon the truth value of the expression it
is applied to.

   The `!' operator is often useful for changing the sense of a flag
variable from false to true and back again. For example, the following
program is one way to print lines in between special bracketing lines:

     $1 == "START"   { interested = ! interested }
     interested == 1 { print }
     $1 == "END"     { interested = ! interested }

The variable `interested', like all `awk' variables, starts out
initialized to zero, which is also false.  When a line is seen whose
first field is `START', the value of `interested' is toggled to true,
using `!'. The next rule prints lines as long as `interested' is true.
When a line is seen whose first field is `END', `interested' is toggled
back to false.


File: gawk.info,  Node: Conditional Exp,  Next: Function Calls,  Prev: Boolean Ops,  Up: Expressions

Conditional Expressions
=======================

   A "conditional expression" is a special kind of expression with
three operands.  It allows you to use one expression's value to select
one of two other expressions.

   The conditional expression is the same as in the C language:

     SELECTOR ? IF-TRUE-EXP : IF-FALSE-EXP

There are three subexpressions.  The first, SELECTOR, is always
computed first.  If it is "true" (not zero and not null) then
IF-TRUE-EXP is computed next and its value becomes the value of the
whole expression.  Otherwise, IF-FALSE-EXP is computed next and its
value becomes the value of the whole expression.

   For example, this expression produces the absolute value of `x':

     x > 0 ? x : -x

   Each time the conditional expression is computed, exactly one of
IF-TRUE-EXP and IF-FALSE-EXP is computed; the other is ignored.  This
is important when the expressions contain side effects.  For example,
this conditional expression examines element `i' of either array `a' or
array `b', and increments `i'.

     x == y ? a[i++] : b[i++]

This is guaranteed to increment `i' exactly once, because each time
only one of the two increment expressions is executed, and the other is
not.  *Note Arrays in `awk': Arrays, for more information about arrays.

   As a minor `gawk' extension, you can continue a statement that uses
`?:' simply by putting a newline after either character.  However, you
cannot put a newline in front of either character without using
backslash continuation (*note `awk' Statements Versus Lines:
Statements/Lines.).


File: gawk.info,  Node: Function Calls,  Next: Precedence,  Prev: Conditional Exp,  Up: Expressions

Function Calls
==============

   A "function" is a name for a particular calculation.  Because it has
a name, you can ask for it by name at any point in the program.  For
example, the function `sqrt' computes the square root of a number.

   A fixed set of functions are "built-in", which means they are
available in every `awk' program.  The `sqrt' function is one of these.
*Note Built-in Functions: Built-in, for a list of built-in functions
and their descriptions.  In addition, you can define your own functions
for use in your program.  *Note User-defined Functions: User-defined,
for how to do this.

   The way to use a function is with a "function call" expression,
which consists of the function name followed immediately by a list of
"arguments" in parentheses.  The arguments are expressions which
provide the raw materials for the function's calculations.  When there
is more than one argument, they are separated by commas.  If there are
no arguments, write just `()' after the function name.  Here are some
examples:

     sqrt(x^2 + y^2)        one argument
     atan2(y, x)            two arguments
     rand()                 no arguments

   *Do not put any space between the function name and the
open-parenthesis!*  A user-defined function name looks just like the
name of a variable, and space would make the expression look like
concatenation of a variable with an expression inside parentheses.
Space before the parenthesis is harmless with built-in functions, but
it is best not to get into the habit of using space to avoid mistakes
with user-defined functions.

   Each function expects a particular number of arguments.  For
example, the `sqrt' function must be called with a single argument, the
number to take the square root of:

     sqrt(ARGUMENT)

   Some of the built-in functions allow you to omit the final argument.
If you do so, they use a reasonable default.  *Note Built-in Functions:
Built-in, for full details.  If arguments are omitted in calls to
user-defined functions, then those arguments are treated as local
variables, initialized to the empty string (*note User-defined
Functions: User-defined.).

   Like every other expression, the function call has a value, which is
computed by the function based on the arguments you give it.  In this
example, the value of `sqrt(ARGUMENT)' is the square root of ARGUMENT.
A function can also have side effects, such as assigning values to
certain variables or doing I/O.

   Here is a command to read numbers, one number per line, and print the
square root of each one:

     $ awk '{ print "The square root of", $1, "is", sqrt($1) }'
     1
     -| The square root of 1 is 1
     3
     -| The square root of 3 is 1.73205
     5
     -| The square root of 5 is 2.23607
     Control-d


File: gawk.info,  Node: Precedence,  Prev: Function Calls,  Up: Expressions

Operator Precedence (How Operators Nest)
========================================

   "Operator precedence" determines how operators are grouped, when
different operators appear close by in one expression.  For example,
`*' has higher precedence than `+'; thus, `a + b * c' means to multiply
`b' and `c', and then add `a' to the product (i.e. `a + (b * c)').

   You can overrule the precedence of the operators by using
parentheses.  You can think of the precedence rules as saying where the
parentheses are assumed to be if you do not write parentheses yourself.
In fact, it is wise to always use parentheses whenever you have an
unusual combination of operators, because other people who read the
program may not remember what the precedence is in this case.  You
might forget, too; then you could make a mistake.  Explicit parentheses
will help prevent any such mistake.

   When operators of equal precedence are used together, the leftmost
operator groups first, except for the assignment, conditional and
exponentiation operators, which group in the opposite order.  Thus, `a
- b + c' groups as `(a - b) + c', and `a = b = c' groups as `a = (b =
c)'.

   The precedence of prefix unary operators does not matter as long as
only unary operators are involved, because there is only one way to
interpret them--innermost first.  Thus, `$++i' means `$(++i)' and
`++$x' means `++($x)'.  However, when another operator follows the
operand, then the precedence of the unary operators can matter.  Thus,
`$x^2' means `($x)^2', but `-x^2' means `-(x^2)', because `-' has lower
precedence than `^' while `$' has higher precedence.

   Here is a table of `awk''s operators, in order from highest
precedence to lowest:

`(...)'
     Grouping.

`$'
     Field.

`++ --'
     Increment, decrement.

`^ **'
     Exponentiation.  These operators group right-to-left.  (The `**'
     operator is not specified by POSIX.)

`+ - !'
     Unary plus, minus, logical "not".

`* / %'
     Multiplication, division, modulus.

`+ -'
     Addition, subtraction.

`Concatenation'
     No special token is used to indicate concatenation.  The operands
     are simply written side by side.

`< <= == !='
`> >= >> |'
     Relational, and redirection.  The relational operators and the
     redirections have the same precedence level.  Characters such as
     `>' serve both as relationals and as redirections; the context
     distinguishes between the two meanings.

     Note that the I/O redirection operators in `print' and `printf'
     statements belong to the statement level, not to expressions.  The
     redirection does not produce an expression which could be the
     operand of another operator.  As a result, it does not make sense
     to use a redirection operator near another operator of lower
     precedence, without parentheses.  Such combinations, for example
     `print foo > a ? b : c', result in syntax errors.  The correct way
     to write this statement is `print foo > (a ? b : c)'.

`~ !~'
     Matching, non-matching.

`in'
     Array membership.

`&&'
     Logical "and".

`||'
     Logical "or".

`?:'
     Conditional.  This operator groups right-to-left.

`= += -= *='
`/= %= ^= **='
     Assignment.  These operators group right-to-left.  (The `**='
     operator is not specified by POSIX.)


File: gawk.info,  Node: Patterns and Actions,  Next: Statements,  Prev: Expressions,  Up: Top

Patterns and Actions
********************

   As you have already seen, each `awk' statement consists of a pattern
with an associated action.  This chapter describes how you build
patterns and actions.

* Menu:

* Pattern Overview::            What goes into a pattern.
* Action Overview::             What goes into an action.


File: gawk.info,  Node: Pattern Overview,  Next: Action Overview,  Prev: Patterns and Actions,  Up: Patterns and Actions

Pattern Elements
================

   Patterns in `awk' control the execution of rules: a rule is executed
when its pattern matches the current input record.  This section
explains all about how to write patterns.

* Menu:

* Kinds of Patterns::           A list of all kinds of patterns.
* Regexp Patterns::             Using regexps as patterns.
* Expression Patterns::         Any expression can be used as a pattern.
* Ranges::                      Pairs of patterns specify record ranges.
* BEGIN/END::                   Specifying initialization and cleanup rules.
* Empty::                       The empty pattern, which matches every record.


File: gawk.info,  Node: Kinds of Patterns,  Next: Regexp Patterns,  Prev: Pattern Overview,  Up: Pattern Overview

Kinds of Patterns
-----------------

   Here is a summary of the types of patterns supported in `awk'.

`/REGULAR EXPRESSION/'
     A regular expression as a pattern.  It matches when the text of the
     input record fits the regular expression.  (*Note Regular
     Expressions: Regexp.)

`EXPRESSION'
     A single expression.  It matches when its value is non-zero (if a
     number) or non-null (if a string).  (*Note Expressions as
     Patterns: Expression Patterns.)

`PAT1, PAT2'
     A pair of patterns separated by a comma, specifying a range of
     records.  The range includes both the initial record that matches
     PAT1, and the final record that matches PAT2.  (*Note Specifying
     Record Ranges with Patterns: Ranges.)

`BEGIN'
`END'
     Special patterns for you to supply start-up or clean-up actions
     for your `awk' program.  (*Note The `BEGIN' and `END' Special
     Patterns: BEGIN/END.)

`EMPTY'
     The empty pattern matches every input record.  (*Note The Empty
     Pattern: Empty.)


File: gawk.info,  Node: Regexp Patterns,  Next: Expression Patterns,  Prev: Kinds of Patterns,  Up: Pattern Overview

Regular Expressions as Patterns
-------------------------------

   We have been using regular expressions as patterns since our early
examples.  This kind of pattern is simply a regexp constant in the
pattern part of a rule.  Its  meaning is `$0 ~ /PATTERN/'.  The pattern
matches when the input record matches the regexp.  For example:

     /foo|bar|baz/  { buzzwords++ }
     END            { print buzzwords, "buzzwords seen" }


File: gawk.info,  Node: Expression Patterns,  Next: Ranges,  Prev: Regexp Patterns,  Up: Pattern Overview

Expressions as Patterns
-----------------------

   Any `awk' expression is valid as an `awk' pattern.  Then the pattern
matches if the expression's value is non-zero (if a number) or non-null
(if a string).

   The expression is reevaluated each time the rule is tested against a
new input record.  If the expression uses fields such as `$1', the
value depends directly on the new input record's text; otherwise, it
depends only on what has happened so far in the execution of the `awk'
program, but that may still be useful.

   A very common kind of expression used as a pattern is the comparison
expression, using the comparison operators described in *Note Variable
Typing and Comparison Expressions: Typing and Comparison.

   Regexp matching and non-matching are also very common expressions.
The left operand of the `~' and `!~' operators is a string.  The right
operand is either a constant regular expression enclosed in slashes
(`/REGEXP/'), or any expression, whose string value is used as a
dynamic regular expression (*note Using Dynamic Regexps: Computed
Regexps.).

   The following example prints the second field of each input record
whose first field is precisely `foo'.

     $ awk '$1 == "foo" { print $2 }' BBS-list

(There is no output, since there is no BBS site named "foo".) Contrast
this with the following regular expression match, which would accept
any record with a first field that contains `foo':

     $ awk '$1 ~ /foo/ { print $2 }' BBS-list
     -| 555-1234
     -| 555-6699
     -| 555-6480
     -| 555-2127

   Boolean expressions are also commonly used as patterns.  Whether the
pattern matches an input record depends on whether its subexpressions
match.

   For example, the following command prints all records in `BBS-list'
that contain both `2400' and `foo'.

     $ awk '/2400/ && /foo/' BBS-list
     -| fooey        555-1234     2400/1200/300     B

   The following command prints all records in `BBS-list' that contain
*either* `2400' or `foo', or both.

     $ awk '/2400/ || /foo/' BBS-list
     -| alpo-net     555-3412     2400/1200/300     A
     -| bites        555-1675     2400/1200/300     A
     -| fooey        555-1234     2400/1200/300     B
     -| foot         555-6699     1200/300          B
     -| macfoo       555-6480     1200/300          A
     -| sdace        555-3430     2400/1200/300     A
     -| sabafoo      555-2127     1200/300          C

   The following command prints all records in `BBS-list' that do *not*
contain the string `foo'.

     $ awk '! /foo/' BBS-list
     -| aardvark     555-5553     1200/300          B
     -| alpo-net     555-3412     2400/1200/300     A
     -| barfly       555-7685     1200/300          A
     -| bites        555-1675     2400/1200/300     A
     -| camelot      555-0542     300               C
     -| core         555-2912     1200/300          C
     -| sdace        555-3430     2400/1200/300     A

   The subexpressions of a boolean operator in a pattern can be
constant regular expressions, comparisons, or any other `awk'
expressions.  Range patterns are not expressions, so they cannot appear
inside boolean patterns.  Likewise, the special patterns `BEGIN' and
`END', which never match any input record, are not expressions and
cannot appear inside boolean patterns.

   A regexp constant as a pattern is also a special case of an
expression pattern.  `/foo/' as an expression has the value one if `foo'
appears in the current input record; thus, as a pattern, `/foo/'
matches any record containing `foo'.


File: gawk.info,  Node: Ranges,  Next: BEGIN/END,  Prev: Expression Patterns,  Up: Pattern Overview

Specifying Record Ranges with Patterns
--------------------------------------

   A "range pattern" is made of two patterns separated by a comma, of
the form `BEGPAT, ENDPAT'.  It matches ranges of consecutive input
records.  The first pattern, BEGPAT, controls where the range begins,
and the second one, ENDPAT, controls where it ends.  For example,

     awk '$1 == "on", $1 == "off"'

prints every record between `on'/`off' pairs, inclusive.

   A range pattern starts out by matching BEGPAT against every input
record; when a record matches BEGPAT, the range pattern becomes "turned
on".  The range pattern matches this record.  As long as it stays
turned on, it automatically matches every input record read.  It also
matches ENDPAT against every input record; when that succeeds, the
range pattern is turned off again for the following record.  Then it
goes back to checking BEGPAT against each record.

   The record that turns on the range pattern and the one that turns it
off both match the range pattern.  If you don't want to operate on
these records, you can write `if' statements in the rule's action to
distinguish them from the records you are interested in.

   It is possible for a pattern to be turned both on and off by the same
record, if the record satisfies both conditions.  Then the action is
executed for just that record.

   For example, suppose you have text between two identical markers (say
the `%' symbol) that you wish to ignore.  You might try to combine a
range pattern that describes the delimited text with the `next'
statement (not discussed yet, *note The `next' Statement: Next
Statement.), which causes `awk' to skip any further processing of the
current record and start over again with the next input record. Such a
program would like this:

     /^%$/,/^%$/    { next }
                    { print }

This program fails because the range pattern is both turned on and
turned off by the first line with just a `%' on it.  To accomplish this
task, you must write the program this way, using a flag:

     /^%$/     { skip = ! skip; next }
     skip == 1 { next } # skip lines with `skip' set

   Note that in a range pattern, the `,' has the lowest precedence (is
evaluated last) of all the operators.  Thus, for example, the following
program attempts to combine a range pattern with another, simpler test.

     echo Yes | awk '/1/,/2/ || /Yes/'

   The author of this program intended it to mean `(/1/,/2/) || /Yes/'.
However, `awk' interprets this as `/1/, (/2/ || /Yes/)'.  This cannot
be changed or worked around; range patterns do not combine with other
patterns.


File: gawk.info,  Node: BEGIN/END,  Next: Empty,  Prev: Ranges,  Up: Pattern Overview

The `BEGIN' and `END' Special Patterns
--------------------------------------

   `BEGIN' and `END' are special patterns.  They are not used to match
input records.  Rather, they supply start-up or clean-up actions for
your `awk' script.

* Menu:

* Using BEGIN/END::             How and why to use BEGIN/END rules.
* I/O And BEGIN/END::           I/O issues in BEGIN/END rules.


File: gawk.info,  Node: Using BEGIN/END,  Next: I/O And BEGIN/END,  Prev: BEGIN/END,  Up: BEGIN/END

Startup and Cleanup Actions
...........................

   A `BEGIN' rule is executed, once, before the first input record has
been read.  An `END' rule is executed, once, after all the input has
been read.  For example:

     $ awk '
     > BEGIN { print "Analysis of \"foo\"" }
     > /foo/ { ++n }
     > END   { print "\"foo\" appears " n " times." }' BBS-list
     -| Analysis of "foo"
     -| "foo" appears 4 times.

   This program finds the number of records in the input file `BBS-list'
that contain the string `foo'.  The `BEGIN' rule prints a title for the
report.  There is no need to use the `BEGIN' rule to initialize the
counter `n' to zero, as `awk' does this automatically (*note
Variables::.).

   The second rule increments the variable `n' every time a record
containing the pattern `foo' is read.  The `END' rule prints the value
of `n' at the end of the run.

   The special patterns `BEGIN' and `END' cannot be used in ranges or
with boolean operators (indeed, they cannot be used with any operators).

   An `awk' program may have multiple `BEGIN' and/or `END' rules.  They
are executed in the order they appear, all the `BEGIN' rules at
start-up and all the `END' rules at termination.  `BEGIN' and `END'
rules may be intermixed with other rules.  This feature was added in
the 1987 version of `awk', and is included in the POSIX standard.  The
original (1978) version of `awk' required you to put the `BEGIN' rule
at the beginning of the program, and the `END' rule at the end, and
only allowed one of each.  This is no longer required, but it is a good
idea in terms of program organization and readability.

   Multiple `BEGIN' and `END' rules are useful for writing library
functions, since each library file can have its own `BEGIN' and/or
`END' rule to do its own initialization and/or cleanup.  Note that the
order in which library functions are named on the command line controls
the order in which their `BEGIN' and `END' rules are executed.
Therefore you have to be careful to write such rules in library files
so that the order in which they are executed doesn't matter.  *Note
Command Line Options: Options, for more information on using library
functions.  *Note A Library of `awk' Functions: Library Functions, for
a number of useful library functions.

   If an `awk' program only has a `BEGIN' rule, and no other rules,
then the program exits after the `BEGIN' rule has been run.  (The
original version of `awk' used to keep reading and ignoring input until
end of file was seen.)  However, if an `END' rule exists, then the
input will be read, even if there are no other rules in the program.
This is necessary in case the `END' rule checks the `FNR' and `NR'
variables (d.c.).

   `BEGIN' and `END' rules must have actions; there is no default
action for these rules since there is no current record when they run.


File: gawk.info,  Node: I/O And BEGIN/END,  Prev: Using BEGIN/END,  Up: BEGIN/END

Input/Output from `BEGIN' and `END' Rules
.........................................

   There are several (sometimes subtle) issues involved when doing I/O
from a `BEGIN' or `END' rule.

   The first has to do with the value of `$0' in a `BEGIN' rule.  Since
`BEGIN' rules are executed before any input is read, there simply is no
input record, and therefore no fields, when executing `BEGIN' rules.
References to `$0' and the fields yield a null string or zero,
depending upon the context.  One way to give `$0' a real value is to
execute a `getline' command without a variable (*note Explicit Input
with `getline': Getline.).  Another way is to simply assign a value to
it.

   The second point is similar to the first, but from the other
direction.  Inside an `END' rule, what is the value of `$0' and `NF'?
Traditionally, due largely to implementation issues, `$0' and `NF' were
*undefined* inside an `END' rule.  The POSIX standard specified that
`NF' was available in an `END' rule, containing the number of fields
from the last input record.  Due most probably to an oversight, the
standard does not say that `$0' is also preserved, although logically
one would think that it should be.  In fact, `gawk' does preserve the
value of `$0' for use in `END' rules.  Be aware, however, that Unix
`awk', and possibly other implementations, do not.

   The third point follows from the first two.  What is the meaning of
`print' inside a `BEGIN' or `END' rule?  The meaning is the same as
always, `print $0'.  If `$0' is the null string, then this prints an
empty line.  Many long time `awk' programmers use `print' in `BEGIN'
and `END' rules, to mean `print ""', relying on `$0' being null.  While
you might generally get away with this in `BEGIN' rules, in `gawk' at
least, it is a very bad idea in `END' rules.  It is also poor style,
since if you want an empty line in the output, you should say so
explicitly in your program.


File: gawk.info,  Node: Empty,  Prev: BEGIN/END,  Up: Pattern Overview

The Empty Pattern
-----------------

   An empty (i.e. non-existent) pattern is considered to match *every*
input record.  For example, the program:

     awk '{ print $1 }' BBS-list

prints the first field of every record.


File: gawk.info,  Node: Action Overview,  Prev: Pattern Overview,  Up: Patterns and Actions

Overview of Actions
===================

   An `awk' program or script consists of a series of rules and
function definitions, interspersed.  (Functions are described later.
*Note User-defined Functions: User-defined.)

   A rule contains a pattern and an action, either of which (but not
both) may be omitted.  The purpose of the "action" is to tell `awk'
what to do once a match for the pattern is found.  Thus, in outline, an
`awk' program generally looks like this:

     [PATTERN] [{ ACTION }]
     [PATTERN] [{ ACTION }]
     ...
     function NAME(ARGS) { ... }
     ...

   An action consists of one or more `awk' "statements", enclosed in
curly braces (`{' and `}').  Each statement specifies one thing to be
done.  The statements are separated by newlines or semicolons.

   The curly braces around an action must be used even if the action
contains only one statement, or even if it contains no statements at
all.  However, if you omit the action entirely, omit the curly braces as
well.  An omitted action is equivalent to `{ print $0 }'.

     /foo/  { }  # match foo, do nothing - empty action
     /foo/       # match foo, print the record - omitted action

   Here are the kinds of statements supported in `awk':

   * Expressions, which can call functions or assign values to variables
     (*note Expressions::.).  Executing this kind of statement simply
     computes the value of the expression.  This is useful when the
     expression has side effects (*note Assignment Expressions:
     Assignment Ops.).

   * Control statements, which specify the control flow of `awk'
     programs.  The `awk' language gives you C-like constructs (`if',
     `for', `while', and `do') as well as a few special ones (*note
     Control Statements in Actions: Statements.).

   * Compound statements, which consist of one or more statements
     enclosed in curly braces.  A compound statement is used in order
     to put several statements together in the body of an `if',
     `while', `do' or `for' statement.

   * Input statements, using the `getline' command (*note Explicit
     Input with `getline': Getline.), the `next' statement (*note The
     `next' Statement: Next Statement.), and the `nextfile' statement
     (*note The `nextfile' Statement: Nextfile Statement.).

   * Output statements, `print' and `printf'.  *Note Printing Output:
     Printing.

   * Deletion statements, for deleting array elements.  *Note The
     `delete' Statement: Delete.


File: gawk.info,  Node: Statements,  Next: Built-in Variables,  Prev: Patterns and Actions,  Up: Top

Control Statements in Actions
*****************************

   "Control statements" such as `if', `while', and so on control the
flow of execution in `awk' programs.  Most of the control statements in
`awk' are patterned on similar statements in C.

   All the control statements start with special keywords such as `if'
and `while', to distinguish them from simple expressions.

   Many control statements contain other statements; for example, the
`if' statement contains another statement which may or may not be
executed.  The contained statement is called the "body".  If you want
to include more than one statement in the body, group them into a
single "compound statement" with curly braces, separating them with
newlines or semicolons.

* Menu:

* If Statement::                Conditionally execute some `awk'
                                statements.
* While Statement::             Loop until some condition is satisfied.
* Do Statement::                Do specified action while looping until some
                                condition is satisfied.
* For Statement::               Another looping statement, that provides
                                initialization and increment clauses.
* Break Statement::             Immediately exit the innermost enclosing loop.
* Continue Statement::          Skip to the end of the innermost enclosing
                                loop.
* Next Statement::              Stop processing the current input record.
* Nextfile Statement::          Stop processing the current file.
* Exit Statement::              Stop execution of `awk'.


File: gawk.info,  Node: If Statement,  Next: While Statement,  Prev: Statements,  Up: Statements

The `if'-`else' Statement
=========================

   The `if'-`else' statement is `awk''s decision-making statement.  It
looks like this:

     if (CONDITION) THEN-BODY [else ELSE-BODY]

The CONDITION is an expression that controls what the rest of the
statement will do.  If CONDITION is true, THEN-BODY is executed;
otherwise, ELSE-BODY is executed.  The `else' part of the statement is
optional.  The condition is considered false if its value is zero or
the null string, and true otherwise.

   Here is an example:

     if (x % 2 == 0)
         print "x is even"
     else
         print "x is odd"

   In this example, if the expression `x % 2 == 0' is true (that is,
the value of `x' is evenly divisible by two), then the first `print'
statement is executed, otherwise the second `print' statement is
executed.

   If the `else' appears on the same line as THEN-BODY, and THEN-BODY
is not a compound statement (i.e. not surrounded by curly braces), then
a semicolon must separate THEN-BODY from `else'.  To illustrate this,
let's rewrite the previous example:

     if (x % 2 == 0) print "x is even"; else
             print "x is odd"

If you forget the `;', `awk' won't be able to interpret the statement,
and you will get a syntax error.

   We would not actually write this example this way, because a human
reader might fail to see the `else' if it were not the first thing on
its line.


File: gawk.info,  Node: While Statement,  Next: Do Statement,  Prev: If Statement,  Up: Statements

The `while' Statement
=====================

   In programming, a "loop" means a part of a program that can be
executed two or more times in succession.

   The `while' statement is the simplest looping statement in `awk'.
It repeatedly executes a statement as long as a condition is true.  It
looks like this:

     while (CONDITION)
       BODY

Here BODY is a statement that we call the "body" of the loop, and
CONDITION is an expression that controls how long the loop keeps
running.

   The first thing the `while' statement does is test CONDITION.  If
CONDITION is true, it executes the statement BODY.  (The CONDITION is
true when the value is not zero and not a null string.) After BODY has
been executed, CONDITION is tested again, and if it is still true, BODY
is executed again.  This process repeats until CONDITION is no longer
true.  If CONDITION is initially false, the body of the loop is never
executed, and `awk' continues with the statement following the loop.

   This example prints the first three fields of each record, one per
line.

     awk '{ i = 1
            while (i <= 3) {
                print $i
                i++
            }
     }' inventory-shipped

Here the body of the loop is a compound statement enclosed in braces,
containing two statements.

   The loop works like this: first, the value of `i' is set to one.
Then, the `while' tests whether `i' is less than or equal to three.
This is true when `i' equals one, so the `i'-th field is printed.  Then
the `i++' increments the value of `i' and the loop repeats.  The loop
terminates when `i' reaches four.

   As you can see, a newline is not required between the condition and
the body; but using one makes the program clearer unless the body is a
compound statement or is very simple.  The newline after the open-brace
that begins the compound statement is not required either, but the
program would be harder to read without it.


File: gawk.info,  Node: Do Statement,  Next: For Statement,  Prev: While Statement,  Up: Statements

The `do'-`while' Statement
==========================

   The `do' loop is a variation of the `while' looping statement.  The
`do' loop executes the BODY once, and then repeats BODY as long as
CONDITION is true.  It looks like this:

     do
       BODY
     while (CONDITION)

   Even if CONDITION is false at the start, BODY is executed at least
once (and only once, unless executing BODY makes CONDITION true).
Contrast this with the corresponding `while' statement:

     while (CONDITION)
       BODY

This statement does not execute BODY even once if CONDITION is false to
begin with.

   Here is an example of a `do' statement:

     awk '{ i = 1
            do {
               print $0
               i++
            } while (i <= 10)
     }'

This program prints each input record ten times.  It isn't a very
realistic example, since in this case an ordinary `while' would do just
as well.  But this reflects actual experience; there is only
occasionally a real use for a `do' statement.


File: gawk.info,  Node: For Statement,  Next: Break Statement,  Prev: Do Statement,  Up: Statements

The `for' Statement
===================

   The `for' statement makes it more convenient to count iterations of a
loop.  The general form of the `for' statement looks like this:

     for (INITIALIZATION; CONDITION; INCREMENT)
       BODY

The INITIALIZATION, CONDITION and INCREMENT parts are arbitrary `awk'
expressions, and BODY stands for any `awk' statement.

   The `for' statement starts by executing INITIALIZATION.  Then, as
long as CONDITION is true, it repeatedly executes BODY and then
INCREMENT.  Typically INITIALIZATION sets a variable to either zero or
one, INCREMENT adds one to it, and CONDITION compares it against the
desired number of iterations.

   Here is an example of a `for' statement:

     awk '{ for (i = 1; i <= 3; i++)
               print $i
     }' inventory-shipped

This prints the first three fields of each input record, one field per
line.

   You cannot set more than one variable in the INITIALIZATION part
unless you use a multiple assignment statement such as `x = y = 0',
which is possible only if all the initial values are equal.  (But you
can initialize additional variables by writing their assignments as
separate statements preceding the `for' loop.)

   The same is true of the INCREMENT part; to increment additional
variables, you must write separate statements at the end of the loop.
The C compound expression, using C's comma operator, would be useful in
this context, but it is not supported in `awk'.

   Most often, INCREMENT is an increment expression, as in the example
above.  But this is not required; it can be any expression whatever.
For example, this statement prints all the powers of two between one
and 100:

     for (i = 1; i <= 100; i *= 2)
       print i

   Any of the three expressions in the parentheses following the `for'
may be omitted if there is nothing to be done there.  Thus,
`for (; x > 0;)' is equivalent to `while (x > 0)'.  If the CONDITION is
omitted, it is treated as TRUE, effectively yielding an "infinite loop"
(i.e. a loop that will never terminate).

   In most cases, a `for' loop is an abbreviation for a `while' loop,
as shown here:

     INITIALIZATION
     while (CONDITION) {
       BODY
       INCREMENT
     }

The only exception is when the `continue' statement (*note The
`continue' Statement: Continue Statement.) is used inside the loop;
changing a `for' statement to a `while' statement in this way can
change the effect of the `continue' statement inside the loop.

   There is an alternate version of the `for' loop, for iterating over
all the indices of an array:

     for (i in array)
         DO SOMETHING WITH array[i]

*Note Scanning All Elements of an Array: Scanning an Array, for more
information on this version of the `for' loop.

   The `awk' language has a `for' statement in addition to a `while'
statement because often a `for' loop is both less work to type and more
natural to think of.  Counting the number of iterations is very common
in loops.  It can be easier to think of this counting as part of
looping rather than as something to do inside the loop.

   The next section has more complicated examples of `for' loops.


File: gawk.info,  Node: Break Statement,  Next: Continue Statement,  Prev: For Statement,  Up: Statements

The `break' Statement
=====================

   The `break' statement jumps out of the innermost `for', `while', or
`do' loop that encloses it.  The following example finds the smallest
divisor of any integer, and also identifies prime numbers:

     awk '# find smallest divisor of num
          { num = $1
            for (div = 2; div*div <= num; div++)
              if (num % div == 0)
                break
            if (num % div == 0)
              printf "Smallest divisor of %d is %d\n", num, div
            else
              printf "%d is prime\n", num
          }'

   When the remainder is zero in the first `if' statement, `awk'
immediately "breaks out" of the containing `for' loop.  This means that
`awk' proceeds immediately to the statement following the loop and
continues processing.  (This is very different from the `exit'
statement which stops the entire `awk' program.  *Note The `exit'
Statement: Exit Statement.)

   Here is another program equivalent to the previous one.  It
illustrates how the CONDITION of a `for' or `while' could just as well
be replaced with a `break' inside an `if':

     awk '# find smallest divisor of num
          { num = $1
            for (div = 2; ; div++) {
              if (num % div == 0) {
                printf "Smallest divisor of %d is %d\n", num, div
                break
              }
              if (div*div > num) {
                printf "%d is prime\n", num
                break
              }
            }
     }'

   As described above, the `break' statement has no meaning when used
outside the body of a loop.  However, although it was never documented,
historical implementations of `awk' have treated the `break' statement
outside of a loop as if it were a `next' statement (*note The `next'
Statement: Next Statement.).  Recent versions of Unix `awk' no longer
allow this usage.  `gawk' will support this use of `break' only if
`--traditional' has been specified on the command line (*note Command
Line Options: Options.).  Otherwise, it will be treated as an error,
since the POSIX standard specifies that `break' should only be used
inside the body of a loop (d.c.).


File: gawk.info,  Node: Continue Statement,  Next: Next Statement,  Prev: Break Statement,  Up: Statements

The `continue' Statement
========================

   The `continue' statement, like `break', is used only inside `for',
`while', and `do' loops.  It skips over the rest of the loop body,
causing the next cycle around the loop to begin immediately.  Contrast
this with `break', which jumps out of the loop altogether.

   The `continue' statement in a `for' loop directs `awk' to skip the
rest of the body of the loop, and resume execution with the
increment-expression of the `for' statement.  The following program
illustrates this fact:

     awk 'BEGIN {
          for (x = 0; x <= 20; x++) {
              if (x == 5)
                  continue
              printf "%d ", x
          }
          print ""
     }'

This program prints all the numbers from zero to 20, except for five,
for which the `printf' is skipped.  Since the increment `x++' is not
skipped, `x' does not remain stuck at five.  Contrast the `for' loop
above with this `while' loop:

     awk 'BEGIN {
          x = 0
          while (x <= 20) {
              if (x == 5)
                  continue
              printf "%d ", x
              x++
          }
          print ""
     }'

This program loops forever once `x' gets to five.

   As described above, the `continue' statement has no meaning when
used outside the body of a loop.  However, although it was never
documented, historical implementations of `awk' have treated the
`continue' statement outside of a loop as if it were a `next' statement
(*note The `next' Statement: Next Statement.).  Recent versions of Unix
`awk' no longer allow this usage.  `gawk' will support this use of
`continue' only if `--traditional' has been specified on the command
line (*note Command Line Options: Options.).  Otherwise, it will be
treated as an error, since the POSIX standard specifies that `continue'
should only be used inside the body of a loop (d.c.).


File: gawk.info,  Node: Next Statement,  Next: Nextfile Statement,  Prev: Continue Statement,  Up: Statements

The `next' Statement
====================

   The `next' statement forces `awk' to immediately stop processing the
current record and go on to the next record.  This means that no
further rules are executed for the current record.  The rest of the
current rule's action is not executed either.

   Contrast this with the effect of the `getline' function (*note
Explicit Input with `getline': Getline.).  That too causes `awk' to
read the next record immediately, but it does not alter the flow of
control in any way.  So the rest of the current action executes with a
new input record.

   At the highest level, `awk' program execution is a loop that reads
an input record and then tests each rule's pattern against it.  If you
think of this loop as a `for' statement whose body contains the rules,
then the `next' statement is analogous to a `continue' statement: it
skips to the end of the body of this implicit loop, and executes the
increment (which reads another record).

   For example, if your `awk' program works only on records with four
fields, and you don't want it to fail when given bad input, you might
use this rule near the beginning of the program:

     NF != 4 {
       err = sprintf("%s:%d: skipped: NF != 4\n", FILENAME, FNR)
       print err > "/dev/stderr"
       next
     }

so that the following rules will not see the bad record.  The error
message is redirected to the standard error output stream, as error
messages should be.  *Note Special File Names in `gawk': Special Files.

   According to the POSIX standard, the behavior is undefined if the
`next' statement is used in a `BEGIN' or `END' rule.  `gawk' will treat
it as a syntax error.  Although POSIX permits it, some other `awk'
implementations don't allow the `next' statement inside function bodies
(*note User-defined Functions: User-defined.).  Just as any other
`next' statement, a `next' inside a function body reads the next record
and starts processing it with the first rule in the program.

   If the `next' statement causes the end of the input to be reached,
then the code in any `END' rules will be executed.  *Note The `BEGIN'
and `END' Special Patterns: BEGIN/END.

   *Caution:* Some `awk' implementations generate a run-time error if
you use the `next' statement inside a user-defined function (*note
User-defined Functions: User-defined.).  `gawk' does not have this
problem.


File: gawk.info,  Node: Nextfile Statement,  Next: Exit Statement,  Prev: Next Statement,  Up: Statements

The `nextfile' Statement
========================

   `gawk' provides the `nextfile' statement, which is similar to the
`next' statement.  However, instead of abandoning processing of the
current record, the `nextfile' statement instructs `gawk' to stop
processing the current data file.

   Upon execution of the `nextfile' statement, `FILENAME' is updated to
the name of the next data file listed on the command line, `FNR' is
reset to one, `ARGIND' is incremented, and processing starts over with
the first rule in the progam.  *Note Built-in Variables::.

   If the `nextfile' statement causes the end of the input to be
reached, then the code in any `END' rules will be executed.  *Note The
`BEGIN' and `END' Special Patterns: BEGIN/END.

   The `nextfile' statement is a `gawk' extension; it is not
(currently) available in any other `awk' implementation.  *Note
Implementing `nextfile' as a Function: Nextfile Function, for a
user-defined function you can use to simulate the `nextfile' statement.

   The `nextfile' statement would be useful if you have many data files
to process, and you expect that you would not want to process every
record in every file.  Normally, in order to move on to the next data
file, you would have to continue scanning the unwanted records.  The
`nextfile' statement accomplishes this much more efficiently.

   *Caution:*  Versions of `gawk' prior to 3.0 used two words (`next
file') for the `nextfile' statement.  This was changed in 3.0 to one
word, since the treatment of `file' was inconsistent. When it appeared
after `next', it was a keyword.  Otherwise, it was a regular
identifier.  The old usage is still accepted. However, `gawk' will
generate a warning message, and support for `next file' will eventually
be discontinued in a future version of `gawk'.


File: gawk.info,  Node: Exit Statement,  Prev: Nextfile Statement,  Up: Statements

The `exit' Statement
====================

   The `exit' statement causes `awk' to immediately stop executing the
current rule and to stop processing input; any remaining input is
ignored.  It looks like this:

     exit [RETURN CODE]

   If an `exit' statement is executed from a `BEGIN' rule the program
stops processing everything immediately.  No input records are read.
However, if an `END' rule is present, it is executed (*note The `BEGIN'
and `END' Special Patterns: BEGIN/END.).

   If `exit' is used as part of an `END' rule, it causes the program to
stop immediately.

   An `exit' statement that is not part of a `BEGIN' or `END' rule
stops the execution of any further automatic rules for the current
record, skips reading any remaining input records, and executes the
`END' rule if there is one.

   If you do not want the `END' rule to do its job in this case, you
can set a variable to non-zero before the `exit' statement, and check
that variable in the `END' rule.  *Note Assertions: Assert Function,
for an example that does this.

   If an argument is supplied to `exit', its value is used as the exit
status code for the `awk' process.  If no argument is supplied, `exit'
returns status zero (success).  In the case where an argument is
supplied to a first `exit' statement, and then `exit' is called a
second time with no argument, the previously supplied exit value is
used (d.c.).

   For example, let's say you've discovered an error condition you
really don't know how to handle.  Conventionally, programs report this
by exiting with a non-zero status.  Your `awk' program can do this
using an `exit' statement with a non-zero argument.  Here is an example:

     BEGIN {
            if (("date" | getline date_now) < 0) {
              print "Can't get system date" > "/dev/stderr"
              exit 1
            }
            print "current date is", date_now
            close("date")
     }


File: gawk.info,  Node: Built-in Variables,  Next: Arrays,  Prev: Statements,  Up: Top

Built-in Variables
******************

   Most `awk' variables are available for you to use for your own
purposes; they never change except when your program assigns values to
them, and never affect anything except when your program examines them.
However, a few variables in `awk' have special built-in meanings.  Some
of them `awk' examines automatically, so that they enable you to tell
`awk' how to do certain things.  Others are set automatically by `awk',
so that they carry information from the internal workings of `awk' to
your program.

   This chapter documents all the built-in variables of `gawk'.  Most
of them are also documented in the chapters describing their areas of
activity.

* Menu:

* User-modified::               Built-in variables that you change to control
                                `awk'.
* Auto-set::                    Built-in variables where `awk' gives you
                                information.
* ARGC and ARGV::               Ways to use `ARGC' and `ARGV'.


File: gawk.info,  Node: User-modified,  Next: Auto-set,  Prev: Built-in Variables,  Up: Built-in Variables

Built-in Variables that Control `awk'
=====================================

   This is an alphabetical list of the variables which you can change to
control how `awk' does certain things. Those variables that are
specific to `gawk' are marked with an asterisk, `*'.

`CONVFMT'
     This string controls conversion of numbers to strings (*note
     Conversion of Strings and Numbers: Conversion.).  It works by
     being passed, in effect, as the first argument to the `sprintf'
     function (*note Built-in Functions for String Manipulation: String
     Functions.).  Its default value is `"%.6g"'.  `CONVFMT' was
     introduced by the POSIX standard.

`FIELDWIDTHS *'
     This is a space separated list of columns that tells `gawk' how to
     split input with fixed, columnar boundaries.  It is an
     experimental feature.  Assigning to `FIELDWIDTHS' overrides the
     use of `FS' for field splitting.  *Note Reading Fixed-width Data:
     Constant Size, for more information.

     If `gawk' is in compatibility mode (*note Command Line Options:
     Options.), then `FIELDWIDTHS' has no special meaning, and field
     splitting operations are done based exclusively on the value of
     `FS'.

`FS'
     `FS' is the input field separator (*note Specifying How Fields are
     Separated: Field Separators.).  The value is a single-character
     string or a multi-character regular expression that matches the
     separations between fields in an input record.  If the value is
     the null string (`""'), then each character in the record becomes
     a separate field.

     The default value is `" "', a string consisting of a single space.
     As a special exception, this value means that any sequence of
     spaces, tabs, and/or newlines is a single separator.(1)  It also
     causes spaces, tabs, and newlines at the beginning and end of a
     record to be ignored.

     You can set the value of `FS' on the command line using the `-F'
     option:

          awk -F, 'PROGRAM' INPUT-FILES

     If `gawk' is using `FIELDWIDTHS' for field-splitting, assigning a
     value to `FS' will cause `gawk' to return to the normal,
     `FS'-based, field splitting. An easy way to do this is to simply
     say `FS = FS', perhaps with an explanatory comment.

`IGNORECASE *'
     If `IGNORECASE' is non-zero or non-null, then all string
     comparisons, and all regular expression matching are
     case-independent.  Thus, regexp matching with `~' and `!~', and
     the `gensub', `gsub', `index', `match', `split' and `sub'
     functions, record termination with `RS', and field splitting with
     `FS' all ignore case when doing their particular regexp operations.
     *Note Case-sensitivity in Matching: Case-sensitivity.

     If `gawk' is in compatibility mode (*note Command Line Options:
     Options.), then `IGNORECASE' has no special meaning, and string
     and regexp operations are always case-sensitive.

`OFMT'
     This string controls conversion of numbers to strings (*note
     Conversion of Strings and Numbers: Conversion.) for printing with
     the `print' statement.  It works by being passed, in effect, as
     the first argument to the `sprintf' function (*note Built-in
     Functions for String Manipulation: String Functions.).  Its
     default value is `"%.6g"'.  Earlier versions of `awk' also used
     `OFMT' to specify the format for converting numbers to strings in
     general expressions; this is now done by `CONVFMT'.

`OFS'
     This is the output field separator (*note Output Separators::.).
     It is output between the fields output by a `print' statement.  Its
     default value is `" "', a string consisting of a single space.

`ORS'
     This is the output record separator.  It is output at the end of
     every `print' statement.  Its default value is `"\n"'.  (*Note
     Output Separators::.)

`RS'
     This is `awk''s input record separator.  Its default value is a
     string containing a single newline character, which means that an
     input record consists of a single line of text.  It can also be
     the null string, in which case records are separated by runs of
     blank lines, or a regexp, in which case records are separated by
     matches of the regexp in the input text.  (*Note How Input is
     Split into Records: Records.)

`SUBSEP'
     `SUBSEP' is the subscript separator.  It has the default value of
     `"\034"', and is used to separate the parts of the indices of a
     multi-dimensional array.  Thus, the expression `foo["A", "B"]'
     really accesses `foo["A\034B"]' (*note Multi-dimensional Arrays:
     Multi-dimensional.).

   ---------- Footnotes ----------

   (1)  In POSIX `awk', newline does not count as whitespace.


File: gawk.info,  Node: Auto-set,  Next: ARGC and ARGV,  Prev: User-modified,  Up: Built-in Variables

Built-in Variables that Convey Information
==========================================

   This is an alphabetical list of the variables that are set
automatically by `awk' on certain occasions in order to provide
information to your program.  Those variables that are specific to
`gawk' are marked with an asterisk, `*'.

`ARGC'
`ARGV'
     The command-line arguments available to `awk' programs are stored
     in an array called `ARGV'.  `ARGC' is the number of command-line
     arguments present.  *Note Other Command Line Arguments: Other
     Arguments.  Unlike most `awk' arrays, `ARGV' is indexed from zero
     to `ARGC' - 1.  For example:

          $ awk 'BEGIN {
          >        for (i = 0; i < ARGC; i++)
          >            print ARGV[i]
          >      }' inventory-shipped BBS-list
          -| awk
          -| inventory-shipped
          -| BBS-list

     In this example, `ARGV[0]' contains `"awk"', `ARGV[1]' contains
     `"inventory-shipped"', and `ARGV[2]' contains `"BBS-list"'.  The
     value of `ARGC' is three, one more than the index of the last
     element in `ARGV', since the elements are numbered from zero.

     The names `ARGC' and `ARGV', as well as the convention of indexing
     the array from zero to `ARGC' - 1, are derived from the C
     language's method of accessing command line arguments.  *Note
     Using `ARGC' and `ARGV': ARGC and ARGV, for information about how
     `awk' uses these variables.

`ARGIND *'
     The index in `ARGV' of the current file being processed.  Every
     time `gawk' opens a new data file for processing, it sets `ARGIND'
     to the index in `ARGV' of the file name.  When `gawk' is
     processing the input files, it is always true that `FILENAME ==
     ARGV[ARGIND]'.

     This variable is useful in file processing; it allows you to tell
     how far along you are in the list of data files, and to
     distinguish between successive instances of the same filename on
     the command line.

     While you can change the value of `ARGIND' within your `awk'
     program, `gawk' will automatically set it to a new value when the
     next file is opened.

     This variable is a `gawk' extension. In other `awk'
     implementations, or if `gawk' is in compatibility mode (*note
     Command Line Options: Options.), it is not special.

`ENVIRON'
     An associative array that contains the values of the environment.
     The array indices are the environment variable names; the values
     are the values of the particular environment variables.  For
     example, `ENVIRON["HOME"]' might be `/home/arnold'.  Changing this
     array does not affect the environment passed on to any programs
     that `awk' may spawn via redirection or the `system' function.
     (In a future version of `gawk', it may do so.)

     Some operating systems may not have environment variables.  On
     such systems, the `ENVIRON' array is empty (except for
     `ENVIRON["AWKPATH"]').

`ERRNO *'
     If a system error occurs either doing a redirection for `getline',
     during a read for `getline', or during a `close' operation, then
     `ERRNO' will contain a string describing the error.

     This variable is a `gawk' extension. In other `awk'
     implementations, or if `gawk' is in compatibility mode (*note
     Command Line Options: Options.), it is not special.

`FILENAME'
     This is the name of the file that `awk' is currently reading.
     When no data files are listed on the command line, `awk' reads
     from the standard input, and `FILENAME' is set to `"-"'.
     `FILENAME' is changed each time a new file is read (*note Reading
     Input Files: Reading Files.).  Inside a `BEGIN' rule, the value of
     `FILENAME' is `""', since there are no input files being processed
     yet.(1) (d.c.)

`FNR'
     `FNR' is the current record number in the current file.  `FNR' is
     incremented each time a new record is read (*note Explicit Input
     with `getline': Getline.).  It is reinitialized to zero each time
     a new input file is started.

`NF'
     `NF' is the number of fields in the current input record.  `NF' is
     set each time a new record is read, when a new field is created,
     or when `$0' changes (*note Examining Fields: Fields.).

`NR'
     This is the number of input records `awk' has processed since the
     beginning of the program's execution (*note How Input is Split
     into Records: Records.).  `NR' is set each time a new record is
     read.

`RLENGTH'
     `RLENGTH' is the length of the substring matched by the `match'
     function (*note Built-in Functions for String Manipulation: String
     Functions.).  `RLENGTH' is set by invoking the `match' function.
     Its value is the length of the matched string, or -1 if no match
     was found.

`RSTART'
     `RSTART' is the start-index in characters of the substring matched
     by the `match' function (*note Built-in Functions for String
     Manipulation: String Functions.).  `RSTART' is set by invoking the
     `match' function.  Its value is the position of the string where
     the matched substring starts, or zero if no match was found.

`RT *'
     `RT' is set each time a record is read. It contains the input text
     that matched the text denoted by `RS', the record separator.

     This variable is a `gawk' extension. In other `awk'
     implementations, or if `gawk' is in compatibility mode (*note
     Command Line Options: Options.), it is not special.

   A side note about `NR' and `FNR'.  `awk' simply increments both of
these variables each time it reads a record, instead of setting them to
the absolute value of the number of records read.  This means that your
program can change these variables, and their new values will be
incremented for each record (d.c.).  For example:

     $ echo '1
     > 2
     > 3
     > 4' | awk 'NR == 2 { NR = 17 }
     > { print NR }'
     -| 1
     -| 17
     -| 18
     -| 19

Before `FNR' was added to the `awk' language (*note Major Changes
between V7 and SVR3.1: V7/SVR3.1.), many `awk' programs used this
feature to track the number of records in a file by resetting `NR' to
zero when `FILENAME' changed.

   ---------- Footnotes ----------

   (1)  Some early implementations of Unix `awk' initialized `FILENAME'
to `"-"', even if there were data files to be processed. This behavior
was incorrect, and should not be relied upon in your programs.


File: gawk.info,  Node: ARGC and ARGV,  Prev: Auto-set,  Up: Built-in Variables

Using `ARGC' and `ARGV'
=======================

   In *Note Built-in Variables that Convey Information: Auto-set, you
saw this program describing the information contained in `ARGC' and
`ARGV':

     $ awk 'BEGIN {
     >        for (i = 0; i < ARGC; i++)
     >            print ARGV[i]
     >      }' inventory-shipped BBS-list
     -| awk
     -| inventory-shipped
     -| BBS-list

In this example, `ARGV[0]' contains `"awk"', `ARGV[1]' contains
`"inventory-shipped"', and `ARGV[2]' contains `"BBS-list"'.

   Notice that the `awk' program is not entered in `ARGV'.  The other
special command line options, with their arguments, are also not
entered.  But variable assignments on the command line *are* treated as
arguments, and do show up in the `ARGV' array.

   Your program can alter `ARGC' and the elements of `ARGV'.  Each time
`awk' reaches the end of an input file, it uses the next element of
`ARGV' as the name of the next input file.  By storing a different
string there, your program can change which files are read.  You can
use `"-"' to represent the standard input.  By storing additional
elements and incrementing `ARGC' you can cause additional files to be
read.

   If you decrease the value of `ARGC', that eliminates input files
from the end of the list.  By recording the old value of `ARGC'
elsewhere, your program can treat the eliminated arguments as something
other than file names.

   To eliminate a file from the middle of the list, store the null
string (`""') into `ARGV' in place of the file's name.  As a special
feature, `awk' ignores file names that have been replaced with the null
string.  You may also use the `delete' statement to remove elements from
`ARGV' (*note The `delete' Statement: Delete.).

   All of these actions are typically done from the `BEGIN' rule,
before actual processing of the input begins.  *Note Splitting a Large
File Into Pieces: Split Program, and see *Note Duplicating Output Into
Multiple Files: Tee Program, for an example of each way of removing
elements from `ARGV'.

   The following fragment processes `ARGV' in order to examine, and
then remove, command line options.

     BEGIN {
         for (i = 1; i < ARGC; i++) {
             if (ARGV[i] == "-v")
                 verbose = 1
             else if (ARGV[i] == "-d")
                 debug = 1
             else if (ARGV[i] ~ /^-?/) {
                 e = sprintf("%s: unrecognized option -- %c",
                         ARGV[0], substr(ARGV[i], 1, ,1))
                 print e > "/dev/stderr"
             } else
                 break
             delete ARGV[i]
         }
     }


File: gawk.info,  Node: Arrays,  Next: Built-in,  Prev: Built-in Variables,  Up: Top

Arrays in `awk'
***************

   An "array" is a table of values, called "elements".  The elements of
an array are distinguished by their indices.  "Indices" may be either
numbers or strings.  `awk' maintains a single set of names that may be
used for naming variables, arrays and functions (*note User-defined
Functions: User-defined.).  Thus, you cannot have a variable and an
array with the same name in the same `awk' program.

* Menu:

* Array Intro::                 Introduction to Arrays
* Reference to Elements::       How to examine one element of an array.
* Assigning Elements::          How to change an element of an array.
* Array Example::               Basic Example of an Array
* Scanning an Array::           A variation of the `for' statement. It
                                loops through the indices of an array's
                                existing elements.
* Delete::                      The `delete' statement removes an element
                                from an array.
* Numeric Array Subscripts::    How to use numbers as subscripts in
                                `awk'.
* Uninitialized Subscripts::    Using Uninitialized variables as subscripts.
* Multi-dimensional::           Emulating multi-dimensional arrays in
                                `awk'.
* Multi-scanning::              Scanning multi-dimensional arrays.


File: gawk.info,  Node: Array Intro,  Next: Reference to Elements,  Prev: Arrays,  Up: Arrays

Introduction to Arrays
======================

   The `awk' language provides one-dimensional "arrays" for storing
groups of related strings or numbers.

   Every `awk' array must have a name.  Array names have the same
syntax as variable names; any valid variable name would also be a valid
array name.  But you cannot use one name in both ways (as an array and
as a variable) in one `awk' program.

   Arrays in `awk' superficially resemble arrays in other programming
languages; but there are fundamental differences.  In `awk', you don't
need to specify the size of an array before you start to use it.
Additionally, any number or string in `awk' may be used as an array
index, not just consecutive integers.

   In most other languages, you have to "declare" an array and specify
how many elements or components it contains.  In such languages, the
declaration causes a contiguous block of memory to be allocated for that
many elements.  An index in the array usually must be a positive
integer; for example, the index zero specifies the first element in the
array, which is actually stored at the beginning of the block of
memory.  Index one specifies the second element, which is stored in
memory right after the first element, and so on.  It is impossible to
add more elements to the array, because it has room for only as many
elements as you declared.  (Some languages allow arbitrary starting and
ending indices, e.g., `15 .. 27', but the size of the array is still
fixed when the array is declared.)

   A contiguous array of four elements might look like this,
conceptually, if the element values are eight, `"foo"', `""' and 30:

     +---------+---------+--------+---------+
     |    8    |  "foo"  |   ""   |    30   |    value
     +---------+---------+--------+---------+
          0         1         2         3        index

Only the values are stored; the indices are implicit from the order of
the values.  Eight is the value at index zero, because eight appears in
the position with zero elements before it.

   Arrays in `awk' are different: they are "associative".  This means
that each array is a collection of pairs: an index, and its
corresponding array element value:

     Element 4     Value 30
     Element 2     Value "foo"
     Element 1     Value 8
     Element 3     Value ""

We have shown the pairs in jumbled order because their order is
irrelevant.

   One advantage of associative arrays is that new pairs can be added
at any time.  For example, suppose we add to the above array a tenth
element whose value is `"number ten"'.  The result is this:

     Element 10    Value "number ten"
     Element 4     Value 30
     Element 2     Value "foo"
     Element 1     Value 8
     Element 3     Value ""

Now the array is "sparse", which just means some indices are missing:
it has elements 1-4 and 10, but doesn't have elements 5, 6, 7, 8, or 9.

   Another consequence of associative arrays is that the indices don't
have to be positive integers.  Any number, or even a string, can be an
index.  For example, here is an array which translates words from
English into French:

     Element "dog" Value "chien"
     Element "cat" Value "chat"
     Element "one" Value "un"
     Element 1     Value "un"

Here we decided to translate the number one in both spelled-out and
numeric form--thus illustrating that a single array can have both
numbers and strings as indices.  (In fact, array subscripts are always
strings; this is discussed in more detail in *Note Using Numbers to
Subscript Arrays: Numeric Array Subscripts.)

   When `awk' creates an array for you, e.g., with the `split' built-in
function, that array's indices are consecutive integers starting at one.
(*Note Built-in Functions for String Manipulation: String Functions.)


File: gawk.info,  Node: Reference to Elements,  Next: Assigning Elements,  Prev: Array Intro,  Up: Arrays

Referring to an Array Element
=============================

   The principal way of using an array is to refer to one of its
elements.  An array reference is an expression which looks like this:

     ARRAY[INDEX]

Here, ARRAY is the name of an array.  The expression INDEX is the index
of the element of the array that you want.

   The value of the array reference is the current value of that array
element.  For example, `foo[4.3]' is an expression for the element of
array `foo' at index `4.3'.

   If you refer to an array element that has no recorded value, the
value of the reference is `""', the null string.  This includes elements
to which you have not assigned any value, and elements that have been
deleted (*note The `delete' Statement: Delete.).  Such a reference
automatically creates that array element, with the null string as its
value.  (In some cases, this is unfortunate, because it might waste
memory inside `awk'.)

   You can find out if an element exists in an array at a certain index
with the expression:

     INDEX in ARRAY

This expression tests whether or not the particular index exists,
without the side effect of creating that element if it is not present.
The expression has the value one (true) if `ARRAY[INDEX]' exists, and
zero (false) if it does not exist.

   For example, to test whether the array `frequencies' contains the
index `2', you could write this statement:

     if (2 in frequencies)
         print "Subscript 2 is present."

   Note that this is *not* a test of whether or not the array
`frequencies' contains an element whose *value* is two.  (There is no
way to do that except to scan all the elements.)  Also, this *does not*
create `frequencies[2]', while the following (incorrect) alternative
would do so:

     if (frequencies[2] != "")
         print "Subscript 2 is present."


File: gawk.info,  Node: Assigning Elements,  Next: Array Example,  Prev: Reference to Elements,  Up: Arrays

Assigning Array Elements
========================

   Array elements are lvalues: they can be assigned values just like
`awk' variables:

     ARRAY[SUBSCRIPT] = VALUE

Here ARRAY is the name of your array.  The expression SUBSCRIPT is the
index of the element of the array that you want to assign a value.  The
expression VALUE is the value you are assigning to that element of the
array.


File: gawk.info,  Node: Array Example,  Next: Scanning an Array,  Prev: Assigning Elements,  Up: Arrays

Basic Array Example
===================

   The following program takes a list of lines, each beginning with a
line number, and prints them out in order of line number.  The line
numbers are not in order, however, when they are first read:  they are
scrambled.  This program sorts the lines by making an array using the
line numbers as subscripts.  It then prints out the lines in sorted
order of their numbers.  It is a very simple program, and gets confused
if it encounters repeated numbers, gaps, or lines that don't begin with
a number.

     {
       if ($1 > max)
         max = $1
       arr[$1] = $0
     }
     
     END {
       for (x = 1; x <= max; x++)
         print arr[x]
     }

   The first rule keeps track of the largest line number seen so far;
it also stores each line into the array `arr', at an index that is the
line's number.

   The second rule runs after all the input has been read, to print out
all the lines.

   When this program is run with the following input:

     5  I am the Five man
     2  Who are you?  The new number two!
     4  . . . And four on the floor
     1  Who is number one?
     3  I three you.

its output is this:

     1  Who is number one?
     2  Who are you?  The new number two!
     3  I three you.
     4  . . . And four on the floor
     5  I am the Five man

   If a line number is repeated, the last line with a given number
overrides the others.

   Gaps in the line numbers can be handled with an easy improvement to
the program's `END' rule:

     END {
       for (x = 1; x <= max; x++)
         if (x in arr)
           print arr[x]
     }


File: gawk.info,  Node: Scanning an Array,  Next: Delete,  Prev: Array Example,  Up: Arrays

Scanning All Elements of an Array
=================================

   In programs that use arrays, you often need a loop that executes
once for each element of an array.  In other languages, where arrays are
contiguous and indices are limited to positive integers, this is easy:
you can find all the valid indices by counting from the lowest index up
to the highest.  This technique won't do the job in `awk', since any
number or string can be an array index.  So `awk' has a special kind of
`for' statement for scanning an array:

     for (VAR in ARRAY)
       BODY

This loop executes BODY once for each index in ARRAY that your program
has previously used, with the variable VAR set to that index.

   Here is a program that uses this form of the `for' statement.  The
first rule scans the input records and notes which words appear (at
least once) in the input, by storing a one into the array `used' with
the word as index.  The second rule scans the elements of `used' to
find all the distinct words that appear in the input.  It prints each
word that is more than 10 characters long, and also prints the number of
such words.  *Note Built-in Functions for String Manipulation: String
Functions, for more information on the built-in function `length'.

     # Record a 1 for each word that is used at least once.
     {
         for (i = 1; i <= NF; i++)
             used[$i] = 1
     }
     
     # Find number of distinct words more than 10 characters long.
     END {
         for (x in used)
             if (length(x) > 10) {
                 ++num_long_words
                 print x
             }
         print num_long_words, "words longer than 10 characters"
     }

*Note Generating Word Usage Counts: Word Sorting, for a more detailed
example of this type.

   The order in which elements of the array are accessed by this
statement is determined by the internal arrangement of the array
elements within `awk' and cannot be controlled or changed.  This can
lead to problems if new elements are added to ARRAY by statements in
the loop body; you cannot predict whether or not the `for' loop will
reach them.  Similarly, changing VAR inside the loop may produce
strange results.  It is best to avoid such things.


File: gawk.info,  Node: Delete,  Next: Numeric Array Subscripts,  Prev: Scanning an Array,  Up: Arrays

The `delete' Statement
======================

   You can remove an individual element of an array using the `delete'
statement:

     delete ARRAY[INDEX]

   Once you have deleted an array element, you can no longer obtain any
value the element once had.  It is as if you had never referred to it
and had never given it any value.

   Here is an example of deleting elements in an array:

     for (i in frequencies)
       delete frequencies[i]

This example removes all the elements from the array `frequencies'.

   If you delete an element, a subsequent `for' statement to scan the
array will not report that element, and the `in' operator to check for
the presence of that element will return zero (i.e. false):

     delete foo[4]
     if (4 in foo)
         print "This will never be printed"

   It is important to note that deleting an element is *not* the same
as assigning it a null value (the empty string, `""').

     foo[4] = ""
     if (4 in foo)
       print "This is printed, even though foo[4] is empty"

   It is not an error to delete an element that does not exist.

   You can delete all the elements of an array with a single statement,
by leaving off the subscript in the `delete' statement.

     delete ARRAY

   This ability is a `gawk' extension; it is not available in
compatibility mode (*note Command Line Options: Options.).

   Using this version of the `delete' statement is about three times
more efficient than the equivalent loop that deletes each element one
at a time.

   The following statement provides a portable, but non-obvious way to
clear out an array.

     # thanks to Michael Brennan for pointing this out
     split("", array)

   The `split' function (*note Built-in Functions for String
Manipulation: String Functions.) clears out the target array first.
This call asks it to split apart the null string. Since there is no
data to split out, the function simply clears the array and then
returns.


File: gawk.info,  Node: Numeric Array Subscripts,  Next: Uninitialized Subscripts,  Prev: Delete,  Up: Arrays

Using Numbers to Subscript Arrays
=================================

   An important aspect of arrays to remember is that *array subscripts
are always strings*.  If you use a numeric value as a subscript, it
will be converted to a string value before it is used for subscripting
(*note Conversion of Strings and Numbers: Conversion.).

   This means that the value of the built-in variable `CONVFMT' can
potentially affect how your program accesses elements of an array.  For
example:

     xyz = 12.153
     data[xyz] = 1
     CONVFMT = "%2.2f"
     if (xyz in data)
         printf "%s is in data\n", xyz
     else
         printf "%s is not in data\n", xyz

This prints `12.15 is not in data'.  The first statement gives `xyz' a
numeric value.  Assigning to `data[xyz]' subscripts `data' with the
string value `"12.153"' (using the default conversion value of
`CONVFMT', `"%.6g"'), and assigns one to `data["12.153"]'.  The program
then changes the value of `CONVFMT'.  The test `(xyz in data)'
generates a new string value from `xyz', this time `"12.15"', since the
value of `CONVFMT' only allows two significant digits.  This test fails,
since `"12.15"' is a different string from `"12.153"'.

   According to the rules for conversions (*note Conversion of Strings
and Numbers: Conversion.), integer values are always converted to
strings as integers, no matter what the value of `CONVFMT' may happen
to be.  So the usual case of:

     for (i = 1; i <= maxsub; i++)
         do something with array[i]

will work, no matter what the value of `CONVFMT'.

   Like many things in `awk', the majority of the time things work as
you would expect them to work.  But it is useful to have a precise
knowledge of the actual rules, since sometimes they can have a subtle
effect on your programs.


File: gawk.info,  Node: Uninitialized Subscripts,  Next: Multi-dimensional,  Prev: Numeric Array Subscripts,  Up: Arrays

Using Uninitialized Variables as Subscripts
===========================================

   Suppose you want to print your input data in reverse order.  A
reasonable attempt at a program to do so (with some test data) might
look like this:

     $ echo 'line 1
     > line 2
     > line 3' | awk '{ l[lines] = $0; ++lines }
     > END {
     >     for (i = lines-1; i >= 0; --i)
     >        print l[i]
     > }'
     -| line 3
     -| line 2

   Unfortunately, the very first line of input data did not come out in
the output!

   At first glance, this program should have worked.  The variable
`lines' is uninitialized, and uninitialized variables have the numeric
value zero.  So, the value of `l[0]' should have been printed.

   The issue here is that subscripts for `awk' arrays are *always*
strings. And uninitialized variables, when used as strings, have the
value `""', not zero.  Thus, `line 1' ended up stored in `l[""]'.

   The following version of the program works correctly:

     { l[lines++] = $0 }
     END {
         for (i = lines - 1; i >= 0; --i)
            print l[i]
     }

   Here, the `++' forces `l' to be numeric, thus making the "old value"
numeric zero, which is then converted to `"0"' as the array subscript.

   As we have just seen, even though it is somewhat unusual, the null
string (`""') is a valid array subscript (d.c.). If `--lint' is provided
on the command line (*note Command Line Options: Options.), `gawk' will
warn about the use of the null string as a subscript.


File: gawk.info,  Node: Multi-dimensional,  Next: Multi-scanning,  Prev: Uninitialized Subscripts,  Up: Arrays

Multi-dimensional Arrays
========================

   A multi-dimensional array is an array in which an element is
identified by a sequence of indices, instead of a single index.  For
example, a two-dimensional array requires two indices.  The usual way
(in most languages, including `awk') to refer to an element of a
two-dimensional array named `grid' is with `grid[X,Y]'.

   Multi-dimensional arrays are supported in `awk' through
concatenation of indices into one string.  What happens is that `awk'
converts the indices into strings (*note Conversion of Strings and
Numbers: Conversion.) and concatenates them together, with a separator
between them.  This creates a single string that describes the values
of the separate indices.  The combined string is used as a single index
into an ordinary, one-dimensional array.  The separator used is the
value of the built-in variable `SUBSEP'.

   For example, suppose we evaluate the expression `foo[5,12] = "value"'
when the value of `SUBSEP' is `"@"'.  The numbers five and 12 are
converted to strings and concatenated with an `@' between them,
yielding `"5@12"'; thus, the array element `foo["5@12"]' is set to
`"value"'.

   Once the element's value is stored, `awk' has no record of whether
it was stored with a single index or a sequence of indices.  The two
expressions `foo[5,12]' and `foo[5 SUBSEP 12]' are always equivalent.

   The default value of `SUBSEP' is the string `"\034"', which contains
a non-printing character that is unlikely to appear in an `awk' program
or in most input data.

   The usefulness of choosing an unlikely character comes from the fact
that index values that contain a string matching `SUBSEP' lead to
combined strings that are ambiguous.  Suppose that `SUBSEP' were `"@"';
then `foo["a@b", "c"]' and `foo["a", "b@c"]' would be indistinguishable
because both would actually be stored as `foo["a@b@c"]'.

   You can test whether a particular index-sequence exists in a
"multi-dimensional" array with the same operator `in' used for single
dimensional arrays.  Instead of a single index as the left-hand operand,
write the whole sequence of indices, separated by commas, in
parentheses:

     (SUBSCRIPT1, SUBSCRIPT2, ...) in ARRAY

   The following example treats its input as a two-dimensional array of
fields; it rotates this array 90 degrees clockwise and prints the
result.  It assumes that all lines have the same number of elements.

     awk '{
          if (max_nf < NF)
               max_nf = NF
          max_nr = NR
          for (x = 1; x <= NF; x++)
               vector[x, NR] = $x
     }
     
     END {
          for (x = 1; x <= max_nf; x++) {
               for (y = max_nr; y >= 1; --y)
                    printf("%s ", vector[x, y])
               printf("\n")
          }
     }'

When given the input:

     1 2 3 4 5 6
     2 3 4 5 6 1
     3 4 5 6 1 2
     4 5 6 1 2 3

it produces:

     4 3 2 1
     5 4 3 2
     6 5 4 3
     1 6 5 4
     2 1 6 5
     3 2 1 6


File: gawk.info,  Node: Multi-scanning,  Prev: Multi-dimensional,  Up: Arrays

Scanning Multi-dimensional Arrays
=================================

   There is no special `for' statement for scanning a
"multi-dimensional" array; there cannot be one, because in truth there
are no multi-dimensional arrays or elements; there is only a
multi-dimensional *way of accessing* an array.

   However, if your program has an array that is always accessed as
multi-dimensional, you can get the effect of scanning it by combining
the scanning `for' statement (*note Scanning All Elements of an Array:
Scanning an Array.) with the `split' built-in function (*note Built-in
Functions for String Manipulation: String Functions.).  It works like
this:

     for (combined in array) {
       split(combined, separate, SUBSEP)
       ...
     }

This sets `combined' to each concatenated, combined index in the array,
and splits it into the individual indices by breaking it apart where
the value of `SUBSEP' appears.  The split-out indices become the
elements of the array `separate'.

   Thus, suppose you have previously stored a value in `array[1,
"foo"]'; then an element with index `"1\034foo"' exists in `array'.
(Recall that the default value of `SUBSEP' is the character with code
034.)  Sooner or later the `for' statement will find that index and do
an iteration with `combined' set to `"1\034foo"'.  Then the `split'
function is called as follows:

     split("1\034foo", separate, "\034")

The result of this is to set `separate[1]' to `"1"' and `separate[2]'
to `"foo"'.  Presto, the original sequence of separate indices has been
recovered.


File: gawk.info,  Node: Built-in,  Next: User-defined,  Prev: Arrays,  Up: Top

Built-in Functions
******************

   "Built-in" functions are functions that are always available for
your `awk' program to call.  This chapter defines all the built-in
functions in `awk'; some of them are mentioned in other sections, but
they are summarized here for your convenience.  (You can also define
new functions yourself.  *Note User-defined Functions: User-defined.)

* Menu:

* Calling Built-in::            How to call built-in functions.
* Numeric Functions::           Functions that work with numbers, including
                                `int', `sin' and `rand'.
* String Functions::            Functions for string manipulation, such as
                                `split', `match', and
                                `sprintf'.
* I/O Functions::               Functions for files and shell commands.
* Time Functions::              Functions for dealing with time stamps.


File: gawk.info,  Node: Calling Built-in,  Next: Numeric Functions,  Prev: Built-in,  Up: Built-in

Calling Built-in Functions
==========================

   To call a built-in function, write the name of the function followed
by arguments in parentheses.  For example, `atan2(y + z, 1)' is a call
to the function `atan2', with two arguments.

   Whitespace is ignored between the built-in function name and the
open-parenthesis, but we recommend that you avoid using whitespace
there.  User-defined functions do not permit whitespace in this way, and
you will find it easier to avoid mistakes by following a simple
convention which always works: no whitespace after a function name.

   Each built-in function accepts a certain number of arguments.  In
some cases, arguments can be omitted. The defaults for omitted
arguments vary from function to function and are described under the
individual functions.  In some `awk' implementations, extra arguments
given to built-in functions are ignored.  However, in `gawk', it is a
fatal error to give extra arguments to a built-in function.

   When a function is called, expressions that create the function's
actual parameters are evaluated completely before the function call is
performed.  For example, in the code fragment:

     i = 4
     j = sqrt(i++)

the variable `i' is set to five before `sqrt' is called with a value of
four for its actual parameter.

   The order of evaluation of the expressions used for the function's
parameters is undefined.  Thus, you should not write programs that
assume that parameters are evaluated from left to right or from right
to left.  For example,

     i = 5
     j = atan2(i++, i *= 2)

   If the order of evaluation is left to right, then `i' first becomes
six, and then 12, and `atan2' is called with the two arguments six and
12.  But if the order of evaluation is right to left, `i' first becomes
10, and then 11, and `atan2' is called with the two arguments 11 and 10.


File: gawk.info,  Node: Numeric Functions,  Next: String Functions,  Prev: Calling Built-in,  Up: Built-in

Numeric Built-in Functions
==========================

   Here is a full list of built-in functions that work with numbers.
Optional parameters are enclosed in square brackets ("[" and "]").

`int(X)'
     This produces the nearest integer to X, located between X and zero,
     truncated toward zero.

     For example, `int(3)' is three, `int(3.9)' is three, `int(-3.9)'
     is -3, and `int(-3)' is -3 as well.

`sqrt(X)'
     This gives you the positive square root of X.  It reports an error
     if X is negative.  Thus, `sqrt(4)' is two.

`exp(X)'
     This gives you the exponential of X (`e ^ X'), or reports an error
     if X is out of range.  The range of values X can have depends on
     your machine's floating point representation.

`log(X)'
     This gives you the natural logarithm of X, if X is positive;
     otherwise, it reports an error.

`sin(X)'
     This gives you the sine of X, with X in radians.

`cos(X)'
     This gives you the cosine of X, with X in radians.

`atan2(Y, X)'
     This gives you the arctangent of `Y / X' in radians.

`rand()'
     This gives you a random number.  The values of `rand' are
     uniformly-distributed between zero and one.  The value is never
     zero and never one.

     Often you want random integers instead.  Here is a user-defined
     function you can use to obtain a random non-negative integer less
     than N:

          function randint(n) {
               return int(n * rand())
          }

     The multiplication produces a random real number greater than zero
     and less than `n'.  We then make it an integer (using `int')
     between zero and `n' - 1, inclusive.

     Here is an example where a similar function is used to produce
     random integers between one and N.  This program prints a new
     random number for each input record.

          awk '
          # Function to roll a simulated die.
          function roll(n) { return 1 + int(rand() * n) }
          
          # Roll 3 six-sided dice and
          # print total number of points.
          {
                printf("%d points\n",
                       roll(6)+roll(6)+roll(6))
          }'

     *Caution:* In most `awk' implementations, including `gawk', `rand'
     starts generating numbers from the same starting number, or
     "seed", each time you run `awk'.  Thus, a program will generate
     the same results each time you run it.  The numbers are random
     within one `awk' run, but predictable from run to run.  This is
     convenient for debugging, but if you want a program to do
     different things each time it is used, you must change the seed to
     a value that will be different in each run.  To do this, use
     `srand'.

`srand([X])'
     The function `srand' sets the starting point, or seed, for
     generating random numbers to the value X.

     Each seed value leads to a particular sequence of random
     numbers.(1) Thus, if you set the seed to the same value a second
     time, you will get the same sequence of random numbers again.

     If you omit the argument X, as in `srand()', then the current date
     and time of day are used for a seed.  This is the way to get random
     numbers that are truly unpredictable.

     The return value of `srand' is the previous seed.  This makes it
     easy to keep track of the seeds for use in consistently reproducing
     sequences of random numbers.

   ---------- Footnotes ----------

   (1)  Computer generated random numbers really are not truly random.
They are technically known as "pseudo-random."  This means that while
the numbers in a sequence appear to be random, you can in fact generate
the same sequence of random numbers over and over again.


File: gawk.info,  Node: String Functions,  Next: I/O Functions,  Prev: Numeric Functions,  Up: Built-in

Built-in Functions for String Manipulation
==========================================

   The functions in this section look at or change the text of one or
more strings.  Optional parameters are enclosed in square brackets ("["
and "]").

`index(IN, FIND)'
     This searches the string IN for the first occurrence of the string
     FIND, and returns the position in characters where that occurrence
     begins in the string IN.  For example:

          $ awk 'BEGIN { print index("peanut", "an") }'
          -| 3

     If FIND is not found, `index' returns zero.  (Remember that string
     indices in `awk' start at one.)

`length([STRING])'
     This gives you the number of characters in STRING.  If STRING is a
     number, the length of the digit string representing that number is
     returned.  For example, `length("abcde")' is five.  By contrast,
     `length(15 * 35)' works out to three.  How?  Well, 15 * 35 = 525,
     and 525 is then converted to the string `"525"', which has three
     characters.

     If no argument is supplied, `length' returns the length of `$0'.

     In older versions of `awk', you could call the `length' function
     without any parentheses.  Doing so is marked as "deprecated" in the
     POSIX standard.  This means that while you can do this in your
     programs, it is a feature that can eventually be removed from a
     future version of the standard.  Therefore, for maximal
     portability of your `awk' programs, you should always supply the
     parentheses.

`match(STRING, REGEXP)'
     The `match' function searches the string, STRING, for the longest,
     leftmost substring matched by the regular expression, REGEXP.  It
     returns the character position, or "index", of where that
     substring begins (one, if it starts at the beginning of STRING).
     If no match is found, it returns zero.

     The `match' function sets the built-in variable `RSTART' to the
     index.  It also sets the built-in variable `RLENGTH' to the length
     in characters of the matched substring.  If no match is found,
     `RSTART' is set to zero, and `RLENGTH' to -1.

     For example:

          awk '{
                 if ($1 == "FIND")
                   regex = $2
                 else {
                   where = match($0, regex)
                   if (where != 0)
                     print "Match of", regex, "found at", \
                               where, "in", $0
                 }
          }'

     This program looks for lines that match the regular expression
     stored in the variable `regex'.  This regular expression can be
     changed.  If the first word on a line is `FIND', `regex' is
     changed to be the second word on that line.  Therefore, given:

          FIND ru+n
          My program runs
          but not very quickly
          FIND Melvin
          JF+KM
          This line is property of Reality Engineering Co.
          Melvin was here.

     `awk' prints:

          Match of ru+n found at 12 in My program runs
          Match of Melvin found at 1 in Melvin was here.

`split(STRING, ARRAY [, FIELDSEP])'
     This divides STRING into pieces separated by FIELDSEP, and stores
     the pieces in ARRAY.  The first piece is stored in `ARRAY[1]', the
     second piece in `ARRAY[2]', and so forth.  The string value of the
     third argument, FIELDSEP, is a regexp describing where to split
     STRING (much as `FS' can be a regexp describing where to split
     input records).  If the FIELDSEP is omitted, the value of `FS' is
     used.  `split' returns the number of elements created.

     The `split' function splits strings into pieces in a manner
     similar to the way input lines are split into fields.  For example:

          split("cul-de-sac", a, "-")

     splits the string `cul-de-sac' into three fields using `-' as the
     separator.  It sets the contents of the array `a' as follows:

          a[1] = "cul"
          a[2] = "de"
          a[3] = "sac"

     The value returned by this call to `split' is three.

     As with input field-splitting, when the value of FIELDSEP is
     `" "', leading and trailing whitespace is ignored, and the elements
     are separated by runs of whitespace.

     Also as with input field-splitting, if FIELDSEP is the null
     string, each individual character in the string is split into its
     own array element.  (This is a `gawk'-specific extension.)

     Recent implementations of `awk', including `gawk', allow the third
     argument to be a regexp constant (`/abc/'), as well as a string
     (d.c.).  The POSIX standard allows this as well.

     Before splitting the string, `split' deletes any previously
     existing elements in the array ARRAY (d.c.).

`sprintf(FORMAT, EXPRESSION1,...)'
     This returns (without printing) the string that `printf' would
     have printed out with the same arguments (*note Using `printf'
     Statements for Fancier Printing: Printf.).  For example:

          sprintf("pi = %.2f (approx.)", 22/7)

     returns the string `"pi = 3.14 (approx.)"'.

`sub(REGEXP, REPLACEMENT [, TARGET])'
     The `sub' function alters the value of TARGET.  It searches this
     value, which is treated as a string, for the leftmost longest
     substring matched by the regular expression, REGEXP, extending
     this match as far as possible.  Then the entire string is changed
     by replacing the matched text with REPLACEMENT.  The modified
     string becomes the new value of TARGET.

     This function is peculiar because TARGET is not simply used to
     compute a value, and not just any expression will do: it must be a
     variable, field or array element, so that `sub' can store a
     modified value there.  If this argument is omitted, then the
     default is to use and alter `$0'.

     For example:

          str = "water, water, everywhere"
          sub(/at/, "ith", str)

     sets `str' to `"wither, water, everywhere"', by replacing the
     leftmost, longest occurrence of `at' with `ith'.

     The `sub' function returns the number of substitutions made (either
     one or zero).

     If the special character `&' appears in REPLACEMENT, it stands for
     the precise substring that was matched by REGEXP.  (If the regexp
     can match more than one string, then this precise substring may
     vary.)  For example:

          awk '{ sub(/candidate/, "& and his wife"); print }'

     changes the first occurrence of `candidate' to `candidate and his
     wife' on each input line.

     Here is another example:

          awk 'BEGIN {
                  str = "daabaaa"
                  sub(/a*/, "c&c", str)
                  print str
          }'
          -| dcaacbaaa

     This shows how `&' can represent a non-constant string, and also
     illustrates the "leftmost, longest" rule in regexp matching (*note
     How Much Text Matches?: Leftmost Longest.).

     The effect of this special character (`&') can be turned off by
     putting a backslash before it in the string.  As usual, to insert
     one backslash in the string, you must write two backslashes.
     Therefore, write `\\&' in a string constant to include a literal
     `&' in the replacement.  For example, here is how to replace the
     first `|' on each line with an `&':

          awk '{ sub(/\|/, "\\&"); print }'

     *Note:* As mentioned above, the third argument to `sub' must be a
     variable, field or array reference.  Some versions of `awk' allow
     the third argument to be an expression which is not an lvalue.  In
     such a case, `sub' would still search for the pattern and return
     zero or one, but the result of the substitution (if any) would be
     thrown away because there is no place to put it.  Such versions of
     `awk' accept expressions like this:

          sub(/USA/, "United States", "the USA and Canada")

     For historical compatibility, `gawk' will accept erroneous code,
     such as in the above example. However, using any other
     non-changeable object as the third parameter will cause a fatal
     error, and your program will not run.

`gsub(REGEXP, REPLACEMENT [, TARGET])'
     This is similar to the `sub' function, except `gsub' replaces
     *all* of the longest, leftmost, *non-overlapping* matching
     substrings it can find.  The `g' in `gsub' stands for "global,"
     which means replace everywhere.  For example:

          awk '{ gsub(/Britain/, "United Kingdom"); print }'

     replaces all occurrences of the string `Britain' with `United
     Kingdom' for all input records.

     The `gsub' function returns the number of substitutions made.  If
     the variable to be searched and altered, TARGET, is omitted, then
     the entire input record, `$0', is used.

     As in `sub', the characters `&' and `\' are special, and the third
     argument must be an lvalue.

`gensub(REGEXP, REPLACEMENT, HOW [, TARGET])'
     `gensub' is a general substitution function.  Like `sub' and
     `gsub', it searches the target string TARGET for matches of the
     regular expression REGEXP.  Unlike `sub' and `gsub', the modified
     string is returned as the result of the function, and the original
     target string is *not* changed.  If HOW is a string beginning with
     `g' or `G', then it replaces all matches of REGEXP with
     REPLACEMENT.  Otherwise, HOW is a number indicating which match of
     REGEXP to replace. If no TARGET is supplied, `$0' is used instead.

     `gensub' provides an additional feature that is not available in
     `sub' or `gsub': the ability to specify components of a regexp in
     the replacement text.  This is done by using parentheses in the
     regexp to mark the components, and then specifying `\N' in the
     replacement text, where N is a digit from one to nine.  For
     example:

          $ gawk '
          > BEGIN {
          >      a = "abc def"
          >      b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)
          >      print b
          > }'
          -| def abc

     As described above for `sub', you must type two backslashes in
     order to get one into the string.

     In the replacement text, the sequence `\0' represents the entire
     matched text, as does the character `&'.

     This example shows how you can use the third argument to control
     which match of the regexp should be changed.

          $ echo a b c a b c |
          > gawk '{ print gensub(/a/, "AA", 2) }'
          -| a b c AA b c

     In this case, `$0' is used as the default target string.  `gensub'
     returns the new string as its result, which is passed directly to
     `print' for printing.

     If the HOW argument is a string that does not begin with `g' or
     `G', or if it is a number that is less than zero, only one
     substitution is performed.

     `gensub' is a `gawk' extension; it is not available in
     compatibility mode (*note Command Line Options: Options.).

`substr(STRING, START [, LENGTH])'
     This returns a LENGTH-character-long substring of STRING, starting
     at character number START.  The first character of a string is
     character number one.  For example, `substr("washington", 5, 3)'
     returns `"ing"'.

     If LENGTH is not present, this function returns the whole suffix of
     STRING that begins at character number START.  For example,
     `substr("washington", 5)' returns `"ington"'.  The whole suffix is
     also returned if LENGTH is greater than the number of characters
     remaining in the string, counting from character number START.

     *Note:* The string returned by `substr' *cannot* be assigned to.
     Thus, it is a mistake to attempt to change a portion of a string,
     like this:

          string = "abcdef"
          # try to get "abCDEf", won't work
          substr(string, 3, 3) = "CDE"

     or to use `substr' as the third agument of `sub' or `gsub':

          gsub(/xyz/, "pdq", substr($0, 5, 20))  # WRONG

`tolower(STRING)'
     This returns a copy of STRING, with each upper-case character in
     the string replaced with its corresponding lower-case character.
     Non-alphabetic characters are left unchanged.  For example,
     `tolower("MiXeD cAsE 123")' returns `"mixed case 123"'.

`toupper(STRING)'
     This returns a copy of STRING, with each lower-case character in
     the string replaced with its corresponding upper-case character.
     Non-alphabetic characters are left unchanged.  For example,
     `toupper("MiXeD cAsE 123")' returns `"MIXED CASE 123"'.

More About `\' and `&' with `sub', `gsub' and `gensub'
------------------------------------------------------

   When using `sub', `gsub' or `gensub', and trying to get literal
backslashes and ampersands into the replacement text, you need to
remember that there are several levels of "escape processing" going on.

   First, there is the "lexical" level, which is when `awk' reads your
program, and builds an internal copy of your program that can be
executed.

   Then there is the run-time level, when `awk' actually scans the
replacement string to determine what to generate.

   At both levels, `awk' looks for a defined set of characters that can
come after a backslash.  At the lexical level, it looks for the escape
sequences listed in *Note Escape Sequences::.  Thus, for every `\' that
`awk' will process at the run-time level, you type two `\'s at the
lexical level.  When a character that is not valid for an escape
sequence follows the `\', Unix `awk' and `gawk' both simply remove the
initial `\', and put the following character into the string. Thus, for
example, `"a\qb"' is treated as `"aqb"'.

   At the run-time level, the various functions handle sequences of `\'
and `&' differently.  The situation is (sadly) somewhat complex.

   Historically, the `sub' and `gsub' functions treated the two
character sequence `\&' specially; this sequence was replaced in the
generated text with a single `&'.  Any other `\' within the REPLACEMENT
string that did not precede an `&' was passed through unchanged.  To
illustrate with a table:

      You type         `sub' sees          `sub' generates
      --------         ----------          ---------------
          `\&'              `&'            the matched text
         `\\&'             `\&'            a literal `&'
        `\\\&'             `\&'            a literal `&'
       `\\\\&'            `\\&'            a literal `\&'
      `\\\\\&'            `\\&'            a literal `\&'
     `\\\\\\&'           `\\\&'            a literal `\\&'
         `\\q'             `\q'            a literal `\q'

This table shows both the lexical level processing, where an odd number
of backslashes becomes an even number at the run time level, and the
run-time processing done by `sub'.  (For the sake of simplicity, the
rest of the tables below only show the case of even numbers of `\'s
entered at the lexical level.)

   The problem with the historical approach is that there is no way to
get a literal `\' followed by the matched text.

   The 1992 POSIX standard attempted to fix this problem. The standard
says that `sub' and `gsub' look for either a `\' or an `&' after the
`\'. If either one follows a `\', that character is output literally.
The interpretation of `\' and `&' then becomes like this:

      You type         `sub' sees          `sub' generates
      --------         ----------          ---------------
           `&'              `&'            the matched text
         `\\&'             `\&'            a literal `&'
       `\\\\&'            `\\&'            a literal `\', then the matched text
     `\\\\\\&'           `\\\&'            a literal `\&'

This would appear to solve the problem.  Unfortunately, the phrasing of
the standard is unusual. It says, in effect, that `\' turns off the
special meaning of any following character, but that for anything other
than `\' and `&', such special meaning is undefined.  This wording
leads to two problems.

  1. Backslashes must now be doubled in the REPLACEMENT string, breaking
     historical `awk' programs.

  2. To make sure that an `awk' program is portable, *every* character
     in the REPLACEMENT string must be preceded with a backslash.(1)

   The POSIX standard is under revision.(2) Because of the above
problems, proposed text for the revised standard reverts to rules that
correspond more closely to the original existing practice. The proposed
rules have special cases that make it possible to produce a `\'
preceding the matched text.

      You type         `sub' sees         `sub' generates
      --------         ----------         ---------------
     `\\\\\\&'           `\\\&'            a literal `\&'
       `\\\\&'            `\\&'            a literal `\', followed by the matched text
         `\\&'             `\&'            a literal `&'
         `\\q'             `\q'            a literal `\q'

   In a nutshell, at the run-time level, there are now three special
sequences of characters, `\\\&', `\\&' and `\&', whereas historically,
there was only one.  However, as in the historical case, any `\' that
is not part of one of these three sequences is not special, and appears
in the output literally.

   `gawk' 3.0 follows these proposed POSIX rules for `sub' and `gsub'.
Whether these proposed rules will actually become codified into the
standard is unknown at this point. Subsequent `gawk' releases will
track the standard and implement whatever the final version specifies;
this Info file will be updated as well.

   The rules for `gensub' are considerably simpler. At the run-time
level, whenever `gawk' sees a `\', if the following character is a
digit, then the text that matched the corresponding parenthesized
subexpression is placed in the generated output.  Otherwise, no matter
what the character after the `\' is, that character will appear in the
generated text, and the `\' will not.

       You type          `gensub' sees         `gensub' generates
       --------          -------------         ------------------
           `&'                    `&'            the matched text
         `\\&'                   `\&'            a literal `&'
        `\\\\'                   `\\'            a literal `\'
       `\\\\&'                  `\\&'            a literal `\', then the matched text
     `\\\\\\&'                 `\\\&'            a literal `\&'
         `\\q'                   `\q'            a literal `q'

   Because of the complexity of the lexical and run-time level
processing, and the special cases for `sub' and `gsub', we recommend
the use of `gawk' and `gensub' for when you have to do substitutions.

   ---------- Footnotes ----------

   (1)  This consequence was certainly unintended.

   (2)  As of December 1995, with final approval and publication
hopefully sometime in 1996.


File: gawk.info,  Node: I/O Functions,  Next: Time Functions,  Prev: String Functions,  Up: Built-in

Built-in Functions for Input/Output
===================================

   The following functions are related to Input/Output (I/O).  Optional
parameters are enclosed in square brackets ("[" and "]").

`close(FILENAME)'
     Close the file FILENAME, for input or output.  The argument may
     alternatively be a shell command that was used for redirecting to
     or from a pipe; then the pipe is closed.  *Note Closing Input and
     Output Files and Pipes: Close Files And Pipes, for more
     information.

`fflush([FILENAME])'
     Flush any buffered output associated FILENAME, which is either a
     file opened for writing, or a shell command for redirecting output
     to a pipe.

     Many utility programs will "buffer" their output; they save
     information to be written to a disk file or terminal in memory,
     until there is enough for it to be worthwhile to send the data to
     the ouput device.  This is often more efficient than writing every
     little bit of information as soon as it is ready.  However,
     sometimes it is necessary to force a program to "flush" its
     buffers; that is, write the information to its destination, even
     if a buffer is not full.  This is the purpose of the `fflush'
     function; `gawk' too buffers its output, and the `fflush' function
     can be used to force `gawk' to flush its buffers.

     `fflush' is a recent (1994) addition to the Bell Labs research
     version of `awk'; it is not part of the POSIX standard, and will
     not be available if `--posix' has been specified on the command
     line (*note Command Line Options: Options.).

     `gawk' extends the `fflush' function in two ways.  The first is to
     allow no argument at all. In this case, the buffer for the
     standard output is flushed.  The second way is to allow the null
     string (`""') as the argument. In this case, the buffers for *all*
     open output files and pipes are flushed.

     `fflush' returns zero if the buffer was successfully flushed, and
     nonzero otherwise.

`system(COMMAND)'
     The system function allows the user to execute operating system
     commands and then return to the `awk' program.  The `system'
     function executes the command given by the string COMMAND.  It
     returns, as its value, the status returned by the command that was
     executed.

     For example, if the following fragment of code is put in your `awk'
     program:

          END {
               system("date | mail -s 'awk run done' root")
          }

     the system administrator will be sent mail when the `awk' program
     finishes processing input and begins its end-of-input processing.

     Note that redirecting `print' or `printf' into a pipe is often
     enough to accomplish your task.  However, if your `awk' program is
     interactive, `system' is useful for cranking up large
     self-contained programs, such as a shell or an editor.

     Some operating systems cannot implement the `system' function.
     `system' causes a fatal error if it is not supported.

Interactive vs. Non-Interactive Buffering
-----------------------------------------

   As a side point, buffering issues can be even more confusing
depending upon whether or not your program is "interactive", i.e.,
communicating with a user sitting at a keyboard.(1)

   Interactive programs generally "line buffer" their output; they
write out every line.  Non-interactive programs wait until they have a
full buffer, which may be many lines of output.

   Here is an example of the difference.

     $ awk '{ print $1 + $2 }'
     1 1
     -| 2
     2 3
     -| 5
     Control-d

Each line of output is printed immediately. Compare that behavior with
this example.

     $ awk '{ print $1 + $2 }' | cat
     1 1
     2 3
     Control-d
     -| 2
     -| 5

Here, no output is printed until after the `Control-d' is typed, since
it is all buffered, and sent down the pipe to `cat' in one shot.

Controlling Output Buffering with `system'
------------------------------------------

   The `fflush' function provides explicit control over output
buffering for individual files and pipes.  However, its use is not
portable to many other `awk' implementations.  An alternative method to
flush output buffers is by calling `system' with a null string as its
argument:

     system("")   # flush output

`gawk' treats this use of the `system' function as a special case, and
is smart enough not to run a shell (or other command interpreter) with
the empty command.  Therefore, with `gawk', this idiom is not only
useful, it is efficient.  While this method should work with other
`awk' implementations, it will not necessarily avoid starting an
unnecessary shell.  (Other implementations may only flush the buffer
associated with the standard output, and not necessarily all buffered
output.)

   If you think about what a programmer expects, it makes sense that
`system' should flush any pending output.  The following program:

     BEGIN {
          print "first print"
          system("echo system echo")
          print "second print"
     }

must print

     first print
     system echo
     second print

and not

     system echo
     first print
     second print

   If `awk' did not flush its buffers before calling `system', the
latter (undesirable) output is what you would see.

   ---------- Footnotes ----------

   (1)  A program is interactive if the standard output is connected to
a terminal device.


File: gawk.info,  Node: Time Functions,  Prev: I/O Functions,  Up: Built-in

Functions for Dealing with Time Stamps
======================================

   A common use for `awk' programs is the processing of log files
containing time stamp information, indicating when a particular log
record was written.  Many programs log their time stamp in the form
returned by the `time' system call, which is the number of seconds
since a particular epoch.  On POSIX systems, it is the number of
seconds since Midnight, January 1, 1970, UTC.

   In order to make it easier to process such log files, and to produce
useful reports, `gawk' provides two functions for working with time
stamps.  Both of these are `gawk' extensions; they are not specified in
the POSIX standard, nor are they in any other known version of `awk'.

   Optional parameters are enclosed in square brackets ("[" and "]").

`systime()'
     This function returns the current time as the number of seconds
     since the system epoch.  On POSIX systems, this is the number of
     seconds since Midnight, January 1, 1970, UTC.  It may be a
     different number on other systems.

`strftime([FORMAT [, TIMESTAMP]])'
     This function returns a string.  It is similar to the function of
     the same name in ANSI C.  The time specified by TIMESTAMP is used
     to produce a string, based on the contents of the FORMAT string.
     The TIMESTAMP is in the same format as the value returned by the
     `systime' function.  If no TIMESTAMP argument is supplied, `gawk'
     will use the current time of day as the time stamp.  If no FORMAT
     argument is supplied, `strftime' uses `"%a %b %d %H:%M:%S %Z %Y"'.
     This format string produces output (almost) equivalent to that of
     the `date' utility.  (Versions of `gawk' prior to 3.0 require the
     FORMAT argument.)

   The `systime' function allows you to compare a time stamp from a log
file with the current time of day.  In particular, it is easy to
determine how long ago a particular record was logged.  It also allows
you to produce log records using the "seconds since the epoch" format.

   The `strftime' function allows you to easily turn a time stamp into
human-readable information.  It is similar in nature to the `sprintf'
function (*note Built-in Functions for String Manipulation: String
Functions.), in that it copies non-format specification characters
verbatim to the returned string, while substituting date and time
values for format specifications in the FORMAT string.

   `strftime' is guaranteed by the ANSI C standard to support the
following date format specifications:

`%a'
     The locale's abbreviated weekday name.

`%A'
     The locale's full weekday name.

`%b'
     The locale's abbreviated month name.

`%B'
     The locale's full month name.

`%c'
     The locale's "appropriate" date and time representation.

`%d'
     The day of the month as a decimal number (01-31).

`%H'
     The hour (24-hour clock) as a decimal number (00-23).

`%I'
     The hour (12-hour clock) as a decimal number (01-12).

`%j'
     The day of the year as a decimal number (001-366).

`%m'
     The month as a decimal number (01-12).

`%M'
     The minute as a decimal number (00-59).

`%p'
     The locale's equivalent of the AM/PM designations associated with
     a 12-hour clock.

`%S'
     The second as a decimal number (00-60).(1)

`%U'
     The week number of the year (the first Sunday as the first day of
     week one) as a decimal number (00-53).

`%w'
     The weekday as a decimal number (0-6).  Sunday is day zero.

`%W'
     The week number of the year (the first Monday as the first day of
     week one) as a decimal number (00-53).

`%x'
     The locale's "appropriate" date representation.

`%X'
     The locale's "appropriate" time representation.

`%y'
     The year without century as a decimal number (00-99).

`%Y'
     The year with century as a decimal number (e.g., 1995).

`%Z'
     The time zone name or abbreviation, or no characters if no time
     zone is determinable.

`%%'
     A literal `%'.

   If a conversion specifier is not one of the above, the behavior is
undefined.(2)

   Informally, a "locale" is the geographic place in which a program is
meant to run.  For example, a common way to abbreviate the date
September 4, 1991 in the United States would be "9/4/91".  In many
countries in Europe, however, it would be abbreviated "4.9.91".  Thus,
the `%x' specification in a `"US"' locale might produce `9/4/91', while
in a `"EUROPE"' locale, it might produce `4.9.91'.  The ANSI C standard
defines a default `"C"' locale, which is an environment that is typical
of what most C programmers are used to.

   A public-domain C version of `strftime' is supplied with `gawk' for
systems that are not yet fully ANSI-compliant.  If that version is used
to compile `gawk' (*note Installing `gawk': Installation.), then the
following additional format specifications are available:

`%D'
     Equivalent to specifying `%m/%d/%y'.

`%e'
     The day of the month, padded with a space if it is only one digit.

`%h'
     Equivalent to `%b', above.

`%n'
     A newline character (ASCII LF).

`%r'
     Equivalent to specifying `%I:%M:%S %p'.

`%R'
     Equivalent to specifying `%H:%M'.

`%T'
     Equivalent to specifying `%H:%M:%S'.

`%t'
     A tab character.

`%k'
     The hour (24-hour clock) as a decimal number (0-23).  Single digit
     numbers are padded with a space.

`%l'
     The hour (12-hour clock) as a decimal number (1-12).  Single digit
     numbers are padded with a space.

`%C'
     The century, as a number between 00 and 99.

`%u'
     The weekday as a decimal number [1 (Monday)-7].

`%V'
     The week number of the year (the first Monday as the first day of
     week one) as a decimal number (01-53).  The method for determining
     the week number is as specified by ISO 8601 (to wit: if the week
     containing January 1 has four or more days in the new year, then
     it is week one, otherwise it is week 53 of the previous year and
     the next week is week one).

`%G'
     The year with century of the ISO week number, as a decimal number.

     For example, January 1, 1993, is in week 53 of 1992. Thus, the year
     of its ISO week number is 1992, even though its year is 1993.
     Similarly, December 31, 1973, is in week 1 of 1974. Thus, the year
     of its ISO week number is 1974, even though its year is 1973.

`%g'
     The year without century of the ISO week number, as a decimal
     number (00-99).

`%Ec %EC %Ex %Ey %EY %Od %Oe %OH %OI'
`%Om %OM %OS %Ou %OU %OV %Ow %OW %Oy'
     These are "alternate representations" for the specifications that
     use only the second letter (`%c', `%C', and so on).  They are
     recognized, but their normal representations are used.(3) (These
     facilitate compliance with the POSIX `date' utility.)

`%v'
     The date in VMS format (e.g., 20-JUN-1991).

`%z'
     The timezone offset in a +HHMM format (e.g., the format necessary
     to produce RFC-822/RFC-1036 date headers).

   This example is an `awk' implementation of the POSIX `date' utility.
Normally, the `date' utility prints the current date and time of day
in a well known format.  However, if you provide an argument to it that
begins with a `+', `date' will copy non-format specifier characters to
the standard output, and will interpret the current time according to
the format specifiers in the string.  For example:

     $ date '+Today is %A, %B %d, %Y.'
     -| Today is Thursday, July 11, 1991.

   Here is the `gawk' version of the `date' utility.  It has a shell
"wrapper", to handle the `-u' option, which requires that `date' run as
if the time zone was set to UTC.

     #! /bin/sh
     #
     # date --- approximate the P1003.2 'date' command
     
     case $1 in
     -u)  TZ=GMT0     # use UTC
          export TZ
          shift ;;
     esac
     
     gawk 'BEGIN  {
         format = "%a %b %d %H:%M:%S %Z %Y"
         exitval = 0
     
         if (ARGC > 2)
             exitval = 1
         else if (ARGC == 2) {
             format = ARGV[1]
             if (format ~ /^\+/)
                 format = substr(format, 2)   # remove leading +
         }
         print strftime(format)
         exit exitval
     }' "$@"

   ---------- Footnotes ----------

   (1)  Occasionally there are minutes in a year with a leap second,
which is why the seconds can go up to 60.

   (2)  This is because ANSI C leaves the behavior of the C version of
`strftime' undefined, and `gawk' will use the system's version of
`strftime' if it's there.  Typically, the conversion specifier will
either not appear in the returned string, or it will appear literally.

   (3)  If you don't understand any of this, don't worry about it;
these facilities are meant to make it easier to "internationalize"
programs.


File: gawk.info,  Node: User-defined,  Next: Invoking Gawk,  Prev: Built-in,  Up: Top

User-defined Functions
**********************

   Complicated `awk' programs can often be simplified by defining your
own functions.  User-defined functions can be called just like built-in
ones (*note Function Calls::.), but it is up to you to define them--to
tell `awk' what they should do.

* Menu:

* Definition Syntax::           How to write definitions and what they mean.
* Function Example::            An example function definition and what it
                                does.
* Function Caveats::            Things to watch out for.
* Return Statement::            Specifying the value a function returns.


File: gawk.info,  Node: Definition Syntax,  Next: Function Example,  Prev: User-defined,  Up: User-defined

Function Definition Syntax
==========================

   Definitions of functions can appear anywhere between the rules of an
`awk' program.  Thus, the general form of an `awk' program is extended
to include sequences of rules *and* user-defined function definitions.
There is no need in `awk' to put the definition of a function before
all uses of the function.  This is because `awk' reads the entire
program before starting to execute any of it.

   The definition of a function named NAME looks like this:

     function NAME(PARAMETER-LIST)
     {
          BODY-OF-FUNCTION
     }

NAME is the name of the function to be defined.  A valid function name
is like a valid variable name: a sequence of letters, digits and
underscores, not starting with a digit.  Within a single `awk' program,
any particular name can only be used as a variable, array or function.

   PARAMETER-LIST is a list of the function's arguments and local
variable names, separated by commas.  When the function is called, the
argument names are used to hold the argument values given in the call.
The local variables are initialized to the empty string.  A function
cannot have two parameters with the same name.

   The BODY-OF-FUNCTION consists of `awk' statements.  It is the most
important part of the definition, because it says what the function
should actually *do*.  The argument names exist to give the body a way
to talk about the arguments; local variables, to give the body places
to keep temporary values.

   Argument names are not distinguished syntactically from local
variable names; instead, the number of arguments supplied when the
function is called determines how many argument variables there are.
Thus, if three argument values are given, the first three names in
PARAMETER-LIST are arguments, and the rest are local variables.

   It follows that if the number of arguments is not the same in all
calls to the function, some of the names in PARAMETER-LIST may be
arguments on some occasions and local variables on others.  Another way
to think of this is that omitted arguments default to the null string.

   Usually when you write a function you know how many names you intend
to use for arguments and how many you intend to use as local variables.
It is conventional to place some extra space between the arguments and
the local variables, to document how your function is supposed to be
used.

   During execution of the function body, the arguments and local
variable values hide or "shadow" any variables of the same names used
in the rest of the program.  The shadowed variables are not accessible
in the function definition, because there is no way to name them while
their names have been taken away for the local variables.  All other
variables used in the `awk' program can be referenced or set normally
in the function's body.

   The arguments and local variables last only as long as the function
body is executing.  Once the body finishes, you can once again access
the variables that were shadowed while the function was running.

   The function body can contain expressions which call functions.  They
can even call this function, either directly or by way of another
function.  When this happens, we say the function is "recursive".

   In many `awk' implementations, including `gawk', the keyword
`function' may be abbreviated `func'.  However, POSIX only specifies
the use of the keyword `function'.  This actually has some practical
implications.  If `gawk' is in POSIX-compatibility mode (*note Command
Line Options: Options.), then the following statement will *not* define
a function:

     func foo() { a = sqrt($1) ; print a }

Instead it defines a rule that, for each record, concatenates the value
of the variable `func' with the return value of the function `foo'.  If
the resulting string is non-null, the action is executed.  This is
probably not what was desired.  (`awk' accepts this input as
syntactically valid, since functions may be used before they are defined
in `awk' programs.)

   To ensure that your `awk' programs are portable, always use the
keyword `function' when defining a function.


File: gawk.info,  Node: Function Example,  Next: Function Caveats,  Prev: Definition Syntax,  Up: User-defined

Function Definition Examples
============================

   Here is an example of a user-defined function, called `myprint', that
takes a number and prints it in a specific format.

     function myprint(num)
     {
          printf "%6.3g\n", num
     }

To illustrate, here is an `awk' rule which uses our `myprint' function:

     $3 > 0     { myprint($3) }

This program prints, in our special format, all the third fields that
contain a positive number in our input.  Therefore, when given:

      1.2   3.4    5.6   7.8
      9.10 11.12 -13.14 15.16
     17.18 19.20  21.22 23.24

this program, using our function to format the results, prints:

        5.6
       21.2

   This function deletes all the elements in an array.

     function delarray(a,    i)
     {
         for (i in a)
            delete a[i]
     }

   When working with arrays, it is often necessary to delete all the
elements in an array and start over with a new list of elements (*note
The `delete' Statement: Delete.).  Instead of having to repeat this
loop everywhere in your program that you need to clear out an array,
your program can just call `delarray'.

   Here is an example of a recursive function.  It takes a string as an
input parameter, and returns the string in backwards order.

     function rev(str, start)
     {
         if (start == 0)
             return ""
     
         return (substr(str, start, 1) rev(str, start - 1))
     }

   If this function is in a file named `rev.awk', we can test it this
way:

     $ echo "Don't Panic!" |
     > gawk --source '{ print rev($0, length($0)) }' -f rev.awk
     -| !cinaP t'noD

   Here is an example that uses the built-in function `strftime'.
(*Note Functions for Dealing with Time Stamps: Time Functions, for more
information on `strftime'.) The C `ctime' function takes a timestamp
and returns it in a string, formatted in a well known fashion.  Here is
an `awk' version:

     # ctime.awk
     #
     # awk version of C ctime(3) function
     
     function ctime(ts,    format)
     {
         format = "%a %b %d %H:%M:%S %Z %Y"
         if (ts == 0)
             ts = systime()       # use current time as default
         return strftime(format, ts)
     }


File: gawk.info,  Node: Function Caveats,  Next: Return Statement,  Prev: Function Example,  Up: User-defined

Calling User-defined Functions
==============================

   "Calling a function" means causing the function to run and do its
job.  A function call is an expression, and its value is the value
returned by the function.

   A function call consists of the function name followed by the
arguments in parentheses.  What you write in the call for the arguments
are `awk' expressions; each time the call is executed, these
expressions are evaluated, and the values are the actual arguments.  For
example, here is a call to `foo' with three arguments (the first being
a string concatenation):

     foo(x y, "lose", 4 * z)

   *Caution:* whitespace characters (spaces and tabs) are not allowed
between the function name and the open-parenthesis of the argument list.
If you write whitespace by mistake, `awk' might think that you mean to
concatenate a variable with an expression in parentheses.  However, it
notices that you used a function name and not a variable name, and
reports an error.

   When a function is called, it is given a *copy* of the values of its
arguments.  This is known as "call by value".  The caller may use a
variable as the expression for the argument, but the called function
does not know this: it only knows what value the argument had.  For
example, if you write this code:

     foo = "bar"
     z = myfunc(foo)

then you should not think of the argument to `myfunc' as being "the
variable `foo'."  Instead, think of the argument as the string value,
`"bar"'.

   If the function `myfunc' alters the values of its local variables,
this has no effect on any other variables.  Thus, if `myfunc' does this:

     function myfunc(str)
     {
       print str
       str = "zzz"
       print str
     }

to change its first argument variable `str', this *does not* change the
value of `foo' in the caller.  The role of `foo' in calling `myfunc'
ended when its value, `"bar"', was computed.  If `str' also exists
outside of `myfunc', the function body cannot alter this outer value,
because it is shadowed during the execution of `myfunc' and cannot be
seen or changed from there.

   However, when arrays are the parameters to functions, they are *not*
copied.  Instead, the array itself is made available for direct
manipulation by the function.  This is usually called "call by
reference".  Changes made to an array parameter inside the body of a
function *are* visible outside that function.  This can be *very*
dangerous if you do not watch what you are doing.  For example:

     function changeit(array, ind, nvalue)
     {
          array[ind] = nvalue
     }
     
     BEGIN {
         a[1] = 1; a[2] = 2; a[3] = 3
         changeit(a, 2, "two")
         printf "a[1] = %s, a[2] = %s, a[3] = %s\n",
                 a[1], a[2], a[3]
     }

This program prints `a[1] = 1, a[2] = two, a[3] = 3', because
`changeit' stores `"two"' in the second element of `a'.

   Some `awk' implementations allow you to call a function that has not
been defined, and only report a problem at run-time when the program
actually tries to call the function. For example:

     BEGIN {
         if (0)
             foo()
         else
             bar()
     }
     function bar() { ... }
     # note that `foo' is not defined

Since the `if' statement will never be true, it is not really a problem
that `foo' has not been defined.  Usually though, it is a problem if a
program calls an undefined function.

   If `--lint' has been specified (*note Command Line Options:
Options.), `gawk' will report about calls to undefined functions.

   Some `awk' implementations generate a run-time error if you use the
`next' statement (*note The `next' Statement: Next Statement.) inside a
user-defined function.  `gawk' does not have this problem.


File: gawk.info,  Node: Return Statement,  Prev: Function Caveats,  Up: User-defined

The `return' Statement
======================

   The body of a user-defined function can contain a `return' statement.
This statement returns control to the rest of the `awk' program.  It
can also be used to return a value for use in the rest of the `awk'
program.  It looks like this:

     return [EXPRESSION]

   The EXPRESSION part is optional.  If it is omitted, then the returned
value is undefined and, therefore, unpredictable.

   A `return' statement with no value expression is assumed at the end
of every function definition.  So if control reaches the end of the
function body, then the function returns an unpredictable value.  `awk'
will *not* warn you if you use the return value of such a function.

   Sometimes, you want to write a function for what it does, not for
what it returns.  Such a function corresponds to a `void' function in C
or to a `procedure' in Pascal.  Thus, it may be appropriate to not
return any value; you should simply bear in mind that if you use the
return value of such a function, you do so at your own risk.

   Here is an example of a user-defined function that returns a value
for the largest number among the elements of an array:

     function maxelt(vec,   i, ret)
     {
          for (i in vec) {
               if (ret == "" || vec[i] > ret)
                    ret = vec[i]
          }
          return ret
     }

You call `maxelt' with one argument, which is an array name.  The local
variables `i' and `ret' are not intended to be arguments; while there
is nothing to stop you from passing two or three arguments to `maxelt',
the results would be strange.  The extra space before `i' in the
function parameter list indicates that `i' and `ret' are not supposed
to be arguments.  This is a convention that you should follow when you
define functions.

   Here is a program that uses our `maxelt' function.  It loads an
array, calls `maxelt', and then reports the maximum number in that
array:

     awk '
     function maxelt(vec,   i, ret)
     {
          for (i in vec) {
               if (ret == "" || vec[i] > ret)
                    ret = vec[i]
          }
          return ret
     }
     
     # Load all fields of each record into nums.
     {
          for(i = 1; i <= NF; i++)
               nums[NR, i] = $i
     }
     
     END {
          print maxelt(nums)
     }'

   Given the following input:

      1 5 23 8 16
     44 3 5 2 8 26
     256 291 1396 2962 100
     -6 467 998 1101
     99385 11 0 225

our program tells us (predictably) that `99385' is the largest number
in our array.


File: gawk.info,  Node: Invoking Gawk,  Next: Library Functions,  Prev: User-defined,  Up: Top

Running `awk'
*************

   There are two ways to run `awk': with an explicit program, or with
one or more program files.  Here are templates for both of them; items
enclosed in `[...]' in these templates are optional.

   Besides traditional one-letter POSIX-style options, `gawk' also
supports GNU long options.

     awk [OPTIONS] -f progfile [`--'] FILE ...
     awk [OPTIONS] [`--'] 'PROGRAM' FILE ...

   It is possible to invoke `awk' with an empty program:

     $ awk '' datafile1 datafile2

Doing so makes little sense though; `awk' will simply exit silently
when given an empty program (d.c.).  If `--lint' has been specified on
the command line, `gawk' will issue a warning that the program is empty.

* Menu:

* Options::                     Command line options and their meanings.
* Other Arguments::             Input file names and variable assignments.
* AWKPATH Variable::            Searching directories for `awk' programs.
* Obsolete::                    Obsolete Options and/or features.
* Undocumented::                Undocumented Options and Features.
* Known Bugs::                  Known Bugs in `gawk'.


File: gawk.info,  Node: Options,  Next: Other Arguments,  Prev: Invoking Gawk,  Up: Invoking Gawk

Command Line Options
====================

   Options begin with a dash, and consist of a single character.  GNU
style long options consist of two dashes and a keyword.  The keyword
can be abbreviated, as long the abbreviation allows the option to be
uniquely identified.  If the option takes an argument, then the keyword
is either immediately followed by an equals sign (`=') and the
argument's value, or the keyword and the argument's value are separated
by whitespace.  For brevity, the discussion below only refers to the
traditional short options; however the long and short options are
interchangeable in all contexts.

   Each long option for `gawk' has a corresponding POSIX-style option.
The options and their meanings are as follows:

`-F FS'
`--field-separator FS'
     Sets the `FS' variable to FS (*note Specifying How Fields are
     Separated: Field Separators.).

`-f SOURCE-FILE'
`--file SOURCE-FILE'
     Indicates that the `awk' program is to be found in SOURCE-FILE
     instead of in the first non-option argument.

`-v VAR=VAL'
`--assign VAR=VAL'
     Sets the variable VAR to the value VAL *before* execution of the
     program begins.  Such variable values are available inside the
     `BEGIN' rule (*note Other Command Line Arguments: Other
     Arguments.).

     The `-v' option can only set one variable, but you can use it more
     than once, setting another variable each time, like this: `awk
     -v foo=1 -v bar=2 ...'.

`-mf NNN'
`-mr NNN'
     Set various memory limits to the value NNN.  The `f' flag sets the
     maximum number of fields, and the `r' flag sets the maximum record
     size.  These two flags and the `-m' option are from the Bell Labs
     research version of Unix `awk'.  They are provided for
     compatibility, but otherwise ignored by `gawk', since `gawk' has
     no predefined limits.

`-W GAWK-OPT'
     Following the POSIX standard, options that are implementation
     specific are supplied as arguments to the `-W' option.  These
     options also have corresponding GNU style long options.  See below.

`--'
     Signals the end of the command line options.  The following
     arguments are not treated as options even if they begin with `-'.
     This interpretation of `--' follows the POSIX argument parsing
     conventions.

     This is useful if you have file names that start with `-', or in
     shell scripts, if you have file names that will be specified by
     the user which could start with `-'.

   The following `gawk'-specific options are available:

`-W traditional'
`-W compat'
`--traditional'
`--compat'
     Specifies "compatibility mode", in which the GNU extensions to the
     `awk' language are disabled, so that `gawk' behaves just like the
     Bell Labs research version of Unix `awk'.  `--traditional' is the
     preferred form of this option.  *Note Extensions in `gawk' Not in
     POSIX `awk': POSIX/GNU, which summarizes the extensions.  Also see
     *Note Downward Compatibility and Debugging: Compatibility Mode.

`-W copyleft'
`-W copyright'
`--copyleft'
`--copyright'
     Print the short version of the General Public License, and then
     exit.  This option may disappear in a future version of `gawk'.

`-W help'
`-W usage'
`--help'
`--usage'
     Print a "usage" message summarizing the short and long style
     options that `gawk' accepts, and then exit.

`-W lint'
`--lint'
     Warn about constructs that are dubious or non-portable to other
     `awk' implementations.  Some warnings are issued when `gawk' first
     reads your program.  Others are issued at run-time, as your
     program executes.

`-W lint-old'
`--lint-old'
     Warn about constructs that are not available in the original
     Version 7 Unix version of `awk' (*note Major Changes between V7
     and SVR3.1: V7/SVR3.1.).

`-W posix'
`--posix'
     Operate in strict POSIX mode.  This disables all `gawk' extensions
     (just like `--traditional'), and adds the following additional
     restrictions:

        * `\x' escape sequences are not recognized (*note Escape
          Sequences::.).

        * Newlines do not act as whitespace to separate fields when
          `FS' is equal to a single space.

        * The synonym `func' for the keyword `function' is not
          recognized (*note Function Definition Syntax: Definition
          Syntax.).

        * The operators `**' and `**=' cannot be used in place of `^'
          and `^=' (*note Arithmetic Operators: Arithmetic Ops., and
          also *note Assignment Expressions: Assignment Ops.).

        * Specifying `-Ft' on the command line does not set the value
          of `FS' to be a single tab character (*note Specifying How
          Fields are Separated: Field Separators.).

        * The `fflush' built-in function is not supported (*note
          Built-in Functions for Input/Output: I/O Functions.).

     If you supply both `--traditional' and `--posix' on the command
     line, `--posix' will take precedence. `gawk' will also issue a
     warning if both options are supplied.

`-W re-interval'
`--re-interval'
     Allow interval expressions (*note Regular Expression Operators:
     Regexp Operators.), in regexps.  Because interval expressions were
     traditionally not available in `awk', `gawk' does not provide them
     by default. This prevents old `awk' programs from breaking.

`-W source PROGRAM-TEXT'
`--source PROGRAM-TEXT'
     Program source code is taken from the PROGRAM-TEXT.  This option
     allows you to mix source code in files with source code that you
     enter on the command line. This is particularly useful when you
     have library functions that you wish to use from your command line
     programs (*note The `AWKPATH' Environment Variable: AWKPATH
     Variable.).

`-W version'
`--version'
     Prints version information for this particular copy of `gawk'.
     This allows you to determine if your copy of `gawk' is up to date
     with respect to whatever the Free Software Foundation is currently
     distributing.  It is also useful for bug reports (*note Reporting
     Problems and Bugs: Bugs.).

   Any other options are flagged as invalid with a warning message, but
are otherwise ignored.

   In compatibility mode, as a special case, if the value of FS supplied
to the `-F' option is `t', then `FS' is set to the tab character
(`"\t"').  This is only true for `--traditional', and not for `--posix'
(*note Specifying How Fields are Separated: Field Separators.).

   The `-f' option may be used more than once on the command line.  If
it is, `awk' reads its program source from all of the named files, as
if they had been concatenated together into one big file.  This is
useful for creating libraries of `awk' functions.  Useful functions can
be written once, and then retrieved from a standard place, instead of
having to be included into each individual program.

   You can type in a program at the terminal and still use library
functions, by specifying `-f /dev/tty'.  `awk' will read a file from
the terminal to use as part of the `awk' program.  After typing your
program, type `Control-d' (the end-of-file character) to terminate it.
(You may also use `-f -' to read program source from the standard
input, but then you will not be able to also use the standard input as a
source of data.)

   Because it is clumsy using the standard `awk' mechanisms to mix
source file and command line `awk' programs, `gawk' provides the
`--source' option.  This does not require you to pre-empt the standard
input for your source code, and allows you to easily mix command line
and library source code (*note The `AWKPATH' Environment Variable:
AWKPATH Variable.).

   If no `-f' or `--source' option is specified, then `gawk' will use
the first non-option command line argument as the text of the program
source code.

   If the environment variable `POSIXLY_CORRECT' exists, then `gawk'
will behave in strict POSIX mode, exactly as if you had supplied the
`--posix' command line option.  Many GNU programs look for this
environment variable to turn on strict POSIX mode. If you supply
`--lint' on the command line, and `gawk' turns on POSIX mode because of
`POSIXLY_CORRECT', then it will print a warning message indicating that
POSIX mode is in effect.

   You would typically set this variable in your shell's startup file.
For a Bourne compatible shell (such as Bash), you would add these lines
to the `.profile' file in your home directory.

     POSIXLY_CORRECT=true
     export POSIXLY_CORRECT

   For a `csh' compatible shell,(1) you would add this line to the
`.login' file in your home directory.

     setenv POSIXLY_CORRECT true

   ---------- Footnotes ----------

   (1)  Not recommended.


File: gawk.info,  Node: Other Arguments,  Next: AWKPATH Variable,  Prev: Options,  Up: Invoking Gawk

Other Command Line Arguments
============================

   Any additional arguments on the command line are normally treated as
input files to be processed in the order specified.   However, an
argument that has the form `VAR=VALUE', assigns the value VALUE to the
variable VAR--it does not specify a file at all.

   All these arguments are made available to your `awk' program in the
`ARGV' array (*note Built-in Variables::.).  Command line options and
the program text (if present) are omitted from `ARGV'.  All other
arguments, including variable assignments, are included.   As each
element of `ARGV' is processed, `gawk' sets the variable `ARGIND' to
the index in `ARGV' of the current element.

   The distinction between file name arguments and variable-assignment
arguments is made when `awk' is about to open the next input file.  At
that point in execution, it checks the "file name" to see whether it is
really a variable assignment; if so, `awk' sets the variable instead of
reading a file.

   Therefore, the variables actually receive the given values after all
previously specified files have been read.  In particular, the values of
variables assigned in this fashion are *not* available inside a `BEGIN'
rule (*note The `BEGIN' and `END' Special Patterns: BEGIN/END.), since
such rules are run before `awk' begins scanning the argument list.

   The variable values given on the command line are processed for
escape sequences (d.c.) (*note Escape Sequences::.).

   In some earlier implementations of `awk', when a variable assignment
occurred before any file names, the assignment would happen *before*
the `BEGIN' rule was executed.  `awk''s behavior was thus inconsistent;
some command line assignments were available inside the `BEGIN' rule,
while others were not.  However, some applications came to depend upon
this "feature."  When `awk' was changed to be more consistent, the `-v'
option was added to accommodate applications that depended upon the old
behavior.

   The variable assignment feature is most useful for assigning to
variables such as `RS', `OFS', and `ORS', which control input and
output formats, before scanning the data files.  It is also useful for
controlling state if multiple passes are needed over a data file.  For
example:

     awk 'pass == 1  { PASS 1 STUFF }
          pass == 2  { PASS 2 STUFF }' pass=1 mydata pass=2 mydata

   Given the variable assignment feature, the `-F' option for setting
the value of `FS' is not strictly necessary.  It remains for historical
compatibility.


File: gawk.info,  Node: AWKPATH Variable,  Next: Obsolete,  Prev: Other Arguments,  Up: Invoking Gawk

The `AWKPATH' Environment Variable
==================================

   The previous section described how `awk' program files can be named
on the command line with the `-f' option.  In most `awk'
implementations, you must supply a precise path name for each program
file, unless the file is in the current directory.

   But in `gawk', if the file name supplied to the `-f' option does not
contain a `/', then `gawk' searches a list of directories (called the
"search path"), one by one, looking for a file with the specified name.

   The search path is a string consisting of directory names separated
by colons.  `gawk' gets its search path from the `AWKPATH' environment
variable.  If that variable does not exist, `gawk' uses a default path,
which is `.:/usr/local/share/awk'.(1) (Programs written for use by
system administrators should use an `AWKPATH' variable that does not
include the current directory, `.'.)

   The search path feature is particularly useful for building up
libraries of useful `awk' functions.  The library files can be placed
in a standard directory that is in the default path, and then specified
on the command line with a short file name.  Otherwise, the full file
name would have to be typed for each file.

   By using both the `--source' and `-f' options, your command line
`awk' programs can use facilities in `awk' library files.  *Note A
Library of `awk' Functions: Library Functions.

   Path searching is not done if `gawk' is in compatibility mode.  This
is true for both `--traditional' and `--posix'.  *Note Command Line
Options: Options.

   *Note:* if you want files in the current directory to be found, you
must include the current directory in the path, either by including `.'
explicitly in the path, or by writing a null entry in the path.  (A
null entry is indicated by starting or ending the path with a colon, or
by placing two colons next to each other (`::').)  If the current
directory is not included in the path, then files cannot be found in
the current directory.  This path search mechanism is identical to the
shell's.

   Starting with version 3.0, if `AWKPATH' is not defined in the
environment, `gawk' will place its default search path into
`ENVIRON["AWKPATH"]'. This makes it easy to determine the actual search
path `gawk' will use.

   ---------- Footnotes ----------

   (1)  Your version of `gawk' may use a directory that is different
than `/usr/local/share/awk'; it will depend upon how `gawk' was built
and installed. The actual directory will be the value of `$(datadir)'
generated when `gawk' was configured.  You probably don't need to worry
about this though.


File: gawk.info,  Node: Obsolete,  Next: Undocumented,  Prev: AWKPATH Variable,  Up: Invoking Gawk

Obsolete Options and/or Features
================================

   This section describes features and/or command line options from
previous releases of `gawk' that are either not available in the
current version, or that are still supported but deprecated (meaning
that they will *not* be in the next release).

   For version 3.0.2 of `gawk', there are no command line options or
other deprecated features from the previous version of `gawk'.  This
node is thus essentially a place holder, in case some option becomes
obsolete in a future version of `gawk'.


File: gawk.info,  Node: Undocumented,  Next: Known Bugs,  Prev: Obsolete,  Up: Invoking Gawk

Undocumented Options and Features
=================================

   This section intentionally left blank.


File: gawk.info,  Node: Known Bugs,  Prev: Undocumented,  Up: Invoking Gawk

Known Bugs in `gawk'
====================

   * The `-F' option for changing the value of `FS' (*note Command Line
     Options: Options.) is not necessary given the command line variable
     assignment feature; it remains only for backwards compatibility.

   * If your system actually has support for `/dev/fd' and the
     associated `/dev/stdin', `/dev/stdout', and `/dev/stderr' files,
     you may get different output from `gawk' than you would get on a
     system without those files.  When `gawk' interprets these files
     internally, it synchronizes output to the standard output with
     output to `/dev/stdout', while on a system with those files, the
     output is actually to different open files (*note Special File
     Names in `gawk': Special Files.).

   * Syntactically invalid single character programs tend to overflow
     the parse stack, generating a rather unhelpful message.  Such
     programs are surprisingly difficult to diagnose in the completely
     general case, and the effort to do so really is not worth it.


File: gawk.info,  Node: Library Functions,  Next: Sample Programs,  Prev: Invoking Gawk,  Up: Top

A Library of `awk' Functions
****************************

   This chapter presents a library of useful `awk' functions.  The
sample programs presented later (*note Practical `awk' Programs: Sample
Programs.) use these functions.  The functions are presented here in a
progression from simple to complex.

   *Note Extracting Programs from Texinfo Source Files: Extract Program,
presents a program that you can use to extract the source code for
these example library functions and programs from the Texinfo source
for this Info file.  (This has already been done as part of the `gawk'
distribution.)

   If you have written one or more useful, general purpose `awk'
functions, and would like to contribute them for a subsequent edition
of this Info file, please contact the author.  *Note Reporting Problems
and Bugs: Bugs, for information on doing this.  Don't just send code,
as you will be required to either place your code in the public domain,
publish it under the GPL (*note GNU GENERAL PUBLIC LICENSE: Copying.),
or assign the copyright in it to the Free Software Foundation.

* Menu:

* Portability Notes::           What to do if you don't have `gawk'.
* Nextfile Function::           Two implementations of a `nextfile'
                                function.
* Assert Function::             A function for assertions in `awk'
                                programs.
* Round Function::              A function for rounding if `sprintf' does
                                not do it correctly.
* Ordinal Functions::           Functions for using characters as numbers and
                                vice versa.
* Join Function::               A function to join an array into a string.
* Mktime Function::             A function to turn a date into a timestamp.
* Gettimeofday Function::       A function to get formatted times.
* Filetrans Function::          A function for handling data file transitions.
* Getopt Function::             A function for processing command line
                                arguments.
* Passwd Functions::            Functions for getting user information.
* Group Functions::             Functions for getting group information.
* Library Names::               How to best name private global variables in
                                library functions.


File: gawk.info,  Node: Portability Notes,  Next: Nextfile Function,  Prev: Library Functions,  Up: Library Functions

Simulating `gawk'-specific Features
===================================

   The programs in this chapter and in *Note Practical `awk' Programs:
Sample Programs, freely use features that are specific to `gawk'.  This
section briefly discusses how you can rewrite these programs for
different implementations of `awk'.

   Diagnostic error messages are sent to `/dev/stderr'.  Use `| "cat
1>&2"' instead of `> "/dev/stderr"', if your system does not have a
`/dev/stderr', or if you cannot use `gawk'.

   A number of programs use `nextfile' (*note The `nextfile' Statement:
Nextfile Statement.), to skip any remaining input in the input file.
*Note Implementing `nextfile' as a Function: Nextfile Function, shows
you how to write a function that will do the same thing.

   Finally, some of the programs choose to ignore upper-case and
lower-case distinctions in their input. They do this by assigning one
to `IGNORECASE'.  You can achieve the same effect by adding the
following rule to the beginning of the program:

     # ignore case
     { $0 = tolower($0) }

Also, verify that all regexp and string constants used in comparisons
only use lower-case letters.


File: gawk.info,  Node: Nextfile Function,  Next: Assert Function,  Prev: Portability Notes,  Up: Library Functions

Implementing `nextfile' as a Function
=====================================

   The `nextfile' statement presented in *Note The `nextfile'
Statement: Nextfile Statement, is a `gawk'-specific extension.  It is
not available in other implementations of `awk'.  This section shows
two versions of a `nextfile' function that you can use to simulate
`gawk''s `nextfile' statement if you cannot use `gawk'.

   Here is a first attempt at writing a `nextfile' function.

     # nextfile --- skip remaining records in current file
     
     # this should be read in before the "main" awk program
     
     function nextfile()    { _abandon_ = FILENAME; next }
     
     _abandon_ == FILENAME  { next }

   This file should be included before the main program, because it
supplies a rule that must be executed first.  This rule compares the
current data file's name (which is always in the `FILENAME' variable)
to a private variable named `_abandon_'.  If the file name matches,
then the action part of the rule executes a `next' statement, to go on
to the next record.  (The use of `_' in the variable name is a
convention.  It is discussed more fully in *Note Naming Library
Function Global Variables: Library Names.)

   The use of the `next' statement effectively creates a loop that reads
all the records from the current data file.  Eventually, the end of the
file is reached, and a new data file is opened, changing the value of
`FILENAME'.  Once this happens, the comparison of `_abandon_' to
`FILENAME' fails, and execution continues with the first rule of the
"real" program.

   The `nextfile' function itself simply sets the value of `_abandon_'
and then executes a `next' statement to start the loop going.(1)

   This initial version has a subtle problem.  What happens if the same
data file is listed *twice* on the command line, one right after the
other, or even with just a variable assignment between the two
occurrences of the file name?

   In such a case, this code will skip right through the file, a second
time, even though it should stop when it gets to the end of the first
occurrence.  Here is a second version of `nextfile' that remedies this
problem.

     # nextfile --- skip remaining records in current file
     # correctly handle successive occurrences of the same file
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May, 1993
     
     # this should be read in before the "main" awk program
     
     function nextfile()   { _abandon_ = FILENAME; next }
     
     _abandon_ == FILENAME {
           if (FNR == 1)
               _abandon_ = ""
           else
               next
     }

   The `nextfile' function has not changed.  It sets `_abandon_' equal
to the current file name and then executes a `next' satement.  The
`next' statement reads the next record and increments `FNR', so `FNR'
is guaranteed to have a value of at least two.  However, if `nextfile'
is called for the last record in the file, then `awk' will close the
current data file and move on to the next one.  Upon doing so,
`FILENAME' will be set to the name of the new file, and `FNR' will be
reset to one.  If this next file is the same as the previous one,
`_abandon_' will still be equal to `FILENAME'.  However, `FNR' will be
equal to one, telling us that this is a new occurrence of the file, and
not the one we were reading when the `nextfile' function was executed.
In that case, `_abandon_' is reset to the empty string, so that further
executions of this rule will fail (until the next time that `nextfile'
is called).

   If `FNR' is not one, then we are still in the original data file,
and the program executes a `next' statement to skip through it.

   An important question to ask at this point is: "Given that the
functionality of `nextfile' can be provided with a library file, why is
it built into `gawk'?"  This is an important question.  Adding features
for little reason leads to larger, slower programs that are harder to
maintain.

   The answer is that building `nextfile' into `gawk' provides
significant gains in efficiency.  If the `nextfile' function is executed
at the beginning of a large data file, `awk' still has to scan the
entire file, splitting it up into records, just to skip over it.  The
built-in `nextfile' can simply close the file immediately and proceed
to the next one, saving a lot of time.  This is particularly important
in `awk', since `awk' programs are generally I/O bound (i.e.  they
spend most of their time doing input and output, instead of performing
computations).

   ---------- Footnotes ----------

   (1)  Some implementations of `awk' do not allow you to execute
`next' from within a function body. Some other work-around will be
necessary if you use such a version.


File: gawk.info,  Node: Assert Function,  Next: Round Function,  Prev: Nextfile Function,  Up: Library Functions

Assertions
==========

   When writing large programs, it is often useful to be able to know
that a condition or set of conditions is true.  Before proceeding with a
particular computation, you make a statement about what you believe to
be the case.  Such a statement is known as an "assertion."  The C
language provides an `<assert.h>' header file and corresponding
`assert' macro that the programmer can use to make assertions.  If an
assertion fails, the `assert' macro arranges to print a diagnostic
message describing the condition that should have been true but was
not, and then it kills the program.  In C, using `assert' looks this:

     #include <assert.h>
     
     int myfunc(int a, double b)
     {
          assert(a <= 5 && b >= 17);
          ...
     }

   If the assertion failed, the program would print a message similar to
this:

     prog.c:5: assertion failed: a <= 5 && b >= 17

   The ANSI C language makes it possible to turn the condition into a
string for use in printing the diagnostic message.  This is not
possible in `awk', so this `assert' function also requires a string
version of the condition that is being tested.

     # assert --- assert that a condition is true. Otherwise exit.
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May, 1993
     
     function assert(condition, string)
     {
         if (! condition) {
             printf("%s:%d: assertion failed: %s\n",
                 FILENAME, FNR, string) > "/dev/stderr"
             _assert_exit = 1
             exit 1
         }
     }
     
     END {
         if (_assert_exit)
             exit 1
     }

   The `assert' function tests the `condition' parameter. If it is
false, it prints a message to standard error, using the `string'
parameter to describe the failed condition.  It then sets the variable
`_assert_exit' to one, and executes the `exit' statement.  The `exit'
statement jumps to the `END' rule. If the `END' rules finds
`_assert_exit' to be true, then it exits immediately.

   The purpose of the `END' rule with its test is to keep any other
`END' rules from running.  When an assertion fails, the program should
exit immediately.  If no assertions fail, then `_assert_exit' will
still be false when the `END' rule is run normally, and the rest of the
program's `END' rules will execute.  For all of this to work correctly,
`assert.awk' must be the first source file read by `awk'.

   You would use this function in your programs this way:

     function myfunc(a, b)
     {
          assert(a <= 5 && b >= 17, "a <= 5 && b >= 17")
          ...
     }

If the assertion failed, you would see a message like this:

     mydata:1357: assertion failed: a <= 5 && b >= 17

   There is a problem with this version of `assert', that it may not be
possible to work around.  An `END' rule is automatically added to the
program calling `assert'.  Normally, if a program consists of just a
`BEGIN' rule, the input files and/or standard input are not read.
However, now that the program has an `END' rule, `awk' will attempt to
read the input data files, or standard input (*note Startup and Cleanup
Actions: Using BEGIN/END.), most likely causing the program to hang,
waiting for input.


File: gawk.info,  Node: Round Function,  Next: Ordinal Functions,  Prev: Assert Function,  Up: Library Functions

Rounding Numbers
================

   The way `printf' and `sprintf' (*note Using `printf' Statements for
Fancier Printing: Printf.) do rounding will often depend upon the
system's C `sprintf' subroutine.  On many machines, `sprintf' rounding
is "unbiased," which means it doesn't always round a trailing `.5' up,
contrary to naive expectations.  In unbiased rounding, `.5' rounds to
even, rather than always up, so 1.5 rounds to 2 but 4.5 rounds to 4.
The result is that if you are using a format that does rounding (e.g.,
`"%.0f"') you should check what your system does.  The following
function does traditional rounding; it might be useful if your awk's
`printf' does unbiased rounding.

     # round --- do normal rounding
     #
     # Arnold Robbins, arnold@gnu.ai.mit.edu, August, 1996
     # Public Domain
     
     function round(x,   ival, aval, fraction)
     {
        ival = int(x)    # integer part, int() truncates
     
        # see if fractional part
        if (ival == x)   # no fraction
           return x
     
        if (x < 0) {
           aval = -x     # absolute value
           ival = int(aval)
           fraction = aval - ival
           if (fraction >= .5)
              return int(x) - 1   # -2.5 --> -3
           else
              return int(x)       # -2.3 --> -2
        } else {
           fraction = x - ival
           if (fraction >= .5)
              return ival + 1
           else
              return ival
        }
     }
     
     # test harness
     { print $0, round($0) }


File: gawk.info,  Node: Ordinal Functions,  Next: Join Function,  Prev: Round Function,  Up: Library Functions

Translating Between Characters and Numbers
==========================================

   One commercial implementation of `awk' supplies a built-in function,
`ord', which takes a character and returns the numeric value for that
character in the machine's character set.  If the string passed to
`ord' has more than one character, only the first one is used.

   The inverse of this function is `chr' (from the function of the same
name in Pascal), which takes a number and returns the corresponding
character.

   Both functions can be written very nicely in `awk'; there is no real
reason to build them into the `awk' interpreter.

     # ord.awk --- do ord and chr
     #
     # Global identifiers:
     #    _ord_:        numerical values indexed by characters
     #    _ord_init:    function to initialize _ord_
     #
     # Arnold Robbins
     # arnold@gnu.ai.mit.edu
     # Public Domain
     # 16 January, 1992
     # 20 July, 1992, revised
     
     BEGIN    { _ord_init() }
     
     function _ord_init(    low, high, i, t)
     {
         low = sprintf("%c", 7) # BEL is ascii 7
         if (low == "\a") {    # regular ascii
             low = 0
             high = 127
         } else if (sprintf("%c", 128 + 7) == "\a") {
             # ascii, mark parity
             low = 128
             high = 255
         } else {        # ebcdic(!)
             low = 0
             high = 255
         }
     
         for (i = low; i <= high; i++) {
             t = sprintf("%c", i)
             _ord_[t] = i
         }
     }

   Some explanation of the numbers used by `chr' is worthwhile.  The
most prominent character set in use today is ASCII. Although an
eight-bit byte can hold 256 distinct values (from zero to 255), ASCII
only defines characters that use the values from zero to 127.(1) At
least one computer manufacturer that we know of uses ASCII, but with
mark parity, meaning that the leftmost bit in the byte is always one.
What this means is that on those systems, characters have numeric
values from 128 to 255.  Finally, large mainframe systems use the
EBCDIC character set, which uses all 256 values.  While there are other
character sets in use on some older systems, they are not really worth
worrying about.

     function ord(str,    c)
     {
         # only first character is of interest
         c = substr(str, 1, 1)
         return _ord_[c]
     }
     
     function chr(c)
     {
         # force c to be numeric by adding 0
         return sprintf("%c", c + 0)
     }
     
     #### test code ####
     # BEGIN    \
     # {
     #    for (;;) {
     #        printf("enter a character: ")
     #        if (getline var <= 0)
     #            break
     #        printf("ord(%s) = %d\n", var, ord(var))
     #    }
     # }

   An obvious improvement to these functions would be to move the code
for the `_ord_init' function into the body of the `BEGIN' rule.  It was
written this way initially for ease of development.

   There is a "test program" in a `BEGIN' rule, for testing the
function.  It is commented out for production use.

   ---------- Footnotes ----------

   (1)  ASCII has been extended in many countries to use the values
from 128 to 255 for country-specific characters.  If your  system uses
these extensions, you can simplify `_ord_init' to simply loop from zero
to 255.


File: gawk.info,  Node: Join Function,  Next: Mktime Function,  Prev: Ordinal Functions,  Up: Library Functions

Merging an Array Into a String
==============================

   When doing string processing, it is often useful to be able to join
all the strings in an array into one long string.  The following
function, `join', accomplishes this task.  It is used later in several
of the application programs (*note Practical `awk' Programs: Sample
Programs.).

   Good function design is important; this function needs to be
general, but it should also have a reasonable default behavior.  It is
called with an array and the beginning and ending indices of the
elements in the array to be merged.  This assumes that the array
indices are numeric--a reasonable assumption since the array was likely
created with `split' (*note Built-in Functions for String Manipulation:
String Functions.).

     # join.awk --- join an array into a string
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     function join(array, start, end, sep,    result, i)
     {
         if (sep == "")
            sep = " "
         else if (sep == SUBSEP) # magic value
            sep = ""
         result = array[start]
         for (i = start + 1; i <= end; i++)
             result = result sep array[i]
         return result
     }

   An optional additional argument is the separator to use when joining
the strings back together.  If the caller supplies a non-empty value,
`join' uses it.  If it is not supplied, it will have a null value.  In
this case, `join' uses a single blank as a default separator for the
strings.  If the value is equal to `SUBSEP', then `join' joins the
strings with no separator between them.  `SUBSEP' serves as a "magic"
value to indicate that there should be no separation between the
component strings.

   It would be nice if `awk' had an assignment operator for
concatenation.  The lack of an explicit operator for concatenation
makes string operations more difficult than they really need to be.


File: gawk.info,  Node: Mktime Function,  Next: Gettimeofday Function,  Prev: Join Function,  Up: Library Functions

Turning Dates Into Timestamps
=============================

   The `systime' function built in to `gawk' returns the current time
of day as a timestamp in "seconds since the Epoch."  This timestamp can
be converted into a printable date of almost infinitely variable format
using the built-in `strftime' function.  (For more information on
`systime' and `strftime', *note Functions for Dealing with Time Stamps:
Time Functions..)

   An interesting but difficult problem is to convert a readable
representation of a date back into a timestamp.  The ANSI C library
provides a `mktime' function that does the basic job, converting a
canonical representation of a date into a timestamp.

   It would appear at first glance that `gawk' would have to supply a
`mktime' built-in function that was simply a "hook" to the C language
version.  In fact though, `mktime' can be implemented entirely in `awk'.

   Here is a version of `mktime' for `awk'.  It takes a simple
representation of the date and time, and converts it into a timestamp.

   The code is presented here intermixed with explanatory prose.  In
*Note Extracting Programs from Texinfo Source Files: Extract Program,
you will see how the Texinfo source file for this Info file can be
processed to extract the code into a single source file.

   The program begins with a descriptive comment and a `BEGIN' rule
that initializes a table `_tm_months'.  This table is a two-dimensional
array that has the lengths of the months.  The first index is zero for
regular years, and one for leap years.  The values are the same for all
the months in both kinds of years, except for February; thus the use of
multiple assignment.

     # mktime.awk --- convert a canonical date representation
     #                into a timestamp
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     BEGIN    \
     {
         # Initialize table of month lengths
         _tm_months[0,1] = _tm_months[1,1] = 31
         _tm_months[0,2] = 28; _tm_months[1,2] = 29
         _tm_months[0,3] = _tm_months[1,3] = 31
         _tm_months[0,4] = _tm_months[1,4] = 30
         _tm_months[0,5] = _tm_months[1,5] = 31
         _tm_months[0,6] = _tm_months[1,6] = 30
         _tm_months[0,7] = _tm_months[1,7] = 31
         _tm_months[0,8] = _tm_months[1,8] = 31
         _tm_months[0,9] = _tm_months[1,9] = 30
         _tm_months[0,10] = _tm_months[1,10] = 31
         _tm_months[0,11] = _tm_months[1,11] = 30
         _tm_months[0,12] = _tm_months[1,12] = 31
     }

   The benefit of merging multiple `BEGIN' rules (*note The `BEGIN' and
`END' Special Patterns: BEGIN/END.) is particularly clear when writing
library files.  Functions in library files can cleanly initialize their
own private data and also provide clean-up actions in private `END'
rules.

   The next function is a simple one that computes whether a given year
is or is not a leap year.  If a year is evenly divisible by four, but
not evenly divisible by 100, or if it is evenly divisible by 400, then
it is a leap year.  Thus, 1904 was a leap year, 1900 was not, but 2000
will be.

     # decide if a year is a leap year
     function _tm_isleap(year,    ret)
     {
         ret = (year % 4 == 0 && year % 100 != 0) ||
                 (year % 400 == 0)
     
         return ret
     }

   This function is only used a few times in this file, and its
computation could have been written "in-line" (at the point where it's
used).  Making it a separate function made the original development
easier, and also avoids the possibility of typing errors when
duplicating the code in multiple places.

   The next function is more interesting.  It does most of the work of
generating a timestamp, which is converting a date and time into some
number of seconds since the Epoch.  The caller passes an array (rather
imaginatively named `a') containing six values: the year including
century, the month as a number between one and 12, the day of the
month, the hour as a number between zero and 23, the minute in the
hour, and the seconds within the minute.

   The function uses several local variables to precompute the number of
seconds in an hour, seconds in a day, and seconds in a year.  Often,
similar C code simply writes out the expression in-line, expecting the
compiler to do "constant folding".  E.g., most C compilers would turn
`60 * 60' into `3600' at compile time, instead of recomputing it every
time at run time.  Precomputing these values makes the function more
efficient.

     # convert a date into seconds
     function _tm_addup(a,    total, yearsecs, daysecs,
                              hoursecs, i, j)
     {
         hoursecs = 60 * 60
         daysecs = 24 * hoursecs
         yearsecs = 365 * daysecs
     
         total = (a[1] - 1970) * yearsecs
     
         # extra day for leap years
         for (i = 1970; i < a[1]; i++)
             if (_tm_isleap(i))
                 total += daysecs
     
         j = _tm_isleap(a[1])
         for (i = 1; i < a[2]; i++)
             total += _tm_months[j, i] * daysecs
     
         total += (a[3] - 1) * daysecs
         total += a[4] * hoursecs
         total += a[5] * 60
         total += a[6]
     
         return total
     }

   The function starts with a first approximation of all the seconds
between Midnight, January 1, 1970,(1) and the beginning of the current
year.  It then goes through all those years, and for every leap year,
adds an additional day's worth of seconds.

   The variable `j' holds either one or zero, if the current year is or
is not a leap year.  For every month in the current year prior to the
current month, it adds the number of seconds in the month, using the
appropriate entry in the `_tm_months' array.

   Finally, it adds in the seconds for the number of days prior to the
current day, and the number of hours, minutes, and seconds in the
current day.

   The result is a count of seconds since January 1, 1970.  This value
is not yet what is needed though.  The reason why is described shortly.

   The main `mktime' function takes a single character string argument.
This string is a representation of a date and time in a "canonical"
(fixed) form.  This string should be `"YEAR MONTH DAY HOUR MINUTE
SECOND"'.

     # mktime --- convert a date into seconds,
     #            compensate for time zone
     
     function mktime(str,    res1, res2, a, b, i, j, t, diff)
     {
         i = split(str, a, " ")    # don't rely on FS
     
         if (i != 6)
             return -1
     
         # force numeric
         for (j in a)
             a[j] += 0
     
         # validate
         if (a[1] < 1970 ||
             a[2] < 1 || a[2] > 12 ||
             a[3] < 1 || a[3] > 31 ||
             a[4] < 0 || a[4] > 23 ||
             a[5] < 0 || a[5] > 59 ||
             a[6] < 0 || a[6] > 60 )
                 return -1
     
         res1 = _tm_addup(a)
         t = strftime("%Y %m %d %H %M %S", res1)
     
         if (_tm_debug)
             printf("(%s) -> (%s)\n", str, t) > "/dev/stderr"
     
         split(t, b, " ")
         res2 = _tm_addup(b)
     
         diff = res1 - res2
     
         if (_tm_debug)
             printf("diff = %d seconds\n", diff) > "/dev/stderr"
     
         res1 += diff
     
         return res1
     }

   The function first splits the string into an array, using spaces and
tabs as separators.  If there are not six elements in the array, it
returns an error, signaled as the value -1.  Next, it forces each
element of the array to be numeric, by adding zero to it.  The
following `if' statement then makes sure that each element is within an
allowable range.  (This checking could be extended further, e.g., to
make sure that the day of the month is within the correct range for the
particular month supplied.)  All of this is essentially preliminary
set-up and error checking.

   Recall that `_tm_addup' generated a value in seconds since Midnight,
January 1, 1970.  This value is not directly usable as the result we
want, *since the calculation does not account for the local timezone*.
In other words, the value represents the count in seconds since the
Epoch, but only for UTC (Universal Coordinated Time).  If the local
timezone is east or west of UTC, then some number of hours should be
either added to, or subtracted from the resulting timestamp.

   For example, 6:23 p.m. in Atlanta, Georgia (USA), is normally five
hours west of (behind) UTC.  It is only four hours behind UTC if
daylight savings time is in effect.  If you are calling `mktime' in
Atlanta, with the argument `"1993 5 23 18 23 12"', the result from
`_tm_addup' will be for 6:23 p.m. UTC, which is only 2:23 p.m. in
Atlanta.  It is necessary to add another four hours worth of seconds to
the result.

   How can `mktime' determine how far away it is from UTC?  This is
surprisingly easy.  The returned timestamp represents the time passed to
`mktime' *as UTC*.  This timestamp can be fed back to `strftime', which
will format it as a *local* time; i.e. as if it already had the UTC
difference added in to it.  This is done by giving
`"%Y %m %d %H %M %S"' to `strftime' as the format argument.  It returns
the computed timestamp in the original string format.  The result
represents a time that accounts for the UTC difference.  When the new
time is converted back to a timestamp, the difference between the two
timestamps is the difference (in seconds) between the local timezone
and UTC.  This difference is then added back to the original result.
An example demonstrating this is presented below.

   Finally, there is a "main" program for testing the function.

     BEGIN  {
         if (_tm_test) {
             printf "Enter date as yyyy mm dd hh mm ss: "
             getline _tm_test_date
     
             t = mktime(_tm_test_date)
             r = strftime("%Y %m %d %H %M %S", t)
             printf "Got back (%s)\n", r
         }
     }

   The entire program uses two variables that can be set on the command
line to control debugging output and to enable the test in the final
`BEGIN' rule.  Here is the result of a test run. (Note that debugging
output is to standard error, and test output is to standard output.)

     $ gawk -f mktime.awk -v _tm_test=1 -v _tm_debug=1
     -| Enter date as yyyy mm dd hh mm ss: 1993 5 23 15 35 10
     error--> (1993 5 23 15 35 10) -> (1993 05 23 11 35 10)
     error--> diff = 14400 seconds
     -| Got back (1993 05 23 15 35 10)

   The time entered was 3:35 p.m. (15:35 on a 24-hour clock), on May
23, 1993.  The first line of debugging output shows the resulting time
as UTC--four hours ahead of the local time zone.  The second line shows
that the difference is 14400 seconds, which is four hours.  (The
difference is only four hours, since daylight savings time is in effect
during May.) The final line of test output shows that the timezone
compensation algorithm works; the returned time is the same as the
entered time.

   This program does not solve the general problem of turning an
arbitrary date representation into a timestamp.  That problem is very
involved.  However, the `mktime' function provides a foundation upon
which to build. Other software can convert month names into numeric
months, and AM/PM times into 24-hour clocks, to generate the
"canonical" format that `mktime' requires.

   ---------- Footnotes ----------

   (1)  This is the Epoch on POSIX systems.  It may be different on
other systems.


File: gawk.info,  Node: Gettimeofday Function,  Next: Filetrans Function,  Prev: Mktime Function,  Up: Library Functions

Managing the Time of Day
========================

   The `systime' and `strftime' functions described in *Note Functions
for Dealing with Time Stamps: Time Functions, provide the minimum
functionality necessary for dealing with the time of day in human
readable form.  While `strftime' is extensive, the control formats are
not necessarily easy to remember or intuitively obvious when reading a
program.

   The following function, `gettimeofday', populates a user-supplied
array with pre-formatted time information.  It returns a string with
the current time formatted in the same way as the `date' utility.

     # gettimeofday --- get the time of day in a usable format
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain, May 1993
     #
     # Returns a string in the format of output of date(1)
     # Populates the array argument time with individual values:
     #    time["second"]       -- seconds (0 - 59)
     #    time["minute"]       -- minutes (0 - 59)
     #    time["hour"]         -- hours (0 - 23)
     #    time["althour"]      -- hours (0 - 12)
     #    time["monthday"]     -- day of month (1 - 31)
     #    time["month"]        -- month of year (1 - 12)
     #    time["monthname"]    -- name of the month
     #    time["shortmonth"]   -- short name of the month
     #    time["year"]         -- year within century (0 - 99)
     #    time["fullyear"]     -- year with century (19xx or 20xx)
     #    time["weekday"]      -- day of week (Sunday = 0)
     #    time["altweekday"]   -- day of week (Monday = 0)
     #    time["weeknum"]      -- week number, Sunday first day
     #    time["altweeknum"]   -- week number, Monday first day
     #    time["dayname"]      -- name of weekday
     #    time["shortdayname"] -- short name of weekday
     #    time["yearday"]      -- day of year (0 - 365)
     #    time["timezone"]     -- abbreviation of timezone name
     #    time["ampm"]         -- AM or PM designation
     
     function gettimeofday(time,    ret, now, i)
     {
         # get time once, avoids unnecessary system calls
         now = systime()
     
         # return date(1)-style output
         ret = strftime("%a %b %d %H:%M:%S %Z %Y", now)
     
         # clear out target array
         for (i in time)
             delete time[i]
     
         # fill in values, force numeric values to be
         # numeric by adding 0
         time["second"]       = strftime("%S", now) + 0
         time["minute"]       = strftime("%M", now) + 0
         time["hour"]         = strftime("%H", now) + 0
         time["althour"]      = strftime("%I", now) + 0
         time["monthday"]     = strftime("%d", now) + 0
         time["month"]        = strftime("%m", now) + 0
         time["monthname"]    = strftime("%B", now)
         time["shortmonth"]   = strftime("%b", now)
         time["year"]         = strftime("%y", now) + 0
         time["fullyear"]     = strftime("%Y", now) + 0
         time["weekday"]      = strftime("%w", now) + 0
         time["altweekday"]   = strftime("%u", now) + 0
         time["dayname"]      = strftime("%A", now)
         time["shortdayname"] = strftime("%a", now)
         time["yearday"]      = strftime("%j", now) + 0
         time["timezone"]     = strftime("%Z", now)
         time["ampm"]         = strftime("%p", now)
         time["weeknum"]      = strftime("%U", now) + 0
         time["altweeknum"]   = strftime("%W", now) + 0
     
         return ret
     }

   The string indices are easier to use and read than the various
formats required by `strftime'.  The `alarm' program presented in *Note
An Alarm Clock Program: Alarm Program, uses this function.

   The `gettimeofday' function is presented above as it was written. A
more general design for this function would have allowed the user to
supply an optional timestamp value that would have been used instead of
the current time.


File: gawk.info,  Node: Filetrans Function,  Next: Getopt Function,  Prev: Gettimeofday Function,  Up: Library Functions

Noting Data File Boundaries
===========================

   The `BEGIN' and `END' rules are each executed exactly once, at the
beginning and end respectively of your `awk' program (*note The `BEGIN'
and `END' Special Patterns: BEGIN/END.).  We (the `gawk' authors) once
had a user who mistakenly thought that the `BEGIN' rule was executed at
the beginning of each data file and the `END' rule was executed at the
end of each data file.  When informed that this was not the case, the
user requested that we add new special patterns to `gawk', named
`BEGIN_FILE' and `END_FILE', that would have the desired behavior.  He
even supplied us the code to do so.

   However, after a little thought, I came up with the following
library program.  It arranges to call two user-supplied functions,
`beginfile' and `endfile', at the beginning and end of each data file.
Besides solving the problem in only nine(!) lines of code, it does so
*portably*; this will work with any implementation of `awk'.

     # transfile.awk
     #
     # Give the user a hook for filename transitions
     #
     # The user must supply functions beginfile() and endfile()
     # that each take the name of the file being started or
     # finished, respectively.
     #
     # Arnold Robbins, arnold@gnu.ai.mit.edu, January 1992
     # Public Domain
     
     FILENAME != _oldfilename \
     {
         if (_oldfilename != "")
             endfile(_oldfilename)
         _oldfilename = FILENAME
         beginfile(FILENAME)
     }
     
     END   { endfile(FILENAME) }

   This file must be loaded before the user's "main" program, so that
the rule it supplies will be executed first.

   This rule relies on `awk''s `FILENAME' variable that automatically
changes for each new data file.  The current file name is saved in a
private variable, `_oldfilename'.  If `FILENAME' does not equal
`_oldfilename', then a new data file is being processed, and it is
necessary to call `endfile' for the old file.  Since `endfile' should
only be called if a file has been processed, the program first checks
to make sure that `_oldfilename' is not the null string.  The program
then assigns the current file name to `_oldfilename', and calls
`beginfile' for the file.  Since, like all `awk' variables,
`_oldfilename' will be initialized to the null string, this rule
executes correctly even for the first data file.

   The program also supplies an `END' rule, to do the final processing
for the last file.  Since this `END' rule comes before any `END' rules
supplied in the "main" program, `endfile' will be called first.  Once
again the value of multiple `BEGIN' and `END' rules should be clear.

   This version has same problem as the first version of `nextfile'
(*note Implementing `nextfile' as a Function: Nextfile Function.).  If
the same data file occurs twice in a row on command line, then
`endfile' and `beginfile' will not be executed at the end of the first
pass and at the beginning of the second pass.  This version solves the
problem.

     # ftrans.awk --- handle data file transitions
     #
     # user supplies beginfile() and endfile() functions
     #
     # Arnold Robbins, arnold@gnu.ai.mit.edu. November 1992
     # Public Domain
     
     FNR == 1 {
         if (_filename_ != "")
             endfile(_filename_)
         _filename_ = FILENAME
         beginfile(FILENAME)
     }
     
     END  { endfile(_filename_) }

   In *Note Counting Things: Wc Program, you will see how this library
function can be used, and how it simplifies writing the main program.


File: gawk.info,  Node: Getopt Function,  Next: Passwd Functions,  Prev: Filetrans Function,  Up: Library Functions

Processing Command Line Options
===============================

   Most utilities on POSIX compatible systems take options or
"switches" on the command line that can be used to change the way a
program behaves.  `awk' is an example of such a program (*note Command
Line Options: Options.).  Often, options take "arguments", data that
the program needs to correctly obey the command line option.  For
example, `awk''s `-F' option requires a string to use as the field
separator.  The first occurrence on the command line of either `--' or a
string that does not begin with `-' ends the options.

   Most Unix systems provide a C function named `getopt' for processing
command line arguments.  The programmer provides a string describing
the one letter options. If an option requires an argument, it is
followed in the string with a colon.  `getopt' is also passed the count
and values of the command line arguments, and is called in a loop.
`getopt' processes the command line arguments for option letters.  Each
time around the loop, it returns a single character representing the
next option letter that it found, or `?' if it found an invalid option.
When it returns -1, there are no options left on the command line.

   When using `getopt', options that do not take arguments can be
grouped together.  Furthermore, options that take arguments require
that the argument be present.  The argument can immediately follow the
option letter, or it can be a separate command line argument.

   Given a hypothetical program that takes three command line options,
`-a', `-b', and `-c', and `-b' requires an argument, all of the
following are valid ways of invoking the program:

     prog -a -b foo -c data1 data2 data3
     prog -ac -bfoo -- data1 data2 data3
     prog -acbfoo data1 data2 data3

   Notice that when the argument is grouped with its option, the rest of
the command line argument is considered to be the option's argument.
In the above example, `-acbfoo' indicates that all of the `-a', `-b',
and `-c' options were supplied, and that `foo' is the argument to the
`-b' option.

   `getopt' provides four external variables that the programmer can
use.

`optind'
     The index in the argument value array (`argv') where the first
     non-option command line argument can be found.

`optarg'
     The string value of the argument to an option.

`opterr'
     Usually `getopt' prints an error message when it finds an invalid
     option.  Setting `opterr' to zero disables this feature.  (An
     application might wish to print its own error message.)

`optopt'
     The letter representing the command line option.  While not
     usually documented, most versions supply this variable.

   The following C fragment shows how `getopt' might process command
line arguments for `awk'.

     int
     main(int argc, char *argv[])
     {
         ...
         /* print our own message */
         opterr = 0;
         while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) {
             switch (c) {
             case 'f':    /* file */
                 ...
                 break;
             case 'F':    /* field separator */
                 ...
                 break;
             case 'v':    /* variable assignment */
                 ...
                 break;
             case 'W':    /* extension */
                 ...
                 break;
             case '?':
             default:
                 usage();
                 break;
             }
         }
         ...
     }

   As a side point, `gawk' actually uses the GNU `getopt_long' function
to process both normal and GNU-style long options (*note Command Line
Options: Options.).

   The abstraction provided by `getopt' is very useful, and would be
quite handy in `awk' programs as well.  Here is an `awk' version of
`getopt'.  This function highlights one of the greatest weaknesses in
`awk', which is that it is very poor at manipulating single characters.
Repeated calls to `substr' are necessary for accessing individual
characters (*note Built-in Functions for String Manipulation: String
Functions.).

   The discussion walks through the code a bit at a time.

     # getopt --- do C library getopt(3) function in awk
     #
     # arnold@gnu.ai.mit.edu
     # Public domain
     #
     # Initial version: March, 1991
     # Revised: May, 1993
     
     # External variables:
     #    Optind -- index of ARGV for first non-option argument
     #    Optarg -- string value of argument to current option
     #    Opterr -- if non-zero, print our own diagnostic
     #    Optopt -- current option letter
     
     # Returns
     #    -1     at end of options
     #    ?      for unrecognized option
     #    <c>    a character representing the current option
     
     # Private Data
     #    _opti  index in multi-flag option, e.g., -abc

   The function starts out with some documentation: who wrote the code,
and when it was revised, followed by a list of the global variables it
uses, what the return values are and what they mean, and any global
variables that are "private" to this library function.  Such
documentation is essential for any program, and particularly for
library functions.

     function getopt(argc, argv, options,    optl, thisopt, i)
     {
         optl = length(options)
         if (optl == 0)        # no options given
             return -1
     
         if (argv[Optind] == "--") {  # all done
             Optind++
             _opti = 0
             return -1
         } else if (argv[Optind] !~ /^-[^: \t\n\f\r\v\b]/) {
             _opti = 0
             return -1
         }

   The function first checks that it was indeed called with a string of
options (the `options' parameter).  If `options' has a zero length,
`getopt' immediately returns -1.

   The next thing to check for is the end of the options.  A `--' ends
the command line options, as does any command line argument that does
not begin with a `-'.  `Optind' is used to step through the array of
command line arguments; it retains its value across calls to `getopt',
since it is a global variable.

   The regexp used, `/^-[^: \t\n\f\r\v\b]/', is perhaps a bit of
overkill; it checks for a `-' followed by anything that is not
whitespace and not a colon.  If the current command line argument does
not match this pattern, it is not an option, and it ends option
processing.

         if (_opti == 0)
             _opti = 2
         thisopt = substr(argv[Optind], _opti, 1)
         Optopt = thisopt
         i = index(options, thisopt)
         if (i == 0) {
             if (Opterr)
                 printf("%c -- invalid option\n",
                                       thisopt) > "/dev/stderr"
             if (_opti >= length(argv[Optind])) {
                 Optind++
                 _opti = 0
             } else
                 _opti++
             return "?"
         }

   The `_opti' variable tracks the position in the current command line
argument (`argv[Optind]').  In the case that multiple options were
grouped together with one `-' (e.g., `-abx'), it is necessary to return
them to the user one at a time.

   If `_opti' is equal to zero, it is set to two, the index in the
string of the next character to look at (we skip the `-', which is at
position one).  The variable `thisopt' holds the character, obtained
with `substr'.  It is saved in `Optopt' for the main program to use.

   If `thisopt' is not in the `options' string, then it is an invalid
option.  If `Opterr' is non-zero, `getopt' prints an error message on
the standard error that is similar to the message from the C version of
`getopt'.

   Since the option is invalid, it is necessary to skip it and move on
to the next option character.  If `_opti' is greater than or equal to
the length of the current command line argument, then it is necessary
to move on to the next one, so `Optind' is incremented and `_opti' is
reset to zero. Otherwise, `Optind' is left alone and `_opti' is merely
incremented.

   In any case, since the option was invalid, `getopt' returns `?'.
The main program can examine `Optopt' if it needs to know what the
invalid option letter actually was.

         if (substr(options, i + 1, 1) == ":") {
             # get option argument
             if (length(substr(argv[Optind], _opti + 1)) > 0)
                 Optarg = substr(argv[Optind], _opti + 1)
             else
                 Optarg = argv[++Optind]
             _opti = 0
         } else
             Optarg = ""

   If the option requires an argument, the option letter is followed by
a colon in the `options' string.  If there are remaining characters in
the current command line argument (`argv[Optind]'), then the rest of
that string is assigned to `Optarg'.  Otherwise, the next command line
argument is used (`-xFOO' vs. `-x FOO'). In either case, `_opti' is
reset to zero, since there are no more characters left to examine in
the current command line argument.

         if (_opti == 0 || _opti >= length(argv[Optind])) {
             Optind++
             _opti = 0
         } else
             _opti++
         return thisopt
     }

   Finally, if `_opti' is either zero or greater than the length of the
current command line argument, it means this element in `argv' is
through being processed, so `Optind' is incremented to point to the
next element in `argv'.  If neither condition is true, then only
`_opti' is incremented, so that the next option letter can be processed
on the next call to `getopt'.

     BEGIN {
         Opterr = 1    # default is to diagnose
         Optind = 1    # skip ARGV[0]
     
         # test program
         if (_getopt_test) {
             while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1)
                 printf("c = <%c>, optarg = <%s>\n",
                                            _go_c, Optarg)
             printf("non-option arguments:\n")
             for (; Optind < ARGC; Optind++)
                 printf("\tARGV[%d] = <%s>\n",
                                         Optind, ARGV[Optind])
         }
     }

   The `BEGIN' rule initializes both `Opterr' and `Optind' to one.
`Opterr' is set to one, since the default behavior is for `getopt' to
print a diagnostic message upon seeing an invalid option.  `Optind' is
set to one, since there's no reason to look at the program name, which
is in `ARGV[0]'.

   The rest of the `BEGIN' rule is a simple test program.  Here is the
result of two sample runs of the test program.

     $ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x
     -| c = <a>, optarg = <>
     -| c = <c>, optarg = <>
     -| c = <b>, optarg = <ARG>
     -| non-option arguments:
     -|         ARGV[3] = <bax>
     -|         ARGV[4] = <-x>
     
     $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc
     -| c = <a>, optarg = <>
     error--> x -- invalid option
     -| c = <?>, optarg = <>
     -| non-option arguments:
     -|         ARGV[4] = <xyz>
     -|         ARGV[5] = <abc>

   The first `--' terminates the arguments to `awk', so that it does
not try to interpret the `-a' etc. as its own options.

   Several of the sample programs presented in *Note Practical `awk'
Programs: Sample Programs, use `getopt' to process their arguments.


File: gawk.info,  Node: Passwd Functions,  Next: Group Functions,  Prev: Getopt Function,  Up: Library Functions

Reading the User Database
=========================

   The `/dev/user' special file (*note Special File Names in `gawk':
Special Files.) provides access to the current user's real and
effective user and group id numbers, and if available, the user's
supplementary group set.  However, since these are numbers, they do not
provide very useful information to the average user.  There needs to be
some way to find the user information associated with the user and
group numbers.  This section presents a suite of functions for
retrieving information from the user database.  *Note Reading the Group
Database: Group Functions, for a similar suite that retrieves
information from the group database.

   The POSIX standard does not define the file where user information is
kept.  Instead, it provides the `<pwd.h>' header file and several C
language subroutines for obtaining user information.  The primary
function is `getpwent', for "get password entry." The "password" comes
from the original user database file, `/etc/passwd', which kept user
information, along with the encrypted passwords (hence the name).

   While an `awk' program could simply read `/etc/passwd' directly (the
format is well known), because of the way password files are handled on
networked systems, this file may not contain complete information about
the system's set of users.

   To be sure of being able to produce a readable, complete version of
the user database, it is necessary to write a small C program that
calls `getpwent'.  `getpwent' is defined to return a pointer to a
`struct passwd'.  Each time it is called, it returns the next entry in
the database.  When there are no more entries, it returns `NULL', the
null pointer.  When this happens, the C program should call `endpwent'
to close the database.  Here is `pwcat', a C program that "cats" the
password database.

     /*
      * pwcat.c
      *
      * Generate a printable version of the password database
      *
      * Arnold Robbins
      * arnold@gnu.ai.mit.edu
      * May 1993
      * Public Domain
      */
     
     #include <stdio.h>
     #include <pwd.h>
     
     int
     main(argc, argv)
     int argc;
     char **argv;
     {
         struct passwd *p;
     
         while ((p = getpwent()) != NULL)
             printf("%s:%s:%d:%d:%s:%s:%s\n",
                 p->pw_name, p->pw_passwd, p->pw_uid,
                 p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell);
     
         endpwent();
         exit(0);
     }

   If you don't understand C, don't worry about it.  The output from
`pwcat' is the user database, in the traditional `/etc/passwd' format
of colon-separated fields.  The fields are:

Login name
     The user's login name.

Encrypted password
     The user's encrypted password.  This may not be available on some
     systems.

User-ID
     The user's numeric user-id number.

Group-ID
     The user's numeric group-id number.

Full name
     The user's full name, and perhaps other information associated
     with the user.

Home directory
     The user's login, or "home" directory (familiar to shell
     programmers as `$HOME').

Login shell
     The program that will be run when the user logs in.  This is
     usually a shell, such as Bash (the Gnu Bourne-Again shell).

   Here are a few lines representative of `pwcat''s output.

     $ pwcat
     -| root:3Ov02d5VaUPB6:0:1:Operator:/:/bin/sh
     -| nobody:*:65534:65534::/:
     -| daemon:*:1:1::/:
     -| sys:*:2:2::/:/bin/csh
     -| bin:*:3:3::/bin:
     -| arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh
     -| miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh
     -| andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh
     ...

   With that introduction, here is a group of functions for getting user
information.  There are several functions here, corresponding to the C
functions of the same name.

     # passwd.awk --- access password file information
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     BEGIN {
         # tailor this to suit your system
         _pw_awklib = "/usr/local/libexec/awk/"
     }
     
     function _pw_init(    oldfs, oldrs, olddol0, pwcat)
     {
         if (_pw_inited)
             return
         oldfs = FS
         oldrs = RS
         olddol0 = $0
         FS = ":"
         RS = "\n"
         pwcat = _pw_awklib "pwcat"
         while ((pwcat | getline) > 0) {
             _pw_byname[$1] = $0
             _pw_byuid[$3] = $0
             _pw_bycount[++_pw_total] = $0
         }
         close(pwcat)
         _pw_count = 0
         _pw_inited = 1
         FS = oldfs
         RS = oldrs
         $0 = olddol0
     }

   The `BEGIN' rule sets a private variable to the directory where
`pwcat' is stored.  Since it is used to help out an `awk' library
routine, we have chosen to put it in `/usr/local/libexec/awk'.  You
might want it to be in a different directory on your system.

   The function `_pw_init' keeps three copies of the user information
in three associative arrays.  The arrays are indexed by user name
(`_pw_byname'), by user-id number (`_pw_byuid'), and by order of
occurrence (`_pw_bycount').

   The variable `_pw_inited' is used for efficiency; `_pw_init' only
needs to be called once.

   Since this function uses `getline' to read information from `pwcat',
it first saves the values of `FS', `RS', and `$0'.  Doing so is
necessary, since these functions could be called from anywhere within a
user's program, and the user may have his or her own values for `FS'
and `RS'.

   The main part of the function uses a loop to read database lines,
split the line into fields, and then store the line into each array as
necessary.  When the loop is done, `_pw_init' cleans up by closing the
pipeline, setting `_pw_inited' to one, and restoring `FS', `RS', and
`$0'.  The use of `_pw_count' will be explained below.

     function getpwnam(name)
     {
         _pw_init()
         if (name in _pw_byname)
             return _pw_byname[name]
         return ""
     }

   The `getpwnam' function takes a user name as a string argument. If
that user is in the database, it returns the appropriate line.
Otherwise it returns the null string.

     function getpwuid(uid)
     {
         _pw_init()
         if (uid in _pw_byuid)
             return _pw_byuid[uid]
         return ""
     }

   Similarly, the `getpwuid' function takes a user-id number argument.
If that user number is in the database, it returns the appropriate
line. Otherwise it returns the null string.

     function getpwent()
     {
         _pw_init()
         if (_pw_count < _pw_total)
             return _pw_bycount[++_pw_count]
         return ""
     }

   The `getpwent' function simply steps through the database, one entry
at a time.  It uses `_pw_count' to track its current position in the
`_pw_bycount' array.

     function endpwent()
     {
         _pw_count = 0
     }

   The `endpwent' function resets `_pw_count' to zero, so that
subsequent calls to `getpwent' will start over again.

   A conscious design decision in this suite is that each subroutine
calls `_pw_init' to initialize the database arrays.  The overhead of
running a separate process to generate the user database, and the I/O
to scan it, will only be incurred if the user's main program actually
calls one of these functions.  If this library file is loaded along
with a user's program, but none of the routines are ever called, then
there is no extra run-time overhead.  (The alternative would be to move
the body of `_pw_init' into a `BEGIN' rule, which would always run
`pwcat'.  This simplifies the code but runs an extra process that may
never be needed.)

   In turn, calling `_pw_init' is not too expensive, since the
`_pw_inited' variable keeps the program from reading the data more than
once.  If you are worried about squeezing every last cycle out of your
`awk' program, the check of `_pw_inited' could be moved out of
`_pw_init' and duplicated in all the other functions.  In practice,
this is not necessary, since most `awk' programs are I/O bound, and it
would clutter up the code.

   The `id' program in *Note Printing Out User Information: Id Program,
uses these functions.


File: gawk.info,  Node: Group Functions,  Next: Library Names,  Prev: Passwd Functions,  Up: Library Functions

Reading the Group Database
==========================

   Much of the discussion presented in *Note Reading the User Database:
Passwd Functions, applies to the group database as well.  Although
there has traditionally been a well known file, `/etc/group', in a well
known format, the POSIX standard only provides a set of C library
routines (`<grp.h>' and `getgrent') for accessing the information.
Even though this file may exist, it likely does not have complete
information.  Therefore, as with the user database, it is necessary to
have a small C program that generates the group database as its output.

   Here is `grcat', a C program that "cats" the group database.

     /*
      * grcat.c
      *
      * Generate a printable version of the group database
      *
      * Arnold Robbins, arnold@gnu.ai.mit.edu
      * May 1993
      * Public Domain
      */
     
     #include <stdio.h>
     #include <grp.h>
     
     int
     main(argc, argv)
     int argc;
     char **argv;
     {
         struct group *g;
         int i;
     
         while ((g = getgrent()) != NULL) {
             printf("%s:%s:%d:", g->gr_name, g->gr_passwd,
                                                 g->gr_gid);
             for (i = 0; g->gr_mem[i] != NULL; i++) {
                 printf("%s", g->gr_mem[i]);
                 if (g->gr_mem[i+1] != NULL)
                     putchar(',');
             }
             putchar('\n');
         }
         endgrent();
         exit(0);
     }

   Each line in the group database represent one group.  The fields are
separated with colons, and represent the following information.

Group Name
     The name of the group.

Group Password
     The encrypted group password. In practice, this field is never
     used. It is usually empty, or set to `*'.

Group ID Number
     The numeric group-id number. This number should be unique within
     the file.

Group Member List
     A comma-separated list of user names.  These users are members of
     the group.  Most Unix systems allow users to be members of several
     groups simultaneously.  If your system does, then reading
     `/dev/user' will return those group-id numbers in `$5' through
     `$NF'.  (Note that `/dev/user' is a `gawk' extension; *note
     Special File Names in `gawk': Special Files..)

   Here is what running `grcat' might produce:

     $ grcat
     -| wheel:*:0:arnold
     -| nogroup:*:65534:
     -| daemon:*:1:
     -| kmem:*:2:
     -| staff:*:10:arnold,miriam,andy
     -| other:*:20:
     ...

   Here are the functions for obtaining information from the group
database.  There are several, modeled after the C library functions of
the same names.

     # group.awk --- functions for dealing with the group file
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     BEGIN    \
     {
         # Change to suit your system
         _gr_awklib = "/usr/local/libexec/awk/"
     }
     
     function _gr_init(    oldfs, oldrs, olddol0, grcat, n, a, i)
     {
         if (_gr_inited)
             return
     
         oldfs = FS
         oldrs = RS
         olddol0 = $0
         FS = ":"
         RS = "\n"
     
         grcat = _gr_awklib "grcat"
         while ((grcat | getline) > 0) {
             if ($1 in _gr_byname)
                 _gr_byname[$1] = _gr_byname[$1] "," $4
             else
                 _gr_byname[$1] = $0
             if ($3 in _gr_bygid)
                 _gr_bygid[$3] = _gr_bygid[$3] "," $4
             else
                 _gr_bygid[$3] = $0
     
             n = split($4, a, "[ \t]*,[ \t]*")
             for (i = 1; i <= n; i++)
                 if (a[i] in _gr_groupsbyuser)
                     _gr_groupsbyuser[a[i]] = \
                         _gr_groupsbyuser[a[i]] " " $1
                 else
                     _gr_groupsbyuser[a[i]] = $1
     
             _gr_bycount[++_gr_count] = $0
         }
         close(grcat)
         _gr_count = 0
         _gr_inited++
         FS = oldfs
         RS = oldrs
         $0 = olddol0
     }

   The `BEGIN' rule sets a private variable to the directory where
`grcat' is stored.  Since it is used to help out an `awk' library
routine, we have chosen to put it in `/usr/local/libexec/awk'.  You
might want it to be in a different directory on your system.

   These routines follow the same general outline as the user database
routines (*note Reading the User Database: Passwd Functions.).  The
`_gr_inited' variable is used to ensure that the database is scanned no
more than once.  The `_gr_init' function first saves `FS', `RS', and
`$0', and then sets `FS' and `RS' to the correct values for scanning
the group information.

   The group information is stored is several associative arrays.  The
arrays are indexed by group name (`_gr_byname'), by group-id number
(`_gr_bygid'), and by position in the database (`_gr_bycount').  There
is an additional array indexed by user name (`_gr_groupsbyuser'), that
is a space separated list of groups that each user belongs to.

   Unlike the user database, it is possible to have multiple records in
the database for the same group.  This is common when a group has a
large number of members.  Such a pair of entries might look like:

     tvpeople:*:101:johny,jay,arsenio
     tvpeople:*:101:david,conan,tom,joan

   For this reason, `_gr_init' looks to see if a group name or group-id
number has already been seen.  If it has, then the user names are
simply concatenated onto the previous list of users.  (There is
actually a subtle problem with the code presented above.  Suppose that
the first time there were no names. This code adds the names with a
leading comma. It also doesn't check that there is a `$4'.)

   Finally, `_gr_init' closes the pipeline to `grcat', restores `FS',
`RS', and `$0', initializes `_gr_count' to zero (it is used later), and
makes `_gr_inited' non-zero.

     function getgrnam(group)
     {
         _gr_init()
         if (group in _gr_byname)
             return _gr_byname[group]
         return ""
     }

   The `getgrnam' function takes a group name as its argument, and if
that group exists, it is returned. Otherwise, `getgrnam' returns the
null string.

     function getgrgid(gid)
     {
         _gr_init()
         if (gid in _gr_bygid)
             return _gr_bygid[gid]
         return ""
     }

   The `getgrgid' function is similar, it takes a numeric group-id, and
looks up the information associated with that group-id.

     function getgruser(user)
     {
         _gr_init()
         if (user in _gr_groupsbyuser)
             return _gr_groupsbyuser[user]
         return ""
     }

   The `getgruser' function does not have a C counterpart. It takes a
user name, and returns the list of groups that have the user as a
member.

     function getgrent()
     {
         _gr_init()
         if (++gr_count in _gr_bycount)
             return _gr_bycount[_gr_count]
         return ""
     }

   The `getgrent' function steps through the database one entry at a
time.  It uses `_gr_count' to track its position in the list.

     function endgrent()
     {
         _gr_count = 0
     }

   `endgrent' resets `_gr_count' to zero so that `getgrent' can start
over again.

   As with the user database routines, each function calls `_gr_init' to
initialize the arrays.  Doing so only incurs the extra overhead of
running `grcat' if these functions are used (as opposed to moving the
body of `_gr_init' into a `BEGIN' rule).

   Most of the work is in scanning the database and building the various
associative arrays.  The functions that the user calls are themselves
very simple, relying on `awk''s associative arrays to do work.

   The `id' program in *Note Printing Out User Information: Id Program,
uses these functions.


File: gawk.info,  Node: Library Names,  Prev: Group Functions,  Up: Library Functions

Naming Library Function Global Variables
========================================

   Due to the way the `awk' language evolved, variables are either
"global" (usable by the entire program), or "local" (usable just by a
specific function).  There is no intermediate state analogous to
`static' variables in C.

   Library functions often need to have global variables that they can
use to preserve state information between calls to the function. For
example, `getopt''s variable `_opti' (*note Processing Command Line
Options: Getopt Function.), and the `_tm_months' array used by `mktime'
(*note Turning Dates Into Timestamps: Mktime Function.).  Such
variables are called "private", since the only functions that need to
use them are the ones in the library.

   When writing a library function, you should try to choose names for
your private variables so that they will not conflict with any
variables used by either another library function or a user's main
program.  For example, a name like `i' or `j' is not a good choice,
since user programs often use variable names like these for their own
purposes.

   The example programs shown in this chapter all start the names of
their private variables with an underscore (`_').  Users generally
don't use leading underscores in their variable names, so this
convention immediately decreases the chances that the variable name
will be accidentally shared with the user's program.

   In addition, several of the library functions use a prefix that helps
indicate what function or set of functions uses the variables. For
example, `_tm_months' in `mktime' (*note Turning Dates Into Timestamps:
Mktime Function.), and `_pw_byname' in the user data base routines
(*note Reading the User Database: Passwd Functions.).  This convention
is recommended, since it even further decreases the chance of
inadvertent conflict among variable names.  Note that this convention
can be used equally well both for variable names and for private
function names too.

   While I could have re-written all the library routines to use this
convention, I did not do so, in order to show how my own `awk'
programming style has evolved, and to provide some basis for this
discussion.

   As a final note on variable naming, if a function makes global
variables available for use by a main program, it is a good convention
to start that variable's name with a capital letter.  For example,
`getopt''s `Opterr' and `Optind' variables (*note Processing Command
Line Options: Getopt Function.).  The leading capital letter indicates
that it is global, while the fact that the variable name is not all
capital letters indicates that the variable is not one of `awk''s
built-in variables, like `FS'.

   It is also important that *all* variables in library functions that
do not need to save state are in fact declared local.  If this is not
done, the variable could accidentally be used in the user's program,
leading to bugs that are very difficult to track down.

     function lib_func(x, y,    l1, l2)
     {
         ...
         USE VARIABLE some_var  # some_var could be local
         ...                   # but is not by oversight
     }

   A different convention, common in the Tcl community, is to use a
single associative array to hold the values needed by the library
function(s), or "package."  This significantly decreases the number of
actual global names in use.  For example, the functions described in
*Note Reading the User Database: Passwd Functions, might have used
`PW_data["inited"]', `PW_data["total"]', `PW_data["count"]' and
`PW_data["awklib"]', instead of `_pw_inited', `_pw_awklib', `_pw_total',
and `_pw_count'.

   The conventions presented in this section are exactly that,
conventions. You are not required to write your programs this way, we
merely recommend that you do so.


File: gawk.info,  Node: Sample Programs,  Next: Language History,  Prev: Library Functions,  Up: Top

Practical `awk' Programs
************************

   This chapter presents a potpourri of `awk' programs for your reading
enjoyment.

   Many of these programs use the library functions presented in *Note
A Library of `awk' Functions: Library Functions.

* Menu:

* Clones::                    Clones of common utilities.
* Miscellaneous Programs::    Some interesting `awk' programs.


File: gawk.info,  Node: Clones,  Next: Miscellaneous Programs,  Prev: Sample Programs,  Up: Sample Programs

Re-inventing Wheels for Fun and Profit
======================================

   This section presents a number of POSIX utilities that are
implemented in `awk'.  Re-inventing these programs in `awk' is often
enjoyable, since the algorithms can be very clearly expressed, and
usually the code is very concise and simple.  This is true because
`awk' does so much for you.

   It should be noted that these programs are not necessarily intended
to replace the installed versions on your system.  Instead, their
purpose is to illustrate `awk' language programming for "real world"
tasks.

   The programs are presented in alphabetical order.

* Menu:

* Cut Program::             The `cut' utility.
* Egrep Program::           The `egrep' utility.
* Id Program::              The `id' utility.
* Split Program::           The `split' utility.
* Tee Program::             The `tee' utility.
* Uniq Program::            The `uniq' utility.
* Wc Program::              The `wc' utility.


File: gawk.info,  Node: Cut Program,  Next: Egrep Program,  Prev: Clones,  Up: Clones

Cutting Out Fields and Columns
------------------------------

   The `cut' utility selects, or "cuts," either characters or fields
from its standard input and sends them to its standard output.  `cut'
can cut out either a list of characters, or a list of fields.  By
default, fields are separated by tabs, but you may supply a command
line option to change the field "delimiter", i.e. the field separator
character. `cut''s definition of fields is less general than `awk''s.

   A common use of `cut' might be to pull out just the login name of
logged-on users from the output of `who'.  For example, the following
pipeline generates a sorted, unique list of the logged on users:

     who | cut -c1-8 | sort | uniq

   The options for `cut' are:

`-c LIST'
     Use LIST as the list of characters to cut out.  Items within the
     list may be separated by commas, and ranges of characters can be
     separated with dashes.  The list `1-8,15,22-35' specifies
     characters one through eight, 15, and 22 through 35.

`-f LIST'
     Use LIST as the list of fields to cut out.

`-d DELIM'
     Use DELIM as the field separator character instead of the tab
     character.

`-s'
     Suppress printing of lines that do not contain the field delimiter.

   The `awk' implementation of `cut' uses the `getopt' library function
(*note Processing Command Line Options: Getopt Function.), and the
`join' library function (*note Merging an Array Into a String: Join
Function.).

   The program begins with a comment describing the options and a
`usage' function which prints out a usage message and exits.  `usage'
is called if invalid arguments are supplied.

     # cut.awk --- implement cut in awk
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     # Options:
     #    -f list        Cut fields
     #    -d c           Field delimiter character
     #    -c list        Cut characters
     #
     #    -s        Suppress lines without the delimiter character
     
     function usage(    e1, e2)
     {
         e1 = "usage: cut [-f list] [-d c] [-s] [files...]"
         e2 = "usage: cut [-c list] [files...]"
         print e1 > "/dev/stderr"
         print e2 > "/dev/stderr"
         exit 1
     }

The variables `e1' and `e2' are used so that the function fits nicely
on the screen.

   Next comes a `BEGIN' rule that parses the command line options.  It
sets `FS' to a single tab character, since that is `cut''s default
field separator.  The output field separator is also set to be the same
as the input field separator.  Then `getopt' is used to step through
the command line options.  One or the other of the variables
`by_fields' or `by_chars' is set to true, to indicate that processing
should be done by fields or by characters respectively.  When cutting
by characters, the output field separator is set to the null string.

     BEGIN    \
     {
         FS = "\t"    # default
         OFS = FS
         while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {
             if (c == "f") {
                 by_fields = 1
                 fieldlist = Optarg
             } else if (c == "c") {
                 by_chars = 1
                 fieldlist = Optarg
                 OFS = ""
             } else if (c == "d") {
                 if (length(Optarg) > 1) {
                     printf("Using first character of %s" \
                     " for delimiter\n", Optarg) > "/dev/stderr"
                     Optarg = substr(Optarg, 1, 1)
                 }
                 FS = Optarg
                 OFS = FS
                 if (FS == " ")    # defeat awk semantics
                     FS = "[ ]"
             } else if (c == "s")
                 suppress++
             else
                 usage()
         }
     
         for (i = 1; i < Optind; i++)
             ARGV[i] = ""

   Special care is taken when the field delimiter is a space. Using
`" "' (a single space) for the value of `FS' is incorrect--`awk' would
separate fields with runs of spaces, tabs and/or newlines, and we want
them to be separated with individual spaces.  Also, note that after
`getopt' is through, we have to clear out all the elements of `ARGV'
from one to `Optind', so that `awk' will not try to process the command
line options as file names.

   After dealing with the command line options, the program verifies
that the options make sense.  Only one or the other of `-c' and `-f'
should be used, and both require a field list.  Then either
`set_fieldlist' or `set_charlist' is called to pull apart the list of
fields or characters.

         if (by_fields && by_chars)
             usage()
     
         if (by_fields == 0 && by_chars == 0)
             by_fields = 1    # default
     
         if (fieldlist == "") {
             print "cut: needs list for -c or -f" > "/dev/stderr"
             exit 1
         }
     
         if (by_fields)
             set_fieldlist()
         else
             set_charlist()
     }

   Here is `set_fieldlist'.  It first splits the field list apart at
the commas, into an array.  Then, for each element of the array, it
looks to see if it is actually a range, and if so splits it apart. The
range is verified to make sure the first number is smaller than the
second.  Each number in the list is added to the `flist' array, which
simply lists the fields that will be printed.  Normal field splitting
is used.  The program lets `awk' handle the job of doing the field
splitting.

     function set_fieldlist(        n, m, i, j, k, f, g)
     {
         n = split(fieldlist, f, ",")
         j = 1    # index in flist
         for (i = 1; i <= n; i++) {
             if (index(f[i], "-") != 0) { # a range
                 m = split(f[i], g, "-")
                 if (m != 2 || g[1] >= g[2]) {
                     printf("bad field list: %s\n",
                                       f[i]) > "/dev/stderr"
                     exit 1
                 }
                 for (k = g[1]; k <= g[2]; k++)
                     flist[j++] = k
             } else
                 flist[j++] = f[i]
         }
         nfields = j - 1
     }

   The `set_charlist' function is more complicated than `set_fieldlist'.
The idea here is to use `gawk''s `FIELDWIDTHS' variable (*note Reading
Fixed-width Data: Constant Size.), which describes constant width
input.  When using a character list, that is exactly what we have.

   Setting up `FIELDWIDTHS' is more complicated than simply listing the
fields that need to be printed.  We have to keep track of the fields to
be printed, and also the intervening characters that have to be skipped.
For example, suppose you wanted characters one through eight, 15, and
22 through 35.  You would use `-c 1-8,15,22-35'.  The necessary value
for `FIELDWIDTHS' would be `"8 6 1 6 14"'.  This gives us five fields,
and what should be printed are `$1', `$3', and `$5'.  The intermediate
fields are "filler," stuff in between the desired data.

   `flist' lists the fields to be printed, and `t' tracks the complete
field list, including filler fields.

     function set_charlist(    field, i, j, f, g, t,
                               filler, last, len)
     {
         field = 1   # count total fields
         n = split(fieldlist, f, ",")
         j = 1       # index in flist
         for (i = 1; i <= n; i++) {
             if (index(f[i], "-") != 0) { # range
                 m = split(f[i], g, "-")
                 if (m != 2 || g[1] >= g[2]) {
                     printf("bad character list: %s\n",
                                    f[i]) > "/dev/stderr"
                     exit 1
                 }
                 len = g[2] - g[1] + 1
                 if (g[1] > 1)  # compute length of filler
                     filler = g[1] - last - 1
                 else
                     filler = 0
                 if (filler)
                     t[field++] = filler
                 t[field++] = len  # length of field
                 last = g[2]
                 flist[j++] = field - 1
             } else {
                 if (f[i] > 1)
                     filler = f[i] - last - 1
                 else
                     filler = 0
                 if (filler)
                     t[field++] = filler
                 t[field++] = 1
                 last = f[i]
                 flist[j++] = field - 1
             }
         }
         FIELDWIDTHS = join(t, 1, field - 1)
         nfields = j - 1
     }

   Here is the rule that actually processes the data.  If the `-s'
option was given, then `suppress' will be true.  The first `if'
statement makes sure that the input record does have the field
separator.  If `cut' is processing fields, `suppress' is true, and the
field separator character is not in the record, then the record is
skipped.

   If the record is valid, then at this point, `gawk' has split the data
into fields, either using the character in `FS' or using fixed-length
fields and `FIELDWIDTHS'.  The loop goes through the list of fields
that should be printed.  If the corresponding field has data in it, it
is printed.  If the next field also has data, then the separator
character is written out in between the fields.

     {
         if (by_fields && suppress && $0 !~ FS)
             next
     
         for (i = 1; i <= nfields; i++) {
             if ($flist[i] != "") {
                 printf "%s", $flist[i]
                 if (i < nfields && $flist[i+1] != "")
                     printf "%s", OFS
             }
         }
         print ""
     }

   This version of `cut' relies on `gawk''s `FIELDWIDTHS' variable to
do the character-based cutting.  While it would be possible in other
`awk' implementations to use `substr' (*note Built-in Functions for
String Manipulation: String Functions.), it would also be extremely
painful to do so.  The `FIELDWIDTHS' variable supplies an elegant
solution to the problem of picking the input line apart by characters.


File: gawk.info,  Node: Egrep Program,  Next: Id Program,  Prev: Cut Program,  Up: Clones

Searching for Regular Expressions in Files
------------------------------------------

   The `egrep' utility searches files for patterns.  It uses regular
expressions that are almost identical to those available in `awk'
(*note Regular Expression Constants: Regexp Constants.).  It is used
this way:

     egrep [ OPTIONS ] 'PATTERN' FILES ...

   The PATTERN is a regexp.  In typical usage, the regexp is quoted to
prevent the shell from expanding any of the special characters as file
name wildcards.  Normally, `egrep' prints the lines that matched.  If
multiple file names are provided on the command line, each output line
is preceded by the name of the file and a colon.

   The options are:

`-c'
     Print out a count of the lines that matched the pattern, instead
     of the lines themselves.

`-s'
     Be silent.  No output is produced, and the exit value indicates
     whether or not the pattern was matched.

`-v'
     Invert the sense of the test. `egrep' prints the lines that do
     *not* match the pattern, and exits successfully if the pattern was
     not matched.

`-i'
     Ignore case distinctions in both the pattern and the input data.

`-l'
     Only print the names of the files that matched, not the lines that
     matched.

`-e PATTERN'
     Use PATTERN as the regexp to match.  The purpose of the `-e'
     option is to allow patterns that start with a `-'.

   This version uses the `getopt' library function (*note Processing
Command Line Options: Getopt Function.), and the file transition
library program (*note Noting Data File Boundaries: Filetrans
Function.).

   The program begins with a descriptive comment, and then a `BEGIN'
rule that processes the command line arguments with `getopt'.  The `-i'
(ignore case) option is particularly easy with `gawk'; we just use the
`IGNORECASE' built in variable (*note Built-in Variables::.).

     # egrep.awk --- simulate egrep in awk
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     # Options:
     #    -c    count of lines
     #    -s    silent - use exit value
     #    -v    invert test, success if no match
     #    -i    ignore case
     #    -l    print filenames only
     #    -e    argument is pattern
     
     BEGIN {
         while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) {
             if (c == "c")
                 count_only++
             else if (c == "s")
                 no_print++
             else if (c == "v")
                 invert++
             else if (c == "i")
                 IGNORECASE = 1
             else if (c == "l")
                 filenames_only++
             else if (c == "e")
                 pattern = Optarg
             else
                 usage()
         }

   Next comes the code that handles the `egrep' specific behavior. If no
pattern was supplied with `-e', the first non-option on the command
line is used.  The `awk' command line arguments up to `ARGV[Optind]'
are cleared, so that `awk' won't try to process them as files.  If no
files were specified, the standard input is used, and if multiple files
were specified, we make sure to note this so that the file names can
precede the matched lines in the output.

   The last two lines are commented out, since they are not needed in
`gawk'.  They should be uncommented if you have to use another version
of `awk'.

         if (pattern == "")
             pattern = ARGV[Optind++]
     
         for (i = 1; i < Optind; i++)
             ARGV[i] = ""
         if (Optind >= ARGC) {
             ARGV[1] = "-"
             ARGC = 2
         } else if (ARGC - Optind > 1)
             do_filenames++
     
     #    if (IGNORECASE)
     #        pattern = tolower(pattern)
     }

   The next set of lines should be uncommented if you are not using
`gawk'.  This rule translates all the characters in the input line into
lower-case if the `-i' option was specified.  The rule is commented out
since it is not necessary with `gawk'.

     #{
     #    if (IGNORECASE)
     #        $0 = tolower($0)
     #}

   The `beginfile' function is called by the rule in `ftrans.awk' when
each new file is processed.  In this case, it is very simple; all it
does is initialize a variable `fcount' to zero. `fcount' tracks how
many lines in the current file matched the pattern.

     function beginfile(junk)
     {
         fcount = 0
     }

   The `endfile' function is called after each file has been processed.
It is used only when the user wants a count of the number of lines that
matched.  `no_print' will be true only if the exit status is desired.
`count_only' will be true if line counts are desired.  `egrep' will
therefore only print line counts if printing and counting are enabled.
The output format must be adjusted depending upon the number of files
to be processed.  Finally, `fcount' is added to `total', so that we
know how many lines altogether matched the pattern.

     function endfile(file)
     {
         if (! no_print && count_only)
             if (do_filenames)
                 print file ":" fcount
             else
                 print fcount
     
         total += fcount
     }

   This rule does most of the work of matching lines. The variable
`matches' will be true if the line matched the pattern. If the user
wants lines that did not match, the sense of the `matches' is inverted
using the `!' operator. `fcount' is incremented with the value of
`matches', which will be either one or zero, depending upon a
successful or unsuccessful match.  If the line did not match, the
`next' statement just moves on to the next record.

   There are several optimizations for performance in the following few
lines of code. If the user only wants exit status (`no_print' is true),
and we don't have to count lines, then it is enough to know that one
line in this file matched, and we can skip on to the next file with
`nextfile'.  Along similar lines, if we are only printing file names,
and we don't need to count lines, we can print the file name, and then
skip to the next file with `nextfile'.

   Finally, each line is printed, with a leading filename and colon if
necessary.

     {
         matches = ($0 ~ pattern)
         if (invert)
             matches = ! matches
     
         fcount += matches    # 1 or 0
     
         if (! matches)
             next
     
         if (no_print && ! count_only)
             nextfile
     
         if (filenames_only && ! count_only) {
             print FILENAME
             nextfile
         }
     
         if (do_filenames && ! count_only)
             print FILENAME ":" $0
         else if (! count_only)
             print
     }

   The `END' rule takes care of producing the correct exit status. If
there were no matches, the exit status is one, otherwise it is zero.

     END    \
     {
         if (total == 0)
             exit 1
         exit 0
     }

   The `usage' function prints a usage message in case of invalid
options and then exits.

     function usage(    e)
     {
         e = "Usage: egrep [-csvil] [-e pat] [files ...]"
         print e > "/dev/stderr"
         exit 1
     }

   The variable `e' is used so that the function fits nicely on the
printed page.

   Just a note on programming style. You may have noticed that the `END'
rule uses backslash continuation, with the open brace on a line by
itself.  This is so that it more closely resembles the way functions
are written.  Many of the examples use this style. You can decide for
yourself if you like writing your `BEGIN' and `END' rules this way, or
not.


File: gawk.info,  Node: Id Program,  Next: Split Program,  Prev: Egrep Program,  Up: Clones

Printing Out User Information
-----------------------------

   The `id' utility lists a user's real and effective user-id numbers,
real and effective group-id numbers, and the user's group set, if any.
`id' will only print the effective user-id and group-id if they are
different from the real ones.  If possible, `id' will also supply the
corresponding user and group names.  The output might look like this:

     $ id
     -| uid=2076(arnold) gid=10(staff) groups=10(staff),4(tty)

   This information is exactly what is provided by `gawk''s `/dev/user'
special file (*note Special File Names in `gawk': Special Files.).
However, the `id' utility provides a more palatable output than just a
string of numbers.

   Here is a simple version of `id' written in `awk'.  It uses the user
database library functions (*note Reading the User Database: Passwd
Functions.), and the group database library functions (*note Reading
the Group Database: Group Functions.).

   The program is fairly straightforward.  All the work is done in the
`BEGIN' rule.  The user and group id numbers are obtained from
`/dev/user'.  If there is no support for `/dev/user', the program gives
up.

   The code is repetitive.  The entry in the user database for the real
user-id number is split into parts at the `:'. The name is the first
field.  Similar code is used for the effective user-id number, and the
group numbers.

     # id.awk --- implement id in awk
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     # output is:
     # uid=12(foo) euid=34(bar) gid=3(baz) \
     #             egid=5(blat) groups=9(nine),2(two),1(one)
     
     BEGIN    \
     {
         if ((getline < "/dev/user") < 0) {
             err = "id: no /dev/user support - cannot run"
             print err > "/dev/stderr"
             exit 1
         }
         close("/dev/user")
     
         uid = $1
         euid = $2
         gid = $3
         egid = $4
     
         printf("uid=%d", uid)
         pw = getpwuid(uid)
         if (pw != "") {
             split(pw, a, ":")
             printf("(%s)", a[1])
         }
     
         if (euid != uid) {
             printf(" euid=%d", euid)
             pw = getpwuid(euid)
             if (pw != "") {
                 split(pw, a, ":")
                 printf("(%s)", a[1])
             }
         }
     
         printf(" gid=%d", gid)
         pw = getgrgid(gid)
         if (pw != "") {
             split(pw, a, ":")
             printf("(%s)", a[1])
         }
     
         if (egid != gid) {
             printf(" egid=%d", egid)
             pw = getgrgid(egid)
             if (pw != "") {
                 split(pw, a, ":")
                 printf("(%s)", a[1])
             }
         }
     
         if (NF > 4) {
             printf(" groups=");
             for (i = 5; i <= NF; i++) {
                 printf("%d", $i)
                 pw = getgrgid($i)
                 if (pw != "") {
                     split(pw, a, ":")
                     printf("(%s)", a[1])
                 }
                 if (i < NF)
                     printf(",")
             }
         }
         print ""
     }


File: gawk.info,  Node: Split Program,  Next: Tee Program,  Prev: Id Program,  Up: Clones

Splitting a Large File Into Pieces
----------------------------------

   The `split' program splits large text files into smaller pieces. By
default, the output files are named `xaa', `xab', and so on. Each file
has 1000 lines in it, with the likely exception of the last file. To
change the number of lines in each file, you supply a number on the
command line preceded with a minus, e.g., `-500' for files with 500
lines in them instead of 1000.  To change the name of the output files
to something like `myfileaa', `myfileab', and so on, you supply an
additional argument that specifies the filename.

   Here is a version of `split' in `awk'. It uses the `ord' and `chr'
functions presented in *Note Translating Between Characters and
Numbers: Ordinal Functions.

   The program first sets its defaults, and then tests to make sure
there are not too many arguments.  It then looks at each argument in
turn.  The first argument could be a minus followed by a number. If it
is, this happens to look like a negative number, so it is made
positive, and that is the count of lines.  The data file name is
skipped over, and the final argument is used as the prefix for the
output file names.

     # split.awk --- do split in awk
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     # usage: split [-num] [file] [outname]
     
     BEGIN {
         outfile = "x"    # default
         count = 1000
         if (ARGC > 4)
             usage()
     
         i = 1
         if (ARGV[i] ~ /^-[0-9]+$/) {
             count = -ARGV[i]
             ARGV[i] = ""
             i++
         }
         # test argv in case reading from stdin instead of file
         if (i in ARGV)
             i++    # skip data file name
         if (i in ARGV) {
             outfile = ARGV[i]
             ARGV[i] = ""
         }
     
         s1 = s2 = "a"
         out = (outfile s1 s2)
     }

   The next rule does most of the work. `tcount' (temporary count)
tracks how many lines have been printed to the output file so far. If
it is greater than `count', it is time to close the current file and
start a new one.  `s1' and `s2' track the current suffixes for the file
name. If they are both `z', the file is just too big.  Otherwise, `s1'
moves to the next letter in the alphabet and `s2' starts over again at
`a'.

     {
         if (++tcount > count) {
             close(out)
             if (s2 == "z") {
                 if (s1 == "z") {
                     printf("split: %s is too large to split\n", \
                            FILENAME) > "/dev/stderr"
                     exit 1
                 }
                 s1 = chr(ord(s1) + 1)
                 s2 = "a"
             } else
                 s2 = chr(ord(s2) + 1)
             out = (outfile s1 s2)
             tcount = 1
         }
         print > out
     }

   The `usage' function simply prints an error message and exits.

     function usage(   e)
     {
         e = "usage: split [-num] [file] [outname]"
         print e > "/dev/stderr"
         exit 1
     }

The variable `e' is used so that the function fits nicely on the screen.

   This program is a bit sloppy; it relies on `awk' to close the last
file for it automatically, instead of doing it in an `END' rule.


File: gawk.info,  Node: Tee Program,  Next: Uniq Program,  Prev: Split Program,  Up: Clones

Duplicating Output Into Multiple Files
--------------------------------------

   The `tee' program is known as a "pipe fitting."  `tee' copies its
standard input to its standard output, and also duplicates it to the
files named on the command line.  Its usage is:

     tee [-a] file ...

   The `-a' option tells `tee' to append to the named files, instead of
truncating them and starting over.

   The `BEGIN' rule first makes a copy of all the command line
arguments, into an array named `copy'.  `ARGV[0]' is not copied, since
it is not needed.  `tee' cannot use `ARGV' directly, since `awk' will
attempt to process each file named in `ARGV' as input data.

   If the first argument is `-a', then the flag variable `append' is
set to true, and both `ARGV[1]' and `copy[1]' are deleted. If `ARGC' is
less than two, then no file names were supplied, and `tee' prints a
usage message and exits.  Finally, `awk' is forced to read the standard
input by setting `ARGV[1]' to `"-"', and `ARGC' to two.

     # tee.awk --- tee in awk
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     # Revised December 1995
     
     BEGIN    \
     {
         for (i = 1; i < ARGC; i++)
             copy[i] = ARGV[i]
     
         if (ARGV[1] == "-a") {
             append = 1
             delete ARGV[1]
             delete copy[1]
             ARGC--
         }
         if (ARGC < 2) {
             print "usage: tee [-a] file ..." > "/dev/stderr"
             exit 1
         }
         ARGV[1] = "-"
         ARGC = 2
     }

   The single rule does all the work.  Since there is no pattern, it is
executed for each line of input.  The body of the rule simply prints the
line into each file on the command line, and then to the standard
output.

     {
         # moving the if outside the loop makes it run faster
         if (append)
             for (i in copy)
                 print >> copy[i]
         else
             for (i in copy)
                 print > copy[i]
         print
     }

   It would have been possible to code the loop this way:

     for (i in copy)
         if (append)
             print >> copy[i]
         else
             print > copy[i]

This is more concise, but it is also less efficient.  The `if' is
tested for each record and for each output file.  By duplicating the
loop body, the `if' is only tested once for each input record.  If
there are N input records and M input files, the first method only
executes N `if' statements, while the second would execute N`*'M `if'
statements.

   Finally, the `END' rule cleans up, by closing all the output files.

     END    \
     {
         for (i in copy)
             close(copy[i])
     }


File: gawk.info,  Node: Uniq Program,  Next: Wc Program,  Prev: Tee Program,  Up: Clones

Printing Non-duplicated Lines of Text
-------------------------------------

   The `uniq' utility reads sorted lines of data on its standard input,
and (by default) removes duplicate lines.  In other words, only unique
lines are printed, hence the name.  `uniq' has a number of options. The
usage is:

     uniq [-udc [-N]] [+N] [ INPUT FILE [ OUTPUT FILE ]]

   The option meanings are:

`-d'
     Only print repeated lines.

`-u'
     Only print non-repeated lines.

`-c'
     Count lines. This option overrides `-d' and `-u'.  Both repeated
     and non-repeated lines are counted.

`-N'
     Skip N fields before comparing lines.  The definition of fields is
     similar to `awk''s default: non-whitespace characters separated by
     runs of spaces and/or tabs.

`+N'
     Skip N characters before comparing lines.  Any fields specified
     with `-N' are skipped first.

`INPUT FILE'
     Data is read from the input file named on the command line,
     instead of from the standard input.

`OUTPUT FILE'
     The generated output is sent to the named output file, instead of
     to the standard output.

   Normally `uniq' behaves as if both the `-d' and `-u' options had
been provided.

   Here is an `awk' implementation of `uniq'. It uses the `getopt'
library function (*note Processing Command Line Options: Getopt
Function.), and the `join' library function (*note Merging an Array
Into a String: Join Function.).

   The program begins with a `usage' function and then a brief outline
of the options and their meanings in a comment.

   The `BEGIN' rule deals with the command line arguments and options.
It uses a trick to get `getopt' to handle options of the form `-25',
treating such an option as the option letter `2' with an argument of
`5'. If indeed two or more digits were supplied (`Optarg' looks like a
number), `Optarg' is concatenated with the option digit, and then
result is added to zero to make it into a number.  If there is only one
digit in the option, then `Optarg' is not needed, and `Optind' must be
decremented so that `getopt' will process it next time.  This code is
admittedly a bit tricky.

   If no options were supplied, then the default is taken, to print both
repeated and non-repeated lines.  The output file, if provided, is
assigned to `outputfile'.  Earlier, `outputfile' was initialized to the
standard output, `/dev/stdout'.

     # uniq.awk --- do uniq in awk
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     function usage(    e)
     {
         e = "Usage: uniq [-udc [-n]] [+n] [ in [ out ]]"
         print e > "/dev/stderr"
         exit 1
     }
     
     # -c    count lines. overrides -d and -u
     # -d    only repeated lines
     # -u    only non-repeated lines
     # -n    skip n fields
     # +n    skip n characters, skip fields first
     
     BEGIN    \
     {
         count = 1
         outputfile = "/dev/stdout"
         opts = "udc0:1:2:3:4:5:6:7:8:9:"
         while ((c = getopt(ARGC, ARGV, opts)) != -1) {
             if (c == "u")
                 non_repeated_only++
             else if (c == "d")
                 repeated_only++
             else if (c == "c")
                 do_count++
             else if (index("0123456789", c) != 0) {
                 # getopt requires args to options
                 # this messes us up for things like -5
                 if (Optarg ~ /^[0-9]+$/)
                     fcount = (c Optarg) + 0
                 else {
                     fcount = c + 0
                     Optind--
                 }
             } else
                 usage()
         }
     
         if (ARGV[Optind] ~ /^\+[0-9]+$/) {
             charcount = substr(ARGV[Optind], 2) + 0
             Optind++
         }
     
         for (i = 1; i < Optind; i++)
             ARGV[i] = ""
     
         if (repeated_only == 0 && non_repeated_only == 0)
             repeated_only = non_repeated_only = 1
     
         if (ARGC - Optind == 2) {
             outputfile = ARGV[ARGC - 1]
             ARGV[ARGC - 1] = ""
         }
     }

   The following function, `are_equal', compares the current line,
`$0', to the previous line, `last'.  It handles skipping fields and
characters.

   If no field count and no character count were specified, `are_equal'
simply returns one or zero depending upon the result of a simple string
comparison of `last' and `$0'.  Otherwise, things get more complicated.

   If fields have to be skipped, each line is broken into an array using
`split' (*note Built-in Functions for String Manipulation: String
Functions.), and then the desired fields are joined back into a line
using `join'.  The joined lines are stored in `clast' and `cline'.  If
no fields are skipped, `clast' and `cline' are set to `last' and `$0'
respectively.

   Finally, if characters are skipped, `substr' is used to strip off the
leading `charcount' characters in `clast' and `cline'.  The two strings
are then compared, and `are_equal' returns the result.

     function are_equal(    n, m, clast, cline, alast, aline)
     {
         if (fcount == 0 && charcount == 0)
             return (last == $0)
     
         if (fcount > 0) {
             n = split(last, alast)
             m = split($0, aline)
             clast = join(alast, fcount+1, n)
             cline = join(aline, fcount+1, m)
         } else {
             clast = last
             cline = $0
         }
         if (charcount) {
             clast = substr(clast, charcount + 1)
             cline = substr(cline, charcount + 1)
         }
     
         return (clast == cline)
     }

   The following two rules are the body of the program.  The first one
is executed only for the very first line of data.  It sets `last' equal
to `$0', so that subsequent lines of text have something to be compared
to.

   The second rule does the work. The variable `equal' will be one or
zero depending upon the results of `are_equal''s comparison. If `uniq'
is counting repeated lines, then the `count' variable is incremented if
the lines are equal. Otherwise the line is printed and `count' is
reset, since the two lines are not equal.

   If `uniq' is not counting, `count' is incremented if the lines are
equal. Otherwise, if `uniq' is counting repeated lines, and more than
one line has been seen, or if `uniq' is counting non-repeated lines,
and only one line has been seen, then the line is printed, and `count'
is reset.

   Finally, similar logic is used in the `END' rule to print the final
line of input data.

     NR == 1 {
         last = $0
         next
     }
     
     {
         equal = are_equal()
     
         if (do_count) {    # overrides -d and -u
             if (equal)
                 count++
             else {
                 printf("%4d %s\n", count, last) > outputfile
                 last = $0
                 count = 1    # reset
             }
             next
         }
     
         if (equal)
             count++
         else {
             if ((repeated_only && count > 1) ||
                 (non_repeated_only && count == 1))
                     print last > outputfile
             last = $0
             count = 1
         }
     }
     
     END {
         if (do_count)
             printf("%4d %s\n", count, last) > outputfile
         else if ((repeated_only && count > 1) ||
                 (non_repeated_only && count == 1))
             print last > outputfile
     }


File: gawk.info,  Node: Wc Program,  Prev: Uniq Program,  Up: Clones

Counting Things
---------------

   The `wc' (word count) utility counts lines, words, and characters in
one or more input files. Its usage is:

     wc [-lwc] [ FILES ... ]

   If no files are specified on the command line, `wc' reads its
standard input. If there are multiple files, it will also print total
counts for all the files.  The options and their meanings are:

`-l'
     Only count lines.

`-w'
     Only count words.  A "word" is a contiguous sequence of
     non-whitespace characters, separated by spaces and/or tabs.
     Happily, this is the normal way `awk' separates fields in its
     input data.

`-c'
     Only count characters.

   Implementing `wc' in `awk' is particularly elegant, since `awk' does
a lot of the work for us; it splits lines into words (i.e.  fields) and
counts them, it counts lines (i.e. records) for us, and it can easily
tell us how long a line is.

   This version uses the `getopt' library function (*note Processing
Command Line Options: Getopt Function.), and the file transition
functions (*note Noting Data File Boundaries: Filetrans Function.).

   This version has one major difference from traditional versions of
`wc'.  Our version always prints the counts in the order lines, words,
and characters.  Traditional versions note the order of the `-l', `-w',
and `-c' options on the command line, and print the counts in that
order.

   The `BEGIN' rule does the argument processing.  The variable
`print_total' will be true if more than one file was named on the
command line.

     # wc.awk --- count lines, words, characters
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     # Options:
     #    -l    only count lines
     #    -w    only count words
     #    -c    only count characters
     #
     # Default is to count lines, words, characters
     
     BEGIN {
         # let getopt print a message about
         # invalid options. we ignore them
         while ((c = getopt(ARGC, ARGV, "lwc")) != -1) {
             if (c == "l")
                 do_lines = 1
             else if (c == "w")
                 do_words = 1
             else if (c == "c")
                 do_chars = 1
         }
         for (i = 1; i < Optind; i++)
             ARGV[i] = ""
     
         # if no options, do all
         if (! do_lines && ! do_words && ! do_chars)
             do_lines = do_words = do_chars = 1
     
         print_total = (ARGC - i > 2)
     }

   The `beginfile' function is simple; it just resets the counts of
lines, words, and characters to zero, and saves the current file name in
`fname'.

   The `endfile' function adds the current file's numbers to the running
totals of lines, words, and characters.  It then prints out those
numbers for the file that was just read. It relies on `beginfile' to
reset the numbers for the following data file.

     function beginfile(file)
     {
         chars = lines = words = 0
         fname = FILENAME
     }
     
     function endfile(file)
     {
         tchars += chars
         tlines += lines
         twords += words
         if (do_lines)
             printf "\t%d", lines
         if (do_words)
             printf "\t%d", words
         if (do_chars)
             printf "\t%d", chars
         printf "\t%s\n", fname
     }

   There is one rule that is executed for each line. It adds the length
of the record to `chars'.  It has to add one, since the newline
character separating records (the value of `RS') is not part of the
record itself.  `lines' is incremented for each line read, and `words'
is incremented by the value of `NF', the number of "words" on this
line.(1)

   Finally, the `END' rule simply prints the totals for all the files.

     # do per line
     {
         chars += length($0) + 1    # get newline
         lines++
         words += NF
     }
     
     END {
         if (print_total) {
             if (do_lines)
                 printf "\t%d", tlines
             if (do_words)
                 printf "\t%d", twords
             if (do_chars)
                 printf "\t%d", tchars
             print "\ttotal"
         }
     }

   ---------- Footnotes ----------

   (1)  Examine the code in *Note Noting Data File Boundaries:
Filetrans Function.  Why must `wc' use a separate `lines' variable,
instead of using the value of `FNR' in `endfile'?


File: gawk.info,  Node: Miscellaneous Programs,  Prev: Clones,  Up: Sample Programs

A Grab Bag of `awk' Programs
============================

   This section is a large "grab bag" of miscellaneous programs.  We
hope you find them both interesting and enjoyable.

* Menu:

* Dupword Program::         Finding duplicated words in a document.
* Alarm Program::           An alarm clock.
* Translate Program::       A program similar to the `tr' utility.
* Labels Program::          Printing mailing labels.
* Word Sorting::            A program to produce a word usage count.
* History Sorting::         Eliminating duplicate entries from a history
                            file.
* Extract Program::         Pulling out programs from Texinfo source
                            files.
* Simple Sed::              A Simple Stream Editor.
* Igawk Program::           A wrapper for `awk' that includes files.


File: gawk.info,  Node: Dupword Program,  Next: Alarm Program,  Prev: Miscellaneous Programs,  Up: Miscellaneous Programs

Finding Duplicated Words in a Document
--------------------------------------

   A common error when writing large amounts of prose is to accidentally
duplicate words.  Often you will see this in text as something like "the
the program does the following ...."  When the text is on-line, often
the duplicated words occur at the end of one line and the beginning of
another, making them very difficult to spot.

   This program, `dupword.awk', scans through a file one line at a time,
and looks for adjacent occurrences of the same word.  It also saves the
last word on a line (in the variable `prev') for comparison with the
first word on the next line.

   The first two statements make sure that the line is all lower-case,
so that, for example, "The" and "the" compare equal to each other.  The
second statement removes all non-alphanumeric and non-whitespace
characters from the line, so that punctuation does not affect the
comparison either.  This sometimes leads to reports of duplicated words
that really are different, but this is unusual.

     # dupword --- find duplicate words in text
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # December 1991
     
     {
         $0 = tolower($0)
         gsub(/[^A-Za-z0-9 \t]/, "");
         if ($1 == prev)
             printf("%s:%d: duplicate %s\n",
                 FILENAME, FNR, $1)
         for (i = 2; i <= NF; i++)
             if ($i == $(i-1))
                 printf("%s:%d: duplicate %s\n",
                     FILENAME, FNR, $i)
         prev = $NF
     }


File: gawk.info,  Node: Alarm Program,  Next: Translate Program,  Prev: Dupword Program,  Up: Miscellaneous Programs

An Alarm Clock Program
----------------------

   The following program is a simple "alarm clock" program.  You give
it a time of day, and an optional message.  At the given time, it
prints the message on the standard output. In addition, you can give it
the number of times to repeat the message, and also a delay between
repetitions.

   This program uses the `gettimeofday' function from *Note Managing
the Time of Day: Gettimeofday Function.

   All the work is done in the `BEGIN' rule.  The first part is argument
checking and setting of defaults; the delay, the count, and the message
to print.  If the user supplied a message, but it does not contain the
ASCII BEL character (known as the "alert" character, `\a'), then it is
added to the message.  (On many systems, printing the ASCII BEL
generates some sort of audible alert. Thus, when the alarm goes off,
the system calls attention to itself, in case the user is not looking
at their computer or terminal.)

     # alarm --- set an alarm
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     # usage: alarm time [ "message" [ count [ delay ] ] ]
     
     BEGIN    \
     {
         # Initial argument sanity checking
         usage1 = "usage: alarm time ['message' [count [delay]]]"
         usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])
     
         if (ARGC < 2) {
             print usage > "/dev/stderr"
             exit 1
         } else if (ARGC == 5) {
             delay = ARGV[4] + 0
             count = ARGV[3] + 0
             message = ARGV[2]
         } else if (ARGC == 4) {
             count = ARGV[3] + 0
             message = ARGV[2]
         } else if (ARGC == 3) {
             message = ARGV[2]
         } else if (ARGV[1] !~ /[0-9]?[0-9]:[0-9][0-9]/) {
             print usage1 > "/dev/stderr"
             print usage2 > "/dev/stderr"
             exit 1
         }
     
         # set defaults for once we reach the desired time
         if (delay == 0)
             delay = 180    # 3 minutes
         if (count == 0)
             count = 5
         if (message == "")
             message = sprintf("\aIt is now %s!\a", ARGV[1])
         else if (index(message, "\a") == 0)
             message = "\a" message "\a"

   The next section of code turns the alarm time into hours and minutes,
and converts it if necessary to a 24-hour clock.  Then it turns that
time into a count of the seconds since midnight.  Next it turns the
current time into a count of seconds since midnight.  The difference
between the two is how long to wait before setting off the alarm.

         # split up dest time
         split(ARGV[1], atime, ":")
         hour = atime[1] + 0    # force numeric
         minute = atime[2] + 0  # force numeric
     
         # get current broken down time
         gettimeofday(now)
     
         # if time given is 12-hour hours and it's after that
         # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
         # then add 12 to real hour
         if (hour < 12 && now["hour"] > hour)
             hour += 12
     
         # set target time in seconds since midnight
         target = (hour * 60 * 60) + (minute * 60)
     
         # get current time in seconds since midnight
         current = (now["hour"] * 60 * 60) + \
                    (now["minute"] * 60) + now["second"]
     
         # how long to sleep for
         naptime = target - current
         if (naptime <= 0) {
             print "time is in the past!" > "/dev/stderr"
             exit 1
         }

   Finally, the program uses the `system' function (*note Built-in
Functions for Input/Output: I/O Functions.) to call the `sleep'
utility.  The `sleep' utility simply pauses for the given number of
seconds.  If the exit status is not zero, the program assumes that
`sleep' was interrupted, and exits. If `sleep' exited with an OK status
(zero), then the program prints the message in a loop, again using
`sleep' to delay for however many seconds are necessary.

         # zzzzzz..... go away if interrupted
         if (system(sprintf("sleep %d", naptime)) != 0)
             exit 1
     
         # time to notify!
         command = sprintf("sleep %d", delay)
         for (i = 1; i <= count; i++) {
             print message
             # if sleep command interrupted, go away
             if (system(command) != 0)
                 break
         }
     
         exit 0
     }


File: gawk.info,  Node: Translate Program,  Next: Labels Program,  Prev: Alarm Program,  Up: Miscellaneous Programs

Transliterating Characters
--------------------------

   The system `tr' utility transliterates characters.  For example, it
is often used to map upper-case letters into lower-case, for further
processing.

     GENERATE DATA | tr '[A-Z]' '[a-z]' | PROCESS DATA ...

   You give `tr' two lists of characters enclosed in square brackets.
Usually, the lists are quoted to keep the shell from attempting to do a
filename expansion.(1)  When processing the input, the first character
in the first list is replaced with the first character in the second
list, the second character in the first list is replaced with the
second character in the second list, and so on.  If there are more
characters in the "from" list than in the "to" list, the last character
of the "to" list is used for the remaining characters in the "from"
list.

   Some time ago, a user proposed to us that we add a transliteration
function to `gawk'.  Being opposed to "creeping featurism," I wrote the
following program to prove that character transliteration could be done
with a user-level function.  This program is not as complete as the
system `tr' utility, but it will do most of the job.

   The `translate' program demonstrates one of the few weaknesses of
standard `awk': dealing with individual characters is very painful,
requiring repeated use of the `substr', `index', and `gsub' built-in
functions (*note Built-in Functions for String Manipulation: String
Functions.).(2)

   There are two functions.  The first, `stranslate', takes three
arguments.

`from'
     A list of characters to translate from.

`to'
     A list of characters to translate to.

`target'
     The string to do the translation on.

   Associative arrays make the translation part fairly easy. `t_ar'
holds the "to" characters, indexed by the "from" characters.  Then a
simple loop goes through `from', one character at a time.  For each
character in `from', if the character appears in `target', `gsub' is
used to change it to the corresponding `to' character.

   The `translate' function simply calls `stranslate' using `$0' as the
target.  The main program sets two global variables, `FROM' and `TO',
from the command line, and then changes `ARGV' so that `awk' will read
from the standard input.

   Finally, the processing rule simply calls `translate' for each
record.

     # translate --- do tr like stuff
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # August 1989
     
     # bugs: does not handle things like: tr A-Z a-z, it has
     # to be spelled out. However, if `to' is shorter than `from',
     # the last character in `to' is used for the rest of `from'.
     
     function stranslate(from, to, target,     lf, lt, t_ar, i, c)
     {
         lf = length(from)
         lt = length(to)
         for (i = 1; i <= lt; i++)
             t_ar[substr(from, i, 1)] = substr(to, i, 1)
         if (lt < lf)
             for (; i <= lf; i++)
                 t_ar[substr(from, i, 1)] = substr(to, lt, 1)
         for (i = 1; i <= lf; i++) {
             c = substr(from, i, 1)
             if (index(target, c) > 0)
                 gsub(c, t_ar[c], target)
         }
         return target
     }
     
     function translate(from, to)
     {
         return $0 = stranslate(from, to, $0)
     }
     
     # main program
     BEGIN {
         if (ARGC < 3) {
             print "usage: translate from to" > "/dev/stderr"
             exit
         }
         FROM = ARGV[1]
         TO = ARGV[2]
         ARGC = 2
         ARGV[1] = "-"
     }
     
     {
         translate(FROM, TO)
         print
     }

   While it is possible to do character transliteration in a user-level
function, it is not necessarily efficient, and we started to consider
adding a built-in function.  However, shortly after writing this
program, we learned that the System V Release 4 `awk' had added the
`toupper' and `tolower' functions.  These functions handle the vast
majority of the cases where character transliteration is necessary, and
so we chose to simply add those functions to `gawk' as well, and then
leave well enough alone.

   An obvious improvement to this program would be to set up the `t_ar'
array only once, in a `BEGIN' rule. However, this assumes that the
"from" and "to" lists will never change throughout the lifetime of the
program.

   ---------- Footnotes ----------

   (1)  On older, non-POSIX systems, `tr' often does not require that
the lists be enclosed in square brackets and quoted.  This is a feature.

   (2)  This program was written before `gawk' acquired the ability to
split each character in a string into separate array elements.  How
might this ability simplify the program?


File: gawk.info,  Node: Labels Program,  Next: Word Sorting,  Prev: Translate Program,  Up: Miscellaneous Programs

Printing Mailing Labels
-----------------------

   Here is a "real world"(1) program.  This script reads lists of names
and addresses, and generates mailing labels.  Each page of labels has
20 labels on it, two across and ten down.  The addresses are guaranteed
to be no more than five lines of data.  Each address is separated from
the next by a blank line.

   The basic idea is to read 20 labels worth of data.  Each line of
each label is stored in the `line' array.  The single rule takes care
of filling the `line' array and printing the page when 20 labels have
been read.

   The `BEGIN' rule simply sets `RS' to the empty string, so that `awk'
will split records at blank lines (*note How Input is Split into
Records: Records.).  It sets `MAXLINES' to 100, since `MAXLINE' is the
maximum number of lines on the page (20 * 5 = 100).

   Most of the work is done in the `printpage' function.  The label
lines are stored sequentially in the `line' array.  But they have to be
printed horizontally; `line[1]' next to `line[6]', `line[2]' next to
`line[7]', and so on.  Two loops are used to accomplish this.  The
outer loop, controlled by `i', steps through every 10 lines of data;
this is each row of labels.  The inner loop, controlled by `j', goes
through the lines within the row.  As `j' goes from zero to four, `i+j'
is the `j''th line in the row, and `i+j+5' is the entry next to it.
The output ends up looking something like this:

     line 1          line 6
     line 2          line 7
     line 3          line 8
     line 4          line 9
     line 5          line 10

   As a final note, at lines 21 and 61, an extra blank line is printed,
to keep the output lined up on the labels.  This is dependent on the
particular brand of labels in use when the program was written.  You
will also note that there are two blank lines at the top and two blank
lines at the bottom.

   The `END' rule arranges to flush the final page of labels; there may
not have been an even multiple of 20 labels in the data.

     # labels.awk
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # June 1992
     
     # Program to print labels.  Each label is 5 lines of data
     # that may have blank lines.  The label sheets have 2
     # blank lines at the top and 2 at the bottom.
     
     BEGIN    { RS = "" ; MAXLINES = 100 }
     
     function printpage(    i, j)
     {
         if (Nlines <= 0)
             return
     
         printf "\n\n"        # header
     
         for (i = 1; i <= Nlines; i += 10) {
             if (i == 21 || i == 61)
                 print ""
             for (j = 0; j < 5; j++) {
                 if (i + j > MAXLINES)
                     break
                 printf "   %-41s %s\n", line[i+j], line[i+j+5]
             }
             print ""
         }
     
         printf "\n\n"        # footer
     
         for (i in line)
             line[i] = ""
     }
     
     # main rule
     {
         if (Count >= 20) {
             printpage()
             Count = 0
             Nlines = 0
         }
         n = split($0, a, "\n")
         for (i = 1; i <= n; i++)
             line[++Nlines] = a[i]
         for (; i <= 5; i++)
             line[++Nlines] = ""
         Count++
     }
     
     END    \
     {
         printpage()
     }

   ---------- Footnotes ----------

   (1)  "Real world" is defined as "a program actually used to get
something done."


File: gawk.info,  Node: Word Sorting,  Next: History Sorting,  Prev: Labels Program,  Up: Miscellaneous Programs

Generating Word Usage Counts
----------------------------

   The following `awk' program prints the number of occurrences of each
word in its input.  It illustrates the associative nature of `awk'
arrays by using strings as subscripts.  It also demonstrates the `for X
in ARRAY' construction.  Finally, it shows how `awk' can be used in
conjunction with other utility programs to do a useful task of some
complexity with a minimum of effort.  Some explanations follow the
program listing.

     awk '
     # Print list of word frequencies
     {
         for (i = 1; i <= NF; i++)
             freq[$i]++
     }
     
     END {
         for (word in freq)
             printf "%s\t%d\n", word, freq[word]
     }'

   The first thing to notice about this program is that it has two
rules.  The first rule, because it has an empty pattern, is executed on
every line of the input.  It uses `awk''s field-accessing mechanism
(*note Examining Fields: Fields.) to pick out the individual words from
the line, and the built-in variable `NF' (*note Built-in Variables::.)
to know how many fields are available.

   For each input word, an element of the array `freq' is incremented to
reflect that the word has been seen an additional time.

   The second rule, because it has the pattern `END', is not executed
until the input has been exhausted.  It prints out the contents of the
`freq' table that has been built up inside the first action.

   This program has several problems that would prevent it from being
useful by itself on real text files:

   * Words are detected using the `awk' convention that fields are
     separated by whitespace and that other characters in the input
     (except newlines) don't have any special meaning to `awk'.  This
     means that punctuation characters count as part of words.

   * The `awk' language considers upper- and lower-case characters to be
     distinct.  Therefore, `bartender' and `Bartender' are not treated
     as the same word.  This is undesirable since, in normal text, words
     are capitalized if they begin sentences, and a frequency analyzer
     should not be sensitive to capitalization.

   * The output does not come out in any useful order.  You're more
     likely to be interested in which words occur most frequently, or
     having an alphabetized table of how frequently each word occurs.

   The way to solve these problems is to use some of the more advanced
features of the `awk' language.  First, we use `tolower' to remove case
distinctions.  Next, we use `gsub' to remove punctuation characters.
Finally, we use the system `sort' utility to process the output of the
`awk' script.  Here is the new version of the program:

     # Print list of word frequencies
     {
         $0 = tolower($0)    # remove case distinctions
         gsub(/[^a-z0-9_ \t]/, "", $0)  # remove punctuation
         for (i = 1; i <= NF; i++)
             freq[$i]++
     }
     
     END {
         for (word in freq)
             printf "%s\t%d\n", word, freq[word]
     }

   Assuming we have saved this program in a file named `wordfreq.awk',
and that the data is in `file1', the following pipeline

     awk -f wordfreq.awk file1 | sort +1 -nr

produces a table of the words appearing in `file1' in order of
decreasing frequency.

   The `awk' program suitably massages the data and produces a word
frequency table, which is not ordered.

   The `awk' script's output is then sorted by the `sort' utility and
printed on the terminal.  The options given to `sort' in this example
specify to sort using the second field of each input line (skipping one
field), that the sort keys should be treated as numeric quantities
(otherwise `15' would come before `5'), and that the sorting should be
done in descending (reverse) order.

   We could have even done the `sort' from within the program, by
changing the `END' action to:

     END {
         sort = "sort +1 -nr"
         for (word in freq)
             printf "%s\t%d\n", word, freq[word] | sort
         close(sort)
     }

   You would have to use this way of sorting on systems that do not
have true pipes.

   See the general operating system documentation for more information
on how to use the `sort' program.


File: gawk.info,  Node: History Sorting,  Next: Extract Program,  Prev: Word Sorting,  Up: Miscellaneous Programs

Removing Duplicates from Unsorted Text
--------------------------------------

   The `uniq' program (*note Printing Non-duplicated Lines of Text:
Uniq Program.), removes duplicate lines from *sorted* data.

   Suppose, however, you need to remove duplicate lines from a data
file, but that you wish to preserve the order the lines are in?  A good
example of this might be a shell history file.  The history file keeps
a copy of all the commands you have entered, and it is not unusual to
repeat a command several times in a row.  Occasionally you might wish
to compact the history by removing duplicate entries.  Yet it is
desirable to maintain the order of the original commands.

   This simple program does the job.  It uses two arrays.  The `data'
array is indexed by the text of each line.  For each line, `data[$0]'
is incremented.

   If a particular line has not been seen before, then `data[$0]' will
be zero.  In that case, the text of the line is stored in
`lines[count]'.  Each element of `lines' is a unique command, and the
indices of `lines' indicate the order in which those lines were
encountered.  The `END' rule simply prints out the lines, in order.

     # histsort.awk --- compact a shell history file
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     # Thanks to Byron Rakitzis for the general idea
     {
         if (data[$0]++ == 0)
             lines[++count] = $0
     }
     
     END {
         for (i = 1; i <= count; i++)
             print lines[i]
     }

   This program also provides a foundation for generating other useful
information.  For example, using the following `print' satement in the
`END' rule would indicate how often a particular command was used.

     print data[lines[i]], lines[i]

   This works because `data[$0]' was incremented each time a line was
seen.


File: gawk.info,  Node: Extract Program,  Next: Simple Sed,  Prev: History Sorting,  Up: Miscellaneous Programs

Extracting Programs from Texinfo Source Files
---------------------------------------------

   The nodes *Note A Library of `awk' Functions: Library Functions, and
*Note Practical `awk' Programs: Sample Programs, are the top level
nodes for a large number of `awk' programs.  If you wish to experiment
with these programs, it is tedious to have to type them in by hand.
Here we present a program that can extract parts of a Texinfo input
file into separate files.

   This Info file is written in Texinfo, the GNU project's document
formatting language.  A single Texinfo source file can be used to
produce both printed and on-line documentation.  The Texinfo language
is described fully, starting with *Note Introduction: (texi)Top.

   For our purposes, it is enough to know three things about Texinfo
input files.

   * The "at" symbol, `@', is special in Texinfo, much like `\' in C or
     `awk'.  Literal `@' symbols are represented in Texinfo source
     files as `@@'.

   * Comments start with either `@c' or `@comment'.  The file
     extraction program will work by using special comments that start
     at the beginning of a line.

   * Example text that should not be split across a page boundary is
     bracketed between lines containing `@group' and `@end group'
     commands.

   The following program, `extract.awk', reads through a Texinfo source
file, and does two things, based on the special comments.  Upon seeing
`@c system ...', it runs a command, by extracting the command text from
the control line and passing it on to the `system' function (*note
Built-in Functions for Input/Output: I/O Functions.).  Upon seeing `@c
file FILENAME', each subsequent line is sent to the file FILENAME,
until `@c endfile' is encountered.  The rules in `extract.awk' will
match either `@c' or `@comment' by letting the `omment' part be
optional.  Lines containing `@group' and `@end group' are simply
removed.  `extract.awk' uses the `join' library function (*note Merging
an Array Into a String: Join Function.).

   The example programs in the on-line Texinfo source for `The GNU Awk
User's Guide' (`gawk.texi') have all been bracketed inside `file', and
`endfile' lines.  The `gawk' distribution uses a copy of `extract.awk'
to extract the sample programs and install many of them in a standard
directory, where `gawk' can find them.

   `extract.awk' begins by setting `IGNORECASE' to one, so that mixed
upper-case and lower-case letters in the directives won't matter.

   The first rule handles calling `system', checking that a command was
given (`NF' is at least three), and also checking that the command
exited with a zero exit status, signifying OK.

     # extract.awk --- extract files and run programs
     #                 from texinfo files
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # May 1993
     
     BEGIN    { IGNORECASE = 1 }
     
     /^@c(omment)?[ \t]+system/    \
     {
         if (NF < 3) {
             e = (FILENAME ":" FNR)
             e = (e  ": badly formed `system' line")
             print e > "/dev/stderr"
             next
         }
         $1 = ""
         $2 = ""
         stat = system($0)
         if (stat != 0) {
             e = (FILENAME ":" FNR)
             e = (e ": warning: system returned " stat)
             print e > "/dev/stderr"
         }
     }

The variable `e' is used so that the function fits nicely on the screen.

   The second rule handles moving data into files.  It verifies that a
file name was given in the directive.  If the file named is not the
current file, then the current file is closed.  This means that an `@c
endfile' was not given for that file.  (We should probably print a
diagnostic in this case, although at the moment we do not.)

   The `for' loop does the work.  It reads lines using `getline' (*note
Explicit Input with `getline': Getline.).  For an unexpected end of
file, it calls the `unexpected_eof' function.  If the line is an
"endfile" line, then it breaks out of the loop.  If the line is an
`@group' or `@end group' line, then it ignores it, and goes on to the
next line.

   Most of the work is in the following few lines.  If the line has no
`@' symbols, it can be printed directly.  Otherwise, each leading `@'
must be stripped off.

   To remove the `@' symbols, the line is split into separate elements
of the array `a', using the `split' function (*note Built-in Functions
for String Manipulation: String Functions.).  Each element of `a' that
is empty indicates two successive `@' symbols in the original line.
For each two empty elements (`@@' in the original file), we have to add
back in a single `@' symbol.

   When the processing of the array is finished, `join' is called with
the value of `SUBSEP', to rejoin the pieces back into a single line.
That line is then printed to the output file.

     /^@c(omment)?[ \t]+file/    \
     {
         if (NF != 3) {
             e = (FILENAME ":" FNR ": badly formed `file' line")
             print e > "/dev/stderr"
             next
         }
         if ($3 != curfile) {
             if (curfile != "")
                 close(curfile)
             curfile = $3
         }
     
         for (;;) {
             if ((getline line) <= 0)
                 unexpected_eof()
             if (line ~ /^@c(omment)?[ \t]+endfile/)
                 break
             else if (line ~ /^@(end[ \t]+)?group/)
                 continue
             if (index(line, "@") == 0) {
                 print line > curfile
                 continue
             }
             n = split(line, a, "@")
             # if a[1] == "", means leading @,
             # don't add one back in.
             for (i = 2; i <= n; i++) {
                 if (a[i] == "") { # was an @@
                     a[i] = "@"
                     if (a[i+1] == "")
                         i++
                 }
             }
             print join(a, 1, n, SUBSEP) > curfile
         }
     }

   An important thing to note is the use of the `>' redirection.
Output done with `>' only opens the file once; it stays open and
subsequent output is appended to the file (*note Redirecting Output of
`print' and `printf': Redirection.).  This allows us to easily mix
program text and explanatory prose for the same sample source file (as
has been done here!) without any hassle.  The file is only closed when
a new data file name is encountered, or at the end of the input file.

   Finally, the function `unexpected_eof' prints an appropriate error
message and then exits.

   The `END' rule handles the final cleanup, closing the open file.

     function unexpected_eof()
     {
         printf("%s:%d: unexpected EOF or error\n", \
             FILENAME, FNR) > "/dev/stderr"
         exit 1
     }
     
     END {
         if (curfile)
             close(curfile)
     }


File: gawk.info,  Node: Simple Sed,  Next: Igawk Program,  Prev: Extract Program,  Up: Miscellaneous Programs

A Simple Stream Editor
----------------------

   The `sed' utility is a "stream editor," a program that reads a
stream of data, makes changes to it, and passes the modified data on.
It is often used to make global changes to a large file, or to a stream
of data generated by a pipeline of commands.

   While `sed' is a complicated program in its own right, its most
common use is to perform global substitutions in the middle of a
pipeline:

     command1 < orig.data | sed 's/old/new/g' | command2 > result

   Here, the `s/old/new/g' tells `sed' to look for the regexp `old' on
each input line, and replace it with the text `new', globally (i.e. all
the occurrences on a line).  This is similar to `awk''s `gsub' function
(*note Built-in Functions for String Manipulation: String Functions.).

   The following program, `awksed.awk', accepts at least two command
line arguments; the pattern to look for and the text to replace it
with. Any additional arguments are treated as data file names to
process. If none are provided, the standard input is used.

     # awksed.awk --- do s/foo/bar/g using just print
     #    Thanks to Michael Brennan for the idea
     
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # August 1995
     
     function usage()
     {
         print "usage: awksed pat repl [files...]" > "/dev/stderr"
         exit 1
     }
     
     BEGIN {
         # validate arguments
         if (ARGC < 3)
             usage()
     
         RS = ARGV[1]
         ORS = ARGV[2]
     
         # don't use arguments as files
         ARGV[1] = ARGV[2] = ""
     }
     
     # look ma, no hands!
     {
         if (RT == "")
             printf "%s", $0
         else
             print
     }

   The program relies on `gawk''s ability to have `RS' be a regexp and
on the setting of `RT' to the actual text that terminated the record
(*note How Input is Split into Records: Records.).

   The idea is to have `RS' be the pattern to look for. `gawk' will
automatically set `$0' to the text between matches of the pattern.
This is text that we wish to keep, unmodified.  Then, by setting `ORS'
to the replacement text, a simple `print' statement will output the
text we wish to keep, followed by the replacement text.

   There is one wrinkle to this scheme, which is what to do if the last
record doesn't end with text that matches `RS'?  Using a `print'
statement unconditionally prints the replacement text, which is not
correct.

   However, if the file did not end in text that matches `RS', `RT'
will be set to the null string.  In this case, we can print `$0' using
`printf' (*note Using `printf' Statements for Fancier Printing:
Printf.).

   The `BEGIN' rule handles the setup, checking for the right number of
arguments, and calling `usage' if there is a problem. Then it sets `RS'
and `ORS' from the command line arguments, and sets `ARGV[1]' and
`ARGV[2]' to the null string, so that they will not be treated as file
names (*note Using `ARGC' and `ARGV': ARGC and ARGV.).

   The `usage' function prints an error message and exits.

   Finally, the single rule handles the printing scheme outlined above,
using `print' or `printf' as appropriate, depending upon the value of
`RT'.


File: gawk.info,  Node: Igawk Program,  Prev: Simple Sed,  Up: Miscellaneous Programs

An Easy Way to Use Library Functions
------------------------------------

   Using library functions in `awk' can be very beneficial. It
encourages code re-use and the writing of general functions. Programs
are smaller, and therefore clearer.  However, using library functions
is only easy when writing `awk' programs; it is painful when running
them, requiring multiple `-f' options.  If `gawk' is unavailable, then
so too is the `AWKPATH' environment variable and the ability to put
`awk' functions into a library directory (*note Command Line Options:
Options.).

   It would be nice to be able to write programs like so:

     # library functions
     @include getopt.awk
     @include join.awk
     ...
     
     # main program
     BEGIN {
         while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
             ...
         ...
     }

   The following program, `igawk.sh', provides this service.  It
simulates `gawk''s searching of the `AWKPATH' variable, and also allows
"nested" includes; i.e. a file that has been included with `@include'
can contain further `@include' statements.  `igawk' will make an effort
to only include files once, so that nested includes don't accidentally
include a library function twice.

   `igawk' should behave externally just like `gawk'.  This means it
should accept all of `gawk''s command line arguments, including the
ability to have multiple source files specified via `-f', and the
ability to mix command line and library source files.

   The program is written using the POSIX Shell (`sh') command language.
The way the program works is as follows:

  1. Loop through the arguments, saving anything that doesn't represent
     `awk' source code for later, when the expanded program is run.

  2. For any arguments that do represent `awk' text, put the arguments
     into a temporary file that will be expanded.  There are two cases.

       a. Literal text, provided with `--source' or `--source='.  This
          text is just echoed directly.  The `echo' program will
          automatically supply a trailing newline.

       b. File names provided with `-f'.  We use a neat trick, and echo
          `@include FILENAME' into the temporary file.  Since the file
          inclusion program will work the way `gawk' does, this will
          get the text of the file included into the program at the
          correct point.

  3. Run an `awk' program (naturally) over the temporary file to expand
     `@include' statements.  The expanded program is placed in a second
     temporary file.

  4. Run the expanded program with `gawk' and any other original
     command line arguments that the user supplied (such as the data
     file names).

   The initial part of the program turns on shell tracing if the first
argument was `debug'.  Otherwise, a shell `trap' statement arranges to
clean up any temporary files on program exit or upon an interrupt.

   The next part loops through all the command line arguments.  There
are several cases of interest.

`--'
     This ends the arguments to `igawk'.  Anything else should be
     passed on to the user's `awk' program without being evaluated.

`-W'
     This indicates that the next option is specific to `gawk'.  To make
     argument processing easier, the `-W' is appended to the front of
     the remaining arguments and the loop continues.  (This is an `sh'
     programming trick.  Don't worry about it if you are not familiar
     with `sh'.)

`-v'
`-F'
     These are saved and passed on to `gawk'.

`-f'
`--file'
`--file='
`-Wfile='
     The file name is saved to the temporary file `/tmp/ig.s.$$' with an
     `@include' statement.  The `sed' utility is used to remove the
     leading option part of the argument (e.g., `--file=').

`--source'
`--source='
`-Wsource='
     The source text is echoed into `/tmp/ig.s.$$'.

`--version'
`--version'
`-Wversion'
     `igawk' prints its version number, and runs `gawk --version' to
     get the `gawk' version information, and then exits.

   If none of `-f', `--file', `-Wfile', `--source', or `-Wsource', were
supplied, then the first non-option argument should be the `awk'
program.  If there are no command line arguments left, `igawk' prints
an error message and exits.  Otherwise, the first argument is echoed
into `/tmp/ig.s.$$'.

   In any case, after the arguments have been processed, `/tmp/ig.s.$$'
contains the complete text of the original `awk' program.

   The `$$' in `sh' represents the current process ID number.  It is
often used in shell programs to generate unique temporary file names.
This allows multiple users to run `igawk' without worrying that the
temporary file names will clash.

   Here's the program:

     #! /bin/sh
     
     # igawk --- like gawk but do @include processing
     # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
     # July 1993
     
     if [ "$1" = debug ]
     then
         set -x
         shift
     else
         # cleanup on exit, hangup, interrupt, quit, termination
         trap 'rm -f /tmp/ig.[se].$$' 0 1 2 3 15
     fi
     
     while [ $# -ne 0 ] # loop over arguments
     do
         case $1 in
         --)     shift; break;;
     
         -W)     shift
                 set -- -W"$@"
                 continue;;
     
         -[vF])  opts="$opts $1 '$2'"
                 shift;;
     
         -[vF]*) opts="$opts '$1'" ;;
     
         -f)     echo @include "$2" >> /tmp/ig.s.$$
                 shift;;
     
         -f*)    f=`echo "$1" | sed 's/-f//'`
                 echo @include "$f" >> /tmp/ig.s.$$ ;;
     
         -?file=*)    # -Wfile or --file
                 f=`echo "$1" | sed 's/-.file=//'`
                 echo @include "$f" >> /tmp/ig.s.$$ ;;
     
         -?file)    # get arg, $2
                 echo @include "$2" >> /tmp/ig.s.$$
                 shift;;
     
         -?source=*)    # -Wsource or --source
                 t=`echo "$1" | sed 's/-.source=//'`
                 echo "$t" >> /tmp/ig.s.$$ ;;
     
         -?source)  # get arg, $2
                 echo "$2" >> /tmp/ig.s.$$
                 shift;;
     
         -?version)
                 echo igawk: version 1.0 1>&2
                 gawk --version
                 exit 0 ;;
     
         -[W-]*)    opts="$opts '$1'" ;;
     
         *)      break;;
         esac
         shift
     done
     
     if [ ! -s /tmp/ig.s.$$ ]
     then
         if [ -z "$1" ]
         then
              echo igawk: no program! 1>&2
              exit 1
         else
             echo "$1" > /tmp/ig.s.$$
             shift
         fi
     fi
     
     # at this point, /tmp/ig.s.$$ has the program

   The `awk' program to process `@include' directives reads through the
program, one line at a time using `getline' (*note Explicit Input with
`getline': Getline.).  The input file names and `@include' statements
are managed using a stack.  As each `@include' is encountered, the
current file name is "pushed" onto the stack, and the file named in the
`@include' directive becomes the current file name.  As each file is
finished, the stack is "popped," and the previous input file becomes
the current input file again.  The process is started by making the
original file the first one on the stack.

   The `pathto' function does the work of finding the full path to a
file.  It simulates `gawk''s behavior when searching the `AWKPATH'
environment variable (*note The `AWKPATH' Environment Variable: AWKPATH
Variable.).  If a file name has a `/' in it, no path search is done.
Otherwise, the file name is concatenated with the name of each
directory in the path, and an attempt is made to open the generated file
name.  The only way in `awk' to test if a file can be read is to go
ahead and try to read it with `getline'; that is what `pathto' does.
If the file can be read, it is closed, and the file name is returned.

     gawk -- '
     # process @include directives
     
     function pathto(file,    i, t, junk)
     {
         if (index(file, "/") != 0)
             return file
     
         for (i = 1; i <= ndirs; i++) {
             t = (pathlist[i] "/" file)
             if ((getline junk < t) > 0) {
                 # found it
                 close(t)
                 return t
             }
         }
         return ""
     }

   The main program is contained inside one `BEGIN' rule.  The first
thing it does is set up the `pathlist' array that `pathto' uses.  After
splitting the path on `:', null elements are replaced with `"."', which
represents the current directory.

     BEGIN {
         path = ENVIRON["AWKPATH"]
         ndirs = split(path, pathlist, ":")
         for (i = 1; i <= ndirs; i++) {
             if (pathlist[i] == "")
                 pathlist[i] = "."
         }

   The stack is initialized with `ARGV[1]', which will be
`/tmp/ig.s.$$'.  The main loop comes next.  Input lines are read in
succession. Lines that do not start with `@include' are printed
verbatim.

   If the line does start with `@include', the file name is in `$2'.
`pathto' is called to generate the full path.  If it could not, then we
print an error message and continue.

   The next thing to check is if the file has been included already.
The `processed' array is indexed by the full file name of each included
file, and it tracks this information for us.  If the file has been
seen, a warning message is printed. Otherwise, the new file name is
pushed onto the stack and processing continues.

   Finally, when `getline' encounters the end of the input file, the
file is closed and the stack is popped.  When `stackptr' is less than
zero, the program is done.

         stackptr = 0
         input[stackptr] = ARGV[1] # ARGV[1] is first file
     
         for (; stackptr >= 0; stackptr--) {
             while ((getline < input[stackptr]) > 0) {
                 if (tolower($1) != "@include") {
                     print
                     continue
                 }
                 fpath = pathto($2)
                 if (fpath == "") {
                     printf("igawk:%s:%d: cannot find %s\n", \
                         input[stackptr], FNR, $2) > "/dev/stderr"
                     continue
                 }
                 if (! (fpath in processed)) {
                     processed[fpath] = input[stackptr]
                     input[++stackptr] = fpath
                 } else
                     print $2, "included in", input[stackptr], \
                         "already included in", \
                         processed[fpath] > "/dev/stderr"
             }
             close(input[stackptr])
         }
     }' /tmp/ig.s.$$ > /tmp/ig.e.$$

   The last step is to call `gawk' with the expanded program and the
original options and command line arguments that the user supplied.
`gawk''s exit status is passed back on to `igawk''s calling program.

     eval gawk -f /tmp/ig.e.$$ $opts -- "$@"
     
     exit $?

   This version of `igawk' represents my third attempt at this program.
There are three key simplifications that made the program work better.

  1. Using `@include' even for the files named with `-f' makes building
     the initial collected `awk' program much simpler; all the
     `@include' processing can be done once.

  2. The `pathto' function doesn't try to save the line read with
     `getline' when testing for the file's accessibility.  Trying to
     save this line for use with the main program complicates things
     considerably.

  3. Using a `getline' loop in the `BEGIN' rule does it all in one
     place.  It is not necessary to call out to a separate loop for
     processing nested `@include' statements.

   Also, this program illustrates that it is often worthwhile to combine
`sh' and `awk' programming together.  You can usually accomplish quite
a lot, without having to resort to low-level programming in C or C++,
and it is frequently easier to do certain kinds of string and argument
manipulation using the shell than it is in `awk'.

   Finally, `igawk' shows that it is not always necessary to add new
features to a program; they can often be layered on top.  With `igawk',
there is no real reason to build `@include' processing into `gawk'
itself.

   As an additional example of this, consider the idea of having two
files in a directory in the search path.

`default.awk'
     This file would contain a set of default library functions, such
     as `getopt' and `assert'.

`site.awk'
     This file would contain library functions that are specific to a
     site or installation, i.e. locally developed functions.  Having a
     separate file allows `default.awk' to change with new `gawk'
     releases, without requiring the system administrator to update it
     each time by adding the local functions.

   One user suggested that `gawk' be modified to automatically read
these files upon startup.  Instead, it would be very simple to modify
`igawk' to do this. Since `igawk' can process nested `@include'
directives, `default.awk' could simply contain `@include' statements
for the desired library functions.


File: gawk.info,  Node: Language History,  Next: Gawk Summary,  Prev: Sample Programs,  Up: Top

The Evolution of the `awk' Language
***********************************

   This Info file describes the GNU implementation of `awk', which
follows the POSIX specification.  Many `awk' users are only familiar
with the original `awk' implementation in Version 7 Unix.  (This
implementation was the basis for `awk' in Berkeley Unix, through
4.3-Reno.  The 4.4 release of Berkeley Unix uses `gawk' 2.15.2 for its
version of `awk'.) This chapter briefly describes the evolution of the
`awk' language, with cross references to other parts of the Info file
where you can find more information.

* Menu:

* V7/SVR3.1::                   The major changes between V7 and System V
                                Release 3.1.
* SVR4::                        Minor changes between System V Releases 3.1
                                and 4.
* POSIX::                       New features from the POSIX standard.
* BTL::                         New features from the Bell Laboratories
                                version of `awk'.
* POSIX/GNU::                   The extensions in `gawk' not in POSIX
                                `awk'.


File: gawk.info,  Node: V7/SVR3.1,  Next: SVR4,  Prev: Language History,  Up: Language History

Major Changes between V7 and SVR3.1
===================================

   The `awk' language evolved considerably between the release of
Version 7 Unix (1978) and the new version first made generally
available in System V Release 3.1 (1987).  This section summarizes the
changes, with cross-references to further details.

   * The requirement for `;' to separate rules on a line (*note `awk'
     Statements Versus Lines: Statements/Lines.).

   * User-defined functions, and the `return' statement (*note
     User-defined Functions: User-defined.).

   * The `delete' statement (*note The `delete' Statement: Delete.).

   * The `do'-`while' statement (*note The `do'-`while' Statement: Do
     Statement.).

   * The built-in functions `atan2', `cos', `sin', `rand' and `srand'
     (*note Numeric Built-in Functions: Numeric Functions.).

   * The built-in functions `gsub', `sub', and `match' (*note Built-in
     Functions for String Manipulation: String Functions.).

   * The built-in functions `close', and `system' (*note Built-in
     Functions for Input/Output: I/O Functions.).

   * The `ARGC', `ARGV', `FNR', `RLENGTH', `RSTART', and `SUBSEP'
     built-in variables (*note Built-in Variables::.).

   * The conditional expression using the ternary operator `?:' (*note
     Conditional Expressions: Conditional Exp.).

   * The exponentiation operator `^' (*note Arithmetic Operators:
     Arithmetic Ops.) and its assignment operator form `^=' (*note
     Assignment Expressions: Assignment Ops.).

   * C-compatible operator precedence, which breaks some old `awk'
     programs (*note Operator Precedence (How Operators Nest):
     Precedence.).

   * Regexps as the value of `FS' (*note Specifying How Fields are
     Separated: Field Separators.), and as the third argument to the
     `split' function (*note Built-in Functions for String
     Manipulation: String Functions.).

   * Dynamic regexps as operands of the `~' and `!~' operators (*note
     How to Use Regular Expressions: Regexp Usage.).

   * The escape sequences `\b', `\f', and `\r' (*note Escape
     Sequences::.).  (Some vendors have updated their old versions of
     `awk' to recognize `\r', `\b', and `\f', but this is not something
     you can rely on.)

   * Redirection of input for the `getline' function (*note Explicit
     Input with `getline': Getline.).

   * Multiple `BEGIN' and `END' rules (*note The `BEGIN' and `END'
     Special Patterns: BEGIN/END.).

   * Multi-dimensional arrays (*note Multi-dimensional Arrays:
     Multi-dimensional.).


File: gawk.info,  Node: SVR4,  Next: POSIX,  Prev: V7/SVR3.1,  Up: Language History

Changes between SVR3.1 and SVR4
===============================

   The System V Release 4 version of Unix `awk' added these features
(some of which originated in `gawk'):

   * The `ENVIRON' variable (*note Built-in Variables::.).

   * Multiple `-f' options on the command line (*note Command Line
     Options: Options.).

   * The `-v' option for assigning variables before program execution
     begins (*note Command Line Options: Options.).

   * The `--' option for terminating command line options.

   * The `\a', `\v', and `\x' escape sequences (*note Escape
     Sequences::.).

   * A defined return value for the `srand' built-in function (*note
     Numeric Built-in Functions: Numeric Functions.).

   * The `toupper' and `tolower' built-in string functions for case
     translation (*note Built-in Functions for String Manipulation:
     String Functions.).

   * A cleaner specification for the `%c' format-control letter in the
     `printf' function (*note Format-Control Letters: Control Letters.).

   * The ability to dynamically pass the field width and precision
     (`"%*.*d"') in the argument list of the `printf' function (*note
     Format-Control Letters: Control Letters.).

   * The use of regexp constants such as `/foo/' as expressions, where
     they are equivalent to using the matching operator, as in `$0 ~
     /foo/' (*note Using Regular Expression Constants: Using Constant
     Regexps.).


File: gawk.info,  Node: POSIX,  Next: BTL,  Prev: SVR4,  Up: Language History

Changes between SVR4 and POSIX `awk'
====================================

   The POSIX Command Language and Utilities standard for `awk'
introduced the following changes into the language:

   * The use of `-W' for implementation-specific options.

   * The use of `CONVFMT' for controlling the conversion of numbers to
     strings (*note Conversion of Strings and Numbers: Conversion.).

   * The concept of a numeric string, and tighter comparison rules to go
     with it (*note Variable Typing and Comparison Expressions: Typing
     and Comparison.).

   * More complete documentation of many of the previously undocumented
     features of the language.

   The following common extensions are not permitted by the POSIX
standard:

   * `\x' escape sequences are not recognized (*note Escape
     Sequences::.).

   * Newlines do not act as whitespace to separate fields when `FS' is
     equal to a single space.

   * The synonym `func' for the keyword `function' is not recognized
     (*note Function Definition Syntax: Definition Syntax.).

   * The operators `**' and `**=' cannot be used in place of `^' and
     `^=' (*note Arithmetic Operators: Arithmetic Ops., and also *note
     Assignment Expressions: Assignment Ops.).

   * Specifying `-Ft' on the command line does not set the value of
     `FS' to be a single tab character (*note Specifying How Fields are
     Separated: Field Separators.).

   * The `fflush' built-in function is not supported (*note Built-in
     Functions for Input/Output: I/O Functions.).


File: gawk.info,  Node: BTL,  Next: POSIX/GNU,  Prev: POSIX,  Up: Language History

Extensions in the Bell Laboratories `awk'
=========================================

   Brian Kernighan, one of the original designers of Unix `awk', has
made his version available via anonymous `ftp' (*note Other Freely
Available `awk' Implementations: Other Versions.).  This section
describes extensions in his version of `awk' that are not in POSIX
`awk'.

   * The `-mf NNN' and `-mr NNN' command line options to set the
     maximum number of fields, and the maximum record size, respectively
     (*note Command Line Options: Options.).

   * The `fflush' built-in function for flushing buffered output (*note
     Built-in Functions for Input/Output: I/O Functions.).


File: gawk.info,  Node: POSIX/GNU,  Prev: BTL,  Up: Language History

Extensions in `gawk' Not in POSIX `awk'
=======================================

   The GNU implementation, `gawk', adds a number of features.  This
sections lists them in the order they were added to `gawk'.  They can
all be disabled with either the `--traditional' or `--posix' options
(*note Command Line Options: Options.).

   Version 2.10 of `gawk' introduced these features:

   * The `AWKPATH' environment variable for specifying a path search for
     the `-f' command line option (*note Command Line Options:
     Options.).

   * The `IGNORECASE' variable and its effects (*note Case-sensitivity
     in Matching: Case-sensitivity.).

   * The `/dev/stdin', `/dev/stdout', `/dev/stderr', and `/dev/fd/N'
     file name interpretation (*note Special File Names in `gawk':
     Special Files.).

   Version 2.13 of `gawk' introduced these features:

   * The `FIELDWIDTHS' variable and its effects (*note Reading
     Fixed-width Data: Constant Size.).

   * The `systime' and `strftime' built-in functions for obtaining and
     printing time stamps (*note Functions for Dealing with Time
     Stamps: Time Functions.).

   * The `-W lint' option to provide source code and run time error and
     portability checking (*note Command Line Options: Options.).

   * The `-W compat' option to turn off these extensions (*note Command
     Line Options: Options.).

   * The `-W posix' option for full POSIX compliance (*note Command
     Line Options: Options.).

   Version 2.14 of `gawk' introduced these features:

   * The `next file' statement for skipping to the next data file
     (*note The `nextfile' Statement: Nextfile Statement.).

   Version 2.15 of `gawk' introduced these features:

   * The `ARGIND' variable, that tracks the movement of `FILENAME'
     through `ARGV'  (*note Built-in Variables::.).

   * The `ERRNO' variable, that contains the system error message when
     `getline' returns -1, or when `close' fails (*note Built-in
     Variables::.).

   * The ability to use GNU-style long named options that start with
     `--' (*note Command Line Options: Options.).

   * The `--source' option for mixing command line and library file
     source code (*note Command Line Options: Options.).

   * The `/dev/pid', `/dev/ppid', `/dev/pgrpid', and `/dev/user' file
     name interpretation (*note Special File Names in `gawk': Special
     Files.).

   Version 3.0 of `gawk' introduced these features:

   * The `next file' statement became `nextfile' (*note The `nextfile'
     Statement: Nextfile Statement.).

   * The `--lint-old' option to warn about constructs that are not
     available in the original Version 7 Unix version of `awk' (*note
     Major Changes between V7 and SVR3.1: V7/SVR3.1.).

   * The `--traditional' option was added as a better name for
     `--compat' (*note Command Line Options: Options.).

   * The ability for `FS' to be a null string, and for the third
     argument to `split' to be the null string (*note Making Each
     Character a Separate Field: Single Character Fields.).

   * The ability for `RS' to be a regexp (*note How Input is Split into
     Records: Records.).

   * The `RT' variable (*note How Input is Split into Records:
     Records.).

   * The `gensub' function for more powerful text manipulation (*note
     Built-in Functions for String Manipulation: String Functions.).

   * The `strftime' function acquired a default time format, allowing
     it to be called with no arguments (*note Functions for Dealing
     with Time Stamps: Time Functions.).

   * Full support for both POSIX and GNU regexps (*note Regular
     Expressions: Regexp.).

   * The `--re-interval' option to provide interval expressions in
     regexps (*note Regular Expression Operators: Regexp Operators.).

   * `IGNORECASE' changed, now applying to string comparison as well as
     regexp operations (*note Case-sensitivity in Matching:
     Case-sensitivity.).

   * The `-m' option and the `fflush' function from the Bell Labs
     research version of `awk' (*note Command Line Options: Options.;
     also *note Built-in Functions for Input/Output: I/O Functions.).

   * The use of GNU Autoconf to control the configuration process
     (*note Compiling `gawk' for Unix: Quick Installation.).

   * Amiga support (*note Installing `gawk' on an Amiga: Amiga
     Installation.).


File: gawk.info,  Node: Gawk Summary,  Next: Installation,  Prev: Language History,  Up: Top

`gawk' Summary
**************

   This appendix provides a brief summary of the `gawk' command line
and the `awk' language.  It is designed to serve as "quick reference."
It is therefore terse, but complete.

* Menu:

* Command Line Summary::        Recapitulation of the command line.
* Language Summary::            A terse review of the language.
* Variables/Fields::            Variables, fields, and arrays.
* Rules Summary::               Patterns and Actions, and their component
                                parts.
* Actions Summary::             Quick overview of actions.
* Functions Summary::           Defining and calling functions.
* Historical Features::         Some undocumented but supported "features".


File: gawk.info,  Node: Command Line Summary,  Next: Language Summary,  Prev: Gawk Summary,  Up: Gawk Summary

Command Line Options Summary
============================

   The command line consists of options to `gawk' itself, the `awk'
program text (if not supplied via the `-f' option), and values to be
made available in the `ARGC' and `ARGV' predefined `awk' variables:

     gawk [POSIX OR GNU STYLE OPTIONS] -f SOURCE-FILE [`--'] FILE ...
     gawk [POSIX OR GNU STYLE OPTIONS] [`--'] 'PROGRAM' FILE ...

   The options that `gawk' accepts are:

`-F FS'
`--field-separator FS'
     Use FS for the input field separator (the value of the `FS'
     predefined variable).

`-f PROGRAM-FILE'
`--file PROGRAM-FILE'
     Read the `awk' program source from the file PROGRAM-FILE, instead
     of from the first command line argument.

`-mf NNN'
`-mr NNN'
     The `f' flag sets the maximum number of fields, and the `r' flag
     sets the maximum record size.  These options are ignored by
     `gawk', since `gawk' has no predefined limits; they are only for
     compatibility with the Bell Labs research version of Unix `awk'.

`-v VAR=VAL'
`--assign VAR=VAL'
     Assign the variable VAR the value VAL before program execution
     begins.

`-W traditional'
`-W compat'
`--traditional'
`--compat'
     Use compatibility mode, in which `gawk' extensions are turned off.

`-W copyleft'
`-W copyright'
`--copyleft'
`--copyright'
     Print the short version of the General Public License on the
     standard output, and exit.  This option may disappear in a future
     version of `gawk'.

`-W help'
`-W usage'
`--help'
`--usage'
     Print a relatively short summary of the available options on the
     standard output, and exit.

`-W lint'
`--lint'
     Give warnings about dubious or non-portable `awk' constructs.

`-W lint-old'
`--lint-old'
     Warn about constructs that are not available in the original
     Version 7 Unix version of `awk'.

`-W posix'
`--posix'
     Use POSIX compatibility mode, in which `gawk' extensions are
     turned off and additional restrictions apply.

`-W re-interval'
`--re-interval'
     Allow interval expressions (*note Regular Expression Operators:
     Regexp Operators.), in regexps.

`-W source=PROGRAM-TEXT'
`--source PROGRAM-TEXT'
     Use PROGRAM-TEXT as `awk' program source code.  This option allows
     mixing command line source code with source code from files, and is
     particularly useful for mixing command line programs with library
     functions.

`-W version'
`--version'
     Print version information for this particular copy of `gawk' on
     the error output.

`--'
     Signal the end of options.  This is useful to allow further
     arguments to the `awk' program itself to start with a `-'.  This
     is mainly for consistency with POSIX argument parsing conventions.

   Any other options are flagged as invalid, but are otherwise ignored.
*Note Command Line Options: Options, for more details.


File: gawk.info,  Node: Language Summary,  Next: Variables/Fields,  Prev: Command Line Summary,  Up: Gawk Summary

Language Summary
================

   An `awk' program consists of a sequence of zero or more
pattern-action statements and optional function definitions.  One or
the other of the pattern and action may be omitted.

     PATTERN    { ACTION STATEMENTS }
     PATTERN
               { ACTION STATEMENTS }
     
     function NAME(PARAMETER LIST)     { ACTION STATEMENTS }

   `gawk' first reads the program source from the PROGRAM-FILE(s), if
specified, or from the first non-option argument on the command line.
The `-f' option may be used multiple times on the command line.  `gawk'
reads the program text from all the PROGRAM-FILE files, effectively
concatenating them in the order they are specified.  This is useful for
building libraries of `awk' functions, without having to include them
in each new `awk' program that uses them.  To use a library function in
a file from a program typed in on the command line, specify `--source
'PROGRAM'', and type your program in between the single quotes.  *Note
Command Line Options: Options.

   The environment variable `AWKPATH' specifies a search path to use
when finding source files named with the `-f' option.  The default
path, which is `.:/usr/local/share/awk'(1) is used if `AWKPATH' is not
set.  If a file name given to the `-f' option contains a `/' character,
no path search is performed.  *Note The `AWKPATH' Environment Variable:
AWKPATH Variable.

   `gawk' compiles the program into an internal form, and then proceeds
to read each file named in the `ARGV' array.  The initial values of
`ARGV' come from the command line arguments.  If there are no files
named on the command line, `gawk' reads the standard input.

   If a "file" named on the command line has the form `VAR=VAL', it is
treated as a variable assignment: the variable VAR is assigned the
value VAL.  If any of the files have a value that is the null string,
that element in the list is skipped.

   For each record in the input, `gawk' tests to see if it matches any
PATTERN in the `awk' program.  For each pattern that the record
matches, the associated ACTION is executed.

   ---------- Footnotes ----------

   (1)  The path may use a directory other than `/usr/local/share/awk',
depending upon how `gawk' was built and installed.


File: gawk.info,  Node: Variables/Fields,  Next: Rules Summary,  Prev: Language Summary,  Up: Gawk Summary

Variables and Fields
====================

   `awk' variables are not declared; they come into existence when they
are first used.  Their values are either floating-point numbers or
strings.  `awk' also has one-dimensional arrays; multiple-dimensional
arrays may be simulated.  There are several predefined variables that
`awk' sets as a program runs; these are summarized below.

* Menu:

* Fields Summary::              Input field splitting.
* Built-in Summary::            `awk''s built-in variables.
* Arrays Summary::              Using arrays.
* Data Type Summary::           Values in `awk' are numbers or strings.


File: gawk.info,  Node: Fields Summary,  Next: Built-in Summary,  Prev: Variables/Fields,  Up: Variables/Fields

Fields
------

   As each input line is read, `gawk' splits the line into FIELDS,
using the value of the `FS' variable as the field separator.  If `FS'
is a single character, fields are separated by that character.
Otherwise, `FS' is expected to be a full regular expression.  In the
special case that `FS' is a single space, fields are separated by runs
of spaces, tabs and/or newlines.(1) If `FS' is the null string (`""'),
then each individual character in the record becomes a separate field.
Note that the value of `IGNORECASE' (*note Case-sensitivity in
Matching: Case-sensitivity.) also affects how fields are split when
`FS' is a regular expression.

   Each field in the input line may be referenced by its position, `$1',
`$2', and so on.  `$0' is the whole line.  The value of a field may be
assigned to as well.  Field numbers need not be constants:

     n = 5
     print $n

prints the fifth field in the input line.  The variable `NF' is set to
the total number of fields in the input line.

   References to non-existent fields (i.e. fields after `$NF') return
the null string.  However, assigning to a non-existent field (e.g.,
`$(NF+2) = 5') increases the value of `NF', creates any intervening
fields with the null string as their value, and causes the value of
`$0' to be recomputed, with the fields being separated by the value of
`OFS'.  Decrementing `NF' causes the values of fields past the new
value to be lost, and the value of `$0' to be recomputed, with the
fields being separated by the value of `OFS'.  *Note Reading Input
Files: Reading Files.

   ---------- Footnotes ----------

   (1)  In POSIX `awk', newline does not separate fields.


File: gawk.info,  Node: Built-in Summary,  Next: Arrays Summary,  Prev: Fields Summary,  Up: Variables/Fields

Built-in Variables
------------------

   `gawk''s built-in variables are:

`ARGC'
     The number of elements in `ARGV'. See below for what is actually
     included in `ARGV'.

`ARGIND'
     The index in `ARGV' of the current file being processed.  When
     `gawk' is processing the input data files, it is always true that
     `FILENAME == ARGV[ARGIND]'.

`ARGV'
     The array of command line arguments.  The array is indexed from
     zero to `ARGC' - 1.  Dynamically changing `ARGC' and the contents
     of `ARGV' can control the files used for data.  A null-valued
     element in `ARGV' is ignored. `ARGV' does not include the options
     to `awk' or the text of the `awk' program itself.

`CONVFMT'
     The conversion format to use when converting numbers to strings.

`FIELDWIDTHS'
     A space separated list of numbers describing the fixed-width input
     data.

`ENVIRON'
     An array of environment variable values.  The array is indexed by
     variable name, each element being the value of that variable.
     Thus, the environment variable `HOME' is `ENVIRON["HOME"]'.  One
     possible value might be `/home/arnold'.

     Changing this array does not affect the environment seen by
     programs which `gawk' spawns via redirection or the `system'
     function.  (This may change in a future version of `gawk'.)

     Some operating systems do not have environment variables.  The
     `ENVIRON' array is empty when running on these systems.

`ERRNO'
     The system error message when an error occurs using `getline' or
     `close'.

`FILENAME'
     The name of the current input file.  If no files are specified on
     the command line, the value of `FILENAME' is the null string.

`FNR'
     The input record number in the current input file.

`FS'
     The input field separator, a space by default.

`IGNORECASE'
     The case-sensitivity flag for string comparisons and regular
     expression operations.  If `IGNORECASE' has a non-zero value, then
     pattern matching in rules, record separating with `RS', field
     splitting with `FS', regular expression matching with `~' and
     `!~', and the `gensub', `gsub', `index', `match', `split' and
     `sub' built-in functions all ignore case when doing regular
     expression operations, and all string comparisons are done
     ignoring case.

`NF'
     The number of fields in the current input record.

`NR'
     The total number of input records seen so far.

`OFMT'
     The output format for numbers for the `print' statement, `"%.6g"'
     by default.

`OFS'
     The output field separator, a space by default.

`ORS'
     The output record separator, by default a newline.

`RS'
     The input record separator, by default a newline.  If `RS' is set
     to the null string, then records are separated by blank lines.
     When `RS' is set to the null string, then the newline character
     always acts as a field separator, in addition to whatever value
     `FS' may have.  If `RS' is set to a multi-character string, it
     denotes a regexp; input text matching the regexp separates records.

`RT'
     The input text that matched the text denoted by `RS', the record
     separator.

`RSTART'
     The index of the first character last matched by `match'; zero if
     no match.

`RLENGTH'
     The length of the string last matched by `match'; -1 if no match.

`SUBSEP'
     The string used to separate multiple subscripts in array elements,
     by default `"\034"'.

   *Note Built-in Variables::, for more information.


File: gawk.info,  Node: Arrays Summary,  Next: Data Type Summary,  Prev: Built-in Summary,  Up: Variables/Fields

Arrays
------

   Arrays are subscripted with an expression between square brackets
(`[' and `]').  Array subscripts are *always* strings; numbers are
converted to strings as necessary, following the standard conversion
rules (*note Conversion of Strings and Numbers: Conversion.).

   If you use multiple expressions separated by commas inside the square
brackets, then the array subscript is a string consisting of the
concatenation of the individual subscript values, converted to strings,
separated by the subscript separator (the value of `SUBSEP').

   The special operator `in' may be used in a conditional context to
see if an array has an index consisting of a particular value.

     if (val in array)
             print array[val]

   If the array has multiple subscripts, use `(i, j, ...) in ARRAY' to
test for existence of an element.

   The `in' construct may also be used in a `for' loop to iterate over
all the elements of an array.  *Note Scanning All Elements of an Array:
Scanning an Array.

   You can remove an element from an array using the `delete' statement.

   You can clear an entire array using `delete ARRAY'.

   *Note Arrays in `awk': Arrays.


File: gawk.info,  Node: Data Type Summary,  Prev: Arrays Summary,  Up: Variables/Fields

Data Types
----------

   The value of an `awk' expression is always either a number or a
string.

   Some contexts (such as arithmetic operators) require numeric values.
They convert strings to numbers by interpreting the text of the string
as a number.  If the string does not look like a number, it converts to
zero.

   Other contexts (such as concatenation) require string values.  They
convert numbers to strings by effectively printing them with `sprintf'.
*Note Conversion of Strings and Numbers: Conversion, for the details.

   To force conversion of a string value to a number, simply add zero
to it.  If the value you start with is already a number, this does not
change it.

   To force conversion of a numeric value to a string, concatenate it
with the null string.

   Comparisons are done numerically if both operands are numeric, or if
one is numeric and the other is a numeric string.  Otherwise one or
both operands are converted to strings and a string comparison is
performed.  Fields, `getline' input, `FILENAME', `ARGV' elements,
`ENVIRON' elements and the elements of an array created by `split' are
the only items that can be numeric strings. String constants, such as
`"3.1415927"' are not numeric strings, they are string constants.  The
full rules for comparisons are described in *Note Variable Typing and
Comparison Expressions: Typing and Comparison.

   Uninitialized variables have the string value `""' (the null, or
empty, string).  In contexts where a number is required, this is
equivalent to zero.

   *Note Variables::, for more information on variable naming and
initialization; *note Conversion of Strings and Numbers: Conversion.,
for more information on how variable values are interpreted.


File: gawk.info,  Node: Rules Summary,  Next: Actions Summary,  Prev: Variables/Fields,  Up: Gawk Summary

Patterns
========

* Menu:

* Pattern Summary::             Quick overview of patterns.
* Regexp Summary::              Quick overview of regular expressions.

   An `awk' program is mostly composed of rules, each consisting of a
pattern followed by an action.  The action is enclosed in `{' and `}'.
Either the pattern may be missing, or the action may be missing, but
not both.  If the pattern is missing, the action is executed for every
input record.  A missing action is equivalent to `{ print }', which
prints the entire line.

   Comments begin with the `#' character, and continue until the end of
the line.  Blank lines may be used to separate statements.  Statements
normally end with a newline; however, this is not the case for lines
ending in a `,', `{', `?', `:', `&&', or `||'.  Lines ending in `do' or
`else' also have their statements automatically continued on the
following line.  In other cases, a line can be continued by ending it
with a `\', in which case the newline is ignored.

   Multiple statements may be put on one line by separating each one
with a `;'.  This applies to both the statements within the action part
of a rule (the usual case), and to the rule statements.

   *Note Comments in `awk' Programs: Comments, for information on
`awk''s commenting convention; *note `awk' Statements Versus Lines:
Statements/Lines., for a description of the line continuation mechanism
in `awk'.


File: gawk.info,  Node: Pattern Summary,  Next: Regexp Summary,  Prev: Rules Summary,  Up: Rules Summary

Pattern Summary
---------------

   `awk' patterns may be one of the following:

     /REGULAR EXPRESSION/
     RELATIONAL EXPRESSION
     PATTERN && PATTERN
     PATTERN || PATTERN
     PATTERN ? PATTERN : PATTERN
     (PATTERN)
     ! PATTERN
     PATTERN1, PATTERN2
     BEGIN
     END

   `BEGIN' and `END' are two special kinds of patterns that are not
tested against the input.  The action parts of all `BEGIN' rules are
concatenated as if all the statements had been written in a single
`BEGIN' rule.  They are executed before any of the input is read.
Similarly, all the `END' rules are concatenated, and executed when all
the input is exhausted (or when an `exit' statement is executed).
`BEGIN' and `END' patterns cannot be combined with other patterns in
pattern expressions.  `BEGIN' and `END' rules cannot have missing
action parts.

   For `/REGULAR-EXPRESSION/' patterns, the associated statement is
executed for each input record that matches the regular expression.
Regular expressions are summarized below.

   A RELATIONAL EXPRESSION may use any of the operators defined below in
the section on actions.  These generally test whether certain fields
match certain regular expressions.

   The `&&', `||', and `!' operators are logical "and," logical "or,"
and logical "not," respectively, as in C.  They do short-circuit
evaluation, also as in C, and are used for combining more primitive
pattern expressions.  As in most languages, parentheses may be used to
change the order of evaluation.

   The `?:' operator is like the same operator in C.  If the first
pattern matches, then the second pattern is matched against the input
record; otherwise, the third is matched.  Only one of the second and
third patterns is matched.

   The `PATTERN1, PATTERN2' form of a pattern is called a range
pattern.  It matches all input lines starting with a line that matches
PATTERN1, and continuing until a line that matches PATTERN2, inclusive.
A range pattern cannot be used as an operand of any of the pattern
operators.

   *Note Pattern Elements: Pattern Overview.


File: gawk.info,  Node: Regexp Summary,  Prev: Pattern Summary,  Up: Rules Summary

Regular Expressions
-------------------

   Regular expressions are based on POSIX EREs (extended regular
expressions).  The escape sequences allowed in string constants are
also valid in regular expressions (*note Escape Sequences::.).  Regexps
are composed of characters as follows:

`C'
     matches the character C (assuming C is none of the characters
     listed below).

`\C'
     matches the literal character C.

`.'
     matches any character, *including* newline.  In strict POSIX mode,
     `.' does not match the NUL character, which is a character with
     all bits equal to zero.

`^'
     matches the beginning of a string.

`$'
     matches the end of a string.

`[ABC...]'
     matches any of the characters ABC... (character list).

`[[:CLASS:]]'
     matches any character in the character class CLASS. Allowable
     classes are `alnum', `alpha', `blank', `cntrl', `digit', `graph',
     `lower', `print', `punct', `space', `upper', and `xdigit'.

`[[.SYMBOL.]]'
     matches the multi-character collating symbol SYMBOL.  `gawk' does
     not currently support collating symbols.

`[[=CLASSNAME=]]'
     matches any of the equivalent characters in the current locale
     named by the equivalence class CLASSNAME.  `gawk' does not
     currently support equivalence classes.

`[^ABC...]'
     matches any character except ABC... (negated character list).

`R1|R2'
     matches either R1 or R2 (alternation).

`R1R2'
     matches R1, and then R2 (concatenation).

`R+'
     matches one or more R's.

`R*'
     matches zero or more R's.

`R?'
     matches zero or one R's.

`(R)'
     matches R (grouping).

`R{N}'
`R{N,}'
`R{N,M}'
     matches at least N, N to any number, or N to M occurrences of R
     (interval expressions).

`\y'
     matches the empty string at either the beginning or the end of a
     word.

`\B'
     matches the empty string within a word.

`\<'
     matches the empty string at the beginning of a word.

`\>'
     matches the empty string at the end of a word.

`\w'
     matches any word-constituent character (alphanumeric characters and
     the underscore).

`\W'
     matches any character that is not word-constituent.

`\`'
     matches the empty string at the beginning of a buffer (same as a
     string in `gawk').

`\''
     matches the empty string at the end of a buffer.

   The various command line options control how `gawk' interprets
characters in regexps.

No options
     In the default case, `gawk' provide all the facilities of POSIX
     regexps and the GNU regexp operators described above.  However,
     interval expressions are not supported.

`--posix'
     Only POSIX regexps are supported, the GNU operators are not special
     (e.g., `\w' matches a literal `w').  Interval expressions are
     allowed.

`--traditional'
     Traditional Unix `awk' regexps are matched. The GNU operators are
     not special, interval expressions are not available, and neither
     are the POSIX character classes (`[[:alnum:]]' and so on).
     Characters described by octal and hexadecimal escape sequences are
     treated literally, even if they represent regexp metacharacters.

`--re-interval'
     Allow interval expressions in regexps, even if `--traditional' has
     been provided.

   *Note Regular Expressions: Regexp.


File: gawk.info,  Node: Actions Summary,  Next: Functions Summary,  Prev: Rules Summary,  Up: Gawk Summary

Actions
=======

   Action statements are enclosed in braces, `{' and `}'.  A missing
action statement is equivalent to `{ print }'.

   Action statements consist of the usual assignment, conditional, and
looping statements found in most languages.  The operators, control
statements, and Input/Output statements available are similar to those
in C.

   Comments begin with the `#' character, and continue until the end of
the line.  Blank lines may be used to separate statements.  Statements
normally end with a newline; however, this is not the case for lines
ending in a `,', `{', `?', `:', `&&', or `||'.  Lines ending in `do' or
`else' also have their statements automatically continued on the
following line.  In other cases, a line can be continued by ending it
with a `\', in which case the newline is ignored.

   Multiple statements may be put on one line by separating each one
with a `;'.  This applies to both the statements within the action part
of a rule (the usual case), and to the rule statements.

   *Note Comments in `awk' Programs: Comments, for information on
`awk''s commenting convention; *note `awk' Statements Versus Lines:
Statements/Lines., for a description of the line continuation mechanism
in `awk'.

* Menu:

* Operator Summary::            `awk' operators.
* Control Flow Summary::        The control statements.
* I/O Summary::                 The I/O statements.
* Printf Summary::              A summary of `printf'.
* Special File Summary::        Special file names interpreted internally.
* Built-in Functions Summary::  Built-in numeric and string functions.
* Time Functions Summary::      Built-in time functions.
* String Constants Summary::    Escape sequences in strings.


File: gawk.info,  Node: Operator Summary,  Next: Control Flow Summary,  Prev: Actions Summary,  Up: Actions Summary

Operators
---------

   The operators in `awk', in order of decreasing precedence, are:

`(...)'
     Grouping.

`$'
     Field reference.

`++ --'
     Increment and decrement, both prefix and postfix.

`^'
     Exponentiation (`**' may also be used, and `**=' for the assignment
     operator, but they are not specified in the POSIX standard).

`+ - !'
     Unary plus, unary minus, and logical negation.

`* / %'
     Multiplication, division, and modulus.

`+ -'
     Addition and subtraction.

`SPACE'
     String concatenation.

`< <= > >= != =='
     The usual relational operators.

`~ !~'
     Regular expression match, negated match.

`in'
     Array membership.

`&&'
     Logical "and".

`||'
     Logical "or".

`?:'
     A conditional expression.  This has the form `EXPR1 ?  EXPR2 :
     EXPR3'.  If EXPR1 is true, the value of the expression is EXPR2;
     otherwise it is EXPR3.  Only one of EXPR2 and EXPR3 is evaluated.

`= += -= *= /= %= ^='
     Assignment.  Both absolute assignment (`VAR=VALUE') and operator
     assignment (the other forms) are supported.

   *Note Expressions::.


File: gawk.info,  Node: Control Flow Summary,  Next: I/O Summary,  Prev: Operator Summary,  Up: Actions Summary

Control Statements
------------------

   The control statements are as follows:

     if (CONDITION) STATEMENT [ else STATEMENT ]
     while (CONDITION) STATEMENT
     do STATEMENT while (CONDITION)
     for (EXPR1; EXPR2; EXPR3) STATEMENT
     for (VAR in ARRAY) STATEMENT
     break
     continue
     delete ARRAY[INDEX]
     delete ARRAY
     exit [ EXPRESSION ]
     { STATEMENTS }

   *Note Control Statements in Actions: Statements.


File: gawk.info,  Node: I/O Summary,  Next: Printf Summary,  Prev: Control Flow Summary,  Up: Actions Summary

I/O Statements
--------------

   The Input/Output statements are as follows:

`getline'
     Set `$0' from next input record; set `NF', `NR', `FNR'.  *Note
     Explicit Input with `getline': Getline.

`getline <FILE'
     Set `$0' from next record of FILE; set `NF'.

`getline VAR'
     Set VAR from next input record; set `NR', `FNR'.

`getline VAR <FILE'
     Set VAR from next record of FILE.

`COMMAND | getline'
     Run COMMAND, piping its output into `getline'; sets `$0', `NF',
     `NR'.

`COMMAND | getline `var''
     Run COMMAND, piping its output into `getline'; sets VAR.

`next'
     Stop processing the current input record.  The next input record
     is read and processing starts over with the first pattern in the
     `awk' program.  If the end of the input data is reached, the `END'
     rule(s), if any, are executed.  *Note The `next' Statement: Next
     Statement.

`nextfile'
     Stop processing the current input file.  The next input record
     read comes from the next input file.  `FILENAME' is updated, `FNR'
     is set to one, `ARGIND' is incremented, and processing starts over
     with the first pattern in the `awk' program.  If the end of the
     input data is reached, the `END' rule(s), if any, are executed.
     Earlier versions of `gawk' used `next file'; this usage is still
     supported, but is considered to be deprecated.  *Note The
     `nextfile' Statement: Nextfile Statement.

`print'
     Prints the current record.  *Note Printing Output: Printing.

`print EXPR-LIST'
     Prints expressions.

`print EXPR-LIST > FILE'
     Prints expressions to FILE. If FILE does not exist, it is created.
     If it does exist, its contents are deleted the first time the
     `print' is executed.

`print EXPR-LIST >> FILE'
     Prints expressions to FILE.  The previous contents of FILE are
     retained, and the output of `print' is appended to the file.

`print EXPR-LIST | COMMAND'
     Prints expressions, sending the output down a pipe to COMMAND.
     The pipeline to the command stays open until the `close' function
     is called.

`printf FMT, EXPR-LIST'
     Format and print.

`printf FMT, EXPR-LIST > file'
     Format and print to FILE. If FILE does not exist, it is created.
     If it does exist, its contents are deleted the first time the
     `printf' is executed.

`printf FMT, EXPR-LIST >> FILE'
     Format and print to FILE.  The previous contents of FILE are
     retained, and the output of `printf' is appended to the file.

`printf FMT, EXPR-LIST | COMMAND'
     Format and print, sending the output down a pipe to COMMAND.  The
     pipeline to the command stays open until the `close' function is
     called.

   `getline' returns zero on end of file, and -1 on an error.  In the
event of an error, `getline' will set `ERRNO' to the value of a
system-dependent string that describes the error.


File: gawk.info,  Node: Printf Summary,  Next: Special File Summary,  Prev: I/O Summary,  Up: Actions Summary

`printf' Summary
----------------

   Conversion specification have the form
`%'[FLAG][WIDTH][`.'PREC]FORMAT.  Items in brackets are optional.

   The `awk' `printf' statement and `sprintf' function accept the
following conversion specification formats:

`%c'
     An ASCII character.  If the argument used for `%c' is numeric, it
     is treated as a character and printed.  Otherwise, the argument is
     assumed to be a string, and the only first character of that
     string is printed.

`%d'
`%i'
     A decimal number (the integer part).

`%e'
`%E'
     A floating point number of the form `[-]d.dddddde[+-]dd'.  The
     `%E' format uses `E' instead of `e'.

`%f'
     A floating point number of the form [`-']`ddd.dddddd'.

`%g'
`%G'
     Use either the `%e' or `%f' formats, whichever produces a shorter
     string, with non-significant zeros suppressed.  `%G' will use `%E'
     instead of `%e'.

`%o'
     An unsigned octal number (again, an integer).

`%s'
     A character string.

`%x'
`%X'
     An unsigned hexadecimal number (an integer).  The `%X' format uses
     `A' through `F' instead of `a' through `f' for decimal 10 through
     15.

`%%'
     A single `%' character; no argument is converted.

   There are optional, additional parameters that may lie between the
`%' and the control letter:

`-'
     The expression should be left-justified within its field.

`SPACE'
     For numeric conversions, prefix positive values with a space, and
     negative values with a minus sign.

`+'
     The plus sign, used before the width modifier (see below), says to
     always supply a sign for numeric conversions, even if the data to
     be formatted is positive. The `+' overrides the space modifier.

`#'
     Use an "alternate form" for certain control letters.  For `o',
     supply a leading zero.  For `x', and `X', supply a leading `0x' or
     `0X' for a non-zero result.  For `e', `E', and `f', the result
     will always contain a decimal point.  For `g', and `G', trailing
     zeros are not removed from the result.

`0'
     A leading `0' (zero) acts as a flag, that indicates output should
     be padded with zeros instead of spaces.  This applies even to
     non-numeric output formats.  This flag only has an effect when the
     field width is wider than the value to be printed.

`WIDTH'
     The field should be padded to this width. The field is normally
     padded with spaces.  If the `0' flag has been used, it is padded
     with zeros.

`.PREC'
     A number that specifies the precision to use when printing.  For
     the `e', `E', and `f' formats, this specifies the number of digits
     you want printed to the right of the decimal point.  For the `g',
     and `G' formats, it specifies the maximum number of significant
     digits.  For the `d', `o', `i', `u', `x', and `X' formats, it
     specifies the minimum number of digits to print.  For the `s'
     format, it specifies the maximum number of characters from the
     string that should be printed.

   Either or both of the WIDTH and PREC values may be specified as `*'.
In that case, the particular value is taken from the argument list.

   *Note Using `printf' Statements for Fancier Printing: Printf.


File: gawk.info,  Node: Special File Summary,  Next: Built-in Functions Summary,  Prev: Printf Summary,  Up: Actions Summary

Special File Names
------------------

   When doing I/O redirection from either `print' or `printf' into a
file, or via `getline' from a file, `gawk' recognizes certain special
file names internally.  These file names allow access to open file
descriptors inherited from `gawk''s parent process (usually the shell).
The file names are:

`/dev/stdin'
     The standard input.

`/dev/stdout'
     The standard output.

`/dev/stderr'
     The standard error output.

`/dev/fd/N'
     The file denoted by the open file descriptor N.

   In addition, reading the following files provides process related
information about the running `gawk' program.  All returned records are
terminated with a newline.

`/dev/pid'
     Returns the process ID of the current process.

`/dev/ppid'
     Returns the parent process ID of the current process.

`/dev/pgrpid'
     Returns the process group ID of the current process.

`/dev/user'
     At least four space-separated fields, containing the return values
     of the `getuid', `geteuid', `getgid', and `getegid' system calls.
     If there are any additional fields, they are the group IDs
     returned by `getgroups' system call.  (Multiple groups may not be
     supported on all systems.)

These file names may also be used on the command line to name data
files.  These file names are only recognized internally if you do not
actually have files with these names on your system.

   *Note Special File Names in `gawk': Special Files, for a longer
description that provides the motivation for this feature.


File: gawk.info,  Node: Built-in Functions Summary,  Next: Time Functions Summary,  Prev: Special File Summary,  Up: Actions Summary

Built-in Functions
------------------

   `awk' provides a number of built-in functions for performing numeric
operations, string related operations, and I/O related operations.

   The built-in arithmetic functions are:

`atan2(Y, X)'
     the arctangent of Y/X in radians.

`cos(EXPR)'
     the cosine of EXPR, which is in radians.

`exp(EXPR)'
     the exponential function (`e ^ EXPR').

`int(EXPR)'
     truncates to integer.

`log(EXPR)'
     the natural logarithm of `expr'.

`rand()'
     a random number between zero and one.

`sin(EXPR)'
     the sine of EXPR, which is in radians.

`sqrt(EXPR)'
     the square root function.

`srand([EXPR])'
     use EXPR as a new seed for the random number generator.  If no EXPR
     is provided, the time of day is used.  The return value is the
     previous seed for the random number generator.

   `awk' has the following built-in string functions:

`gensub(REGEX, SUBST, HOW [, TARGET])'
     If HOW is a string beginning with `g' or `G', then replace each
     match of REGEX in TARGET with SUBST.  Otherwise, replace the
     HOW'th occurrence. If TARGET is not supplied, use `$0'.  The
     return value is the changed string; the original TARGET is not
     modified. Within SUBST, `\N', where N is a digit from one to nine,
     can be used to indicate the text that matched the N'th
     parenthesized subexpression.  This function is `gawk'-specific.

`gsub(REGEX, SUBST [, TARGET])'
     for each substring matching the regular expression REGEX in the
     string TARGET, substitute the string SUBST, and return the number
     of substitutions. If TARGET is not supplied, use `$0'.

`index(STR, SEARCH)'
     returns the index of the string SEARCH in the string STR, or zero
     if SEARCH is not present.

`length([STR])'
     returns the length of the string STR.  The length of `$0' is
     returned if no argument is supplied.

`match(STR, REGEX)'
     returns the position in STR where the regular expression REGEX
     occurs, or zero if REGEX is not present, and sets the values of
     `RSTART' and `RLENGTH'.

`split(STR, ARR [, REGEX])'
     splits the string STR into the array ARR on the regular expression
     REGEX, and returns the number of elements.  If REGEX is omitted,
     `FS' is used instead. REGEX can be the null string, causing each
     character to be placed into its own array element.  The array ARR
     is cleared first.

`sprintf(FMT, EXPR-LIST)'
     prints EXPR-LIST according to FMT, and returns the resulting
     string.

`sub(REGEX, SUBST [, TARGET])'
     just like `gsub', but only the first matching substring is
     replaced.

`substr(STR, INDEX [, LEN])'
     returns the LEN-character substring of STR starting at INDEX.  If
     LEN is omitted, the rest of STR is used.

`tolower(STR)'
     returns a copy of the string STR, with all the upper-case
     characters in STR translated to their corresponding lower-case
     counterparts.  Non-alphabetic characters are left unchanged.

`toupper(STR)'
     returns a copy of the string STR, with all the lower-case
     characters in STR translated to their corresponding upper-case
     counterparts.  Non-alphabetic characters are left unchanged.

   The I/O related functions are:

`close(EXPR)'
     Close the open file or pipe denoted by EXPR.

`fflush([EXPR])'
     Flush any buffered output for the output file or pipe denoted by
     EXPR.  If EXPR is omitted, standard output is flushed.  If EXPR is
     the null string (`""'), all output buffers are flushed.

`system(CMD-LINE)'
     Execute the command CMD-LINE, and return the exit status.  If your
     operating system does not support `system', calling it will
     generate a fatal error.

     `system("")' can be used to force `awk' to flush any pending
     output.  This is more portable, but less obvious, than calling
     `fflush'.


File: gawk.info,  Node: Time Functions Summary,  Next: String Constants Summary,  Prev: Built-in Functions Summary,  Up: Actions Summary

Time Functions
--------------

   The following two functions are available for getting the current
time of day, and for formatting time stamps.  They are specific to
`gawk'.

`systime()'
     returns the current time of day as the number of seconds since a
     particular epoch (Midnight, January 1, 1970 UTC, on POSIX systems).

`strftime([FORMAT[, TIMESTAMP]])'
     formats TIMESTAMP according to the specification in FORMAT.  The
     current time of day is used if no TIMESTAMP is supplied.  A
     default format equivalent to the output of the `date' utility is
     used if no FORMAT is supplied.  *Note Functions for Dealing with
     Time Stamps: Time Functions, for the details on the conversion
     specifiers that `strftime' accepts.


File: gawk.info,  Node: String Constants Summary,  Prev: Time Functions Summary,  Up: Actions Summary

String Constants
----------------

   String constants in `awk' are sequences of characters enclosed in
double quotes (`"').  Within strings, certain "escape sequences" are
recognized, as in C.  These are:

`\\'
     A literal backslash.

`\a'
     The "alert" character; usually the ASCII BEL character.

`\b'
     Backspace.

`\f'
     Formfeed.

`\n'
     Newline.

`\r'
     Carriage return.

`\t'
     Horizontal tab.

`\v'
     Vertical tab.

`\xHEX DIGITS'
     The character represented by the string of hexadecimal digits
     following the `\x'.  As in ANSI C, all following hexadecimal
     digits are considered part of the escape sequence.  E.g., `"\x1B"'
     is a string containing the ASCII ESC (escape) character.  (The `\x'
     escape sequence is not in POSIX `awk'.)

`\DDD'
     The character represented by the one, two, or three digit sequence
     of octal digits.  Thus, `"\033"' is also a string containing the
     ASCII ESC (escape) character.

`\C'
     The literal character C, if C is not one of the above.

   The escape sequences may also be used inside constant regular
expressions (e.g., the regexp `/[ \t\f\n\r\v]/' matches whitespace
characters).

   *Note Escape Sequences::.


File: gawk.info,  Node: Functions Summary,  Next: Historical Features,  Prev: Actions Summary,  Up: Gawk Summary

User-defined Functions
======================

   Functions in `awk' are defined as follows:

     function NAME(PARAMETER LIST) { STATEMENTS }

   Actual parameters supplied in the function call are used to
instantiate the formal parameters declared in the function.  Arrays are
passed by reference, other variables are passed by value.

   If there are fewer arguments passed than there are names in
PARAMETER-LIST, the extra names are given the null string as their
value.  Extra names have the effect of local variables.

   The open-parenthesis in a function call of a user-defined function
must immediately follow the function name, without any intervening
white space.  This is to avoid a syntactic ambiguity with the
concatenation operator.

   The word `func' may be used in place of `function' (but not in POSIX
`awk').

   Use the `return' statement to return a value from a function.

   *Note User-defined Functions: User-defined.


File: gawk.info,  Node: Historical Features,  Prev: Functions Summary,  Up: Gawk Summary

Historical Features
===================

   There are two features of historical `awk' implementations that
`gawk' supports.

   First, it is possible to call the `length' built-in function not only
with no arguments, but even without parentheses!

     a = length

is the same as either of

     a = length()
     a = length($0)

For example:

     $ echo abcdef | awk '{ print length }'
     -| 6

This feature is marked as "deprecated" in the POSIX standard, and
`gawk' will issue a warning about its use if `--lint' is specified on
the command line.  (The ability to use `length' this way was actually
an accident of the original Unix `awk' implementation.  If any built-in
function used `$0' as its default argument, it was possible to call
that function without the parentheses.  In particular, it was common
practice to use the `length' function in this fashion, and this usage
was documented in the `awk' manual page.)

   The other historical feature is the use of either the `break'
statement, or the `continue' statement outside the body of a `while',
`for', or `do' loop.  Traditional `awk' implementations have treated
such usage as equivalent to the `next' statement.  More recent versions
of Unix `awk' do not allow it. `gawk' supports this usage if
`--traditional' has been specified.

   *Note Command Line Options: Options, for more information about the
`--posix' and `--lint' options.


File: gawk.info,  Node: Installation,  Next: Notes,  Prev: Gawk Summary,  Up: Top

Installing `gawk'
*****************

   This appendix provides instructions for installing `gawk' on the
various platforms that are supported by the developers.  The primary
developers support Unix (and one day, GNU), while the other ports were
contributed.  The file `ACKNOWLEDGMENT' in the `gawk' distribution
lists the electronic mail addresses of the people who did the
respective ports, and they are also provided in *Note Reporting
Problems and Bugs: Bugs.

* Menu:

* Gawk Distribution::           What is in the `gawk' distribution.
* Unix Installation::           Installing `gawk' under various versions
                                of Unix.
* VMS Installation::            Installing `gawk' on VMS.
* PC Installation::             Installing and Compiling `gawk' on MS-DOS
                                and OS/2
* Atari Installation::          Installing `gawk' on the Atari ST.
* Amiga Installation::          Installing `gawk' on an Amiga.
* Bugs::                        Reporting Problems and Bugs.
* Other Versions::              Other freely available `awk'
                                implementations.


File: gawk.info,  Node: Gawk Distribution,  Next: Unix Installation,  Prev: Installation,  Up: Installation

The `gawk' Distribution
=======================

   This section first describes how to get the `gawk' distribution, how
to extract it, and then what is in the various files and subdirectories.

* Menu:

* Getting::                     How to get the distribution.
* Extracting::                  How to extract the distribution.
* Distribution contents::       What is in the distribution.


File: gawk.info,  Node: Getting,  Next: Extracting,  Prev: Gawk Distribution,  Up: Gawk Distribution

Getting the `gawk' Distribution
-------------------------------

   There are three ways you can get GNU software.

  1. You can copy it from someone else who already has it.

  2. You can order `gawk' directly from the Free Software Foundation.
     Software distributions are available for Unix, MS-DOS, and VMS, on
     tape and CD-ROM.  The address is:

          Free Software Foundation
          59 Temple Place--Suite 330
          Boston, MA  02111-1307 USA
          Phone: +1-617-542-5942
          Fax (including Japan): +1-617-542-2652
          E-mail: `gnu@prep.ai.mit.edu'

     Ordering from the FSF directly contributes to the support of the
     foundation and to the production of more free software.

  3. You can get `gawk' by using anonymous `ftp' to the Internet host
     `ftp.gnu.ai.mit.edu', in the directory `/pub/gnu'.

     Here is a list of alternate `ftp' sites from which you can obtain
     GNU software.  When a site is listed as "SITE`:'DIRECTORY" the
     DIRECTORY indicates the directory where GNU software is kept.  You
     should use a site that is geographically close to you.

    Asia:
         `cair-archive.kaist.ac.kr:/pub/gnu'
         `ftp.cs.titech.ac.jp'
         `ftp.nectec.or.th:/pub/mirrors/gnu'
         `utsun.s.u-tokyo.ac.jp:/ftpsync/prep'
    Australia:
         `archie.au:/gnu'
               (`archie.oz' or `archie.oz.au' for ACSnet)

    Africa:
         `ftp.sun.ac.za:/pub/gnu'
    Middle East:
         `ftp.technion.ac.il:/pub/unsupported/gnu'
    Europe:
         `archive.eu.net'
         `ftp.denet.dk'
         `ftp.eunet.ch'
         `ftp.funet.fi:/pub/gnu'
         `ftp.ieunet.ie:pub/gnu'
         `ftp.informatik.rwth-aachen.de:/pub/gnu'
         `ftp.informatik.tu-muenchen.de'
         `ftp.luth.se:/pub/unix/gnu'
         `ftp.mcc.ac.uk'
         `ftp.stacken.kth.se'
         `ftp.sunet.se:/pub/gnu'
         `ftp.univ-lyon1.fr:pub/gnu'
         `ftp.win.tue.nl:/pub/gnu'
         `irisa.irisa.fr:/pub/gnu'
         `isy.liu.se'
         `nic.switch.ch:/mirror/gnu'
         `src.doc.ic.ac.uk:/gnu'
         `unix.hensa.ac.uk:/pub/uunet/systems/gnu'
    South America:
         `ftp.inf.utfsm.cl:/pub/gnu'
         `ftp.unicamp.br:/pub/gnu'
    Western Canada:
         `ftp.cs.ubc.ca:/mirror2/gnu'
    USA:
         `col.hp.com:/mirrors/gnu'
         `f.ms.uky.edu:/pub3/gnu'
         `ftp.cc.gatech.edu:/pub/gnu'
         `ftp.cs.columbia.edu:/archives/gnu/prep'
         `ftp.digex.net:/pub/gnu'
         `ftp.hawaii.edu:/mirrors/gnu'
         `ftp.kpc.com:/pub/mirror/gnu'
    USA (continued):
         `ftp.uu.net:/systems/gnu'
         `gatekeeper.dec.com:/pub/GNU'
         `jaguar.utah.edu:/gnustuff'
         `labrea.stanford.edu'
         `mrcnext.cso.uiuc.edu:/pub/gnu'
         `vixen.cso.uiuc.edu:/gnu'
         `wuarchive.wustl.edu:/systems/gnu'


File: gawk.info,  Node: Extracting,  Next: Distribution contents,  Prev: Getting,  Up: Gawk Distribution

Extracting the Distribution
---------------------------

   `gawk' is distributed as a `tar' file compressed with the GNU Zip
program, `gzip'.

   Once you have the distribution (for example, `gawk-3.0.2.tar.gz'),
first use `gzip' to expand the file, and then use `tar' to extract it.
You can use the following pipeline to produce the `gawk' distribution:

     # Under System V, add 'o' to the tar flags
     gzip -d -c gawk-3.0.2.tar.gz | tar -xvpf -

This will create a directory named `gawk-3.0.2' in the current
directory.

   The distribution file name is of the form `gawk-V.R.N.tar.gz'.  The
V represents the major version of `gawk', the R represents the current
release of version V, and the N represents a "patch level", meaning
that minor bugs have been fixed in the release.  The current patch
level is 0, but when retrieving distributions, you should get the
version with the highest version, release, and patch level.  (Note that
release levels greater than or equal to 90 denote "beta," or
non-production software; you may not wish to retrieve such a version
unless you don't mind experimenting.)

   If you are not on a Unix system, you will need to make other
arrangements for getting and extracting the `gawk' distribution.  You
should consult a local expert.


File: gawk.info,  Node: Distribution contents,  Prev: Extracting,  Up: Gawk Distribution

Contents of the `gawk' Distribution
-----------------------------------

   The `gawk' distribution has a number of C source files,
documentation files, subdirectories and files related to the
configuration process (*note Compiling and Installing `gawk' on Unix:
Unix Installation.), and several subdirectories related to different,
non-Unix, operating systems.

various `.c', `.y', and `.h' files
     These files are the actual `gawk' source code.

`README'
`README_d/README.*'
     Descriptive files: `README' for `gawk' under Unix, and the rest
     for the various hardware and software combinations.

`INSTALL'
     A file providing an overview of the configuration and installation
     process.

`PORTS'
     A list of systems to which `gawk' has been ported, and which have
     successfully run the test suite.

`ACKNOWLEDGMENT'
     A list of the people who contributed major parts of the code or
     documentation.

`ChangeLog'
     A detailed list of source code changes as bugs are fixed or
     improvements made.

`NEWS'
     A list of changes to `gawk' since the last release or patch.

`COPYING'
     The GNU General Public License.

`FUTURES'
     A brief list of features and/or changes being contemplated for
     future releases, with some indication of the time frame for the
     feature, based on its difficulty.

`LIMITATIONS'
     A list of those factors that limit `gawk''s performance.  Most of
     these depend on the hardware or operating system software, and are
     not limits in `gawk' itself.

`POSIX.STD'
     A description of one area where the POSIX standard for `awk' is
     incorrect, and how `gawk' handles the problem.

`PROBLEMS'
     A file describing known problems with the current release.

`doc/awkforai.txt'
     A short article describing why `gawk' is a good language for AI
     (Artificial Intelligence) programming.

`doc/README.card'
`doc/ad.block'
`doc/awkcard.in'
`doc/cardfonts'
`doc/colors'
`doc/macros'
`doc/no.colors'
`doc/setter.outline'
     The `troff' source for a five-color `awk' reference card.  A
     modern version of `troff', such as GNU Troff (`groff') is needed
     to produce the color version. See the file `README.card' for
     instructions if you have an older `troff'.

`doc/gawk.1'
     The `troff' source for a manual page describing `gawk'.  This is
     distributed for the convenience of Unix users.

`doc/gawk.texi'
     The Texinfo source file for this Info file.  It should be
     processed with TeX to produce a printed document, and with
     `makeinfo' to produce an Info file.

`doc/gawk.info'
     The generated Info file for this Info file.

`doc/igawk.1'
     The `troff' source for a manual page describing the `igawk'
     program presented in *Note An Easy Way to Use Library Functions:
     Igawk Program.

`doc/Makefile.in'
     The input file used during the configuration process to generate
     the actual `Makefile' for creating the documentation.

`Makefile.in'
`acconfig.h'
`aclocal.m4'
`configh.in'
`configure.in'
`configure'
`custom.h'
`missing/*'
     These files and subdirectory are used when configuring `gawk' for
     various Unix systems.  They are explained in detail in *Note
     Compiling and Installing `gawk' on Unix: Unix Installation.

`awklib/extract.awk'
`awklib/Makefile.in'
     The `awklib' directory contains a copy of `extract.awk' (*note
     Extracting Programs from Texinfo Source Files: Extract Program.),
     which can be used to extract the sample programs from the Texinfo
     source file for this Info file, and a `Makefile.in' file, which
     `configure' uses to generate a `Makefile'.  As part of the process
     of building `gawk', the library functions from *Note A Library of
     `awk' Functions: Library Functions, and the `igawk' program from
     *Note An Easy Way to Use Library Functions: Igawk Program, are
     extracted into ready to use files.  They are installed as part of
     the installation process.

`amiga/*'
     Files needed for building `gawk' on an Amiga.  *Note Installing
     `gawk' on an Amiga: Amiga Installation, for details.

`atari/*'
     Files needed for building `gawk' on an Atari ST.  *Note Installing
     `gawk' on the Atari ST: Atari Installation, for details.

`pc/*'
     Files needed for building `gawk' under MS-DOS and OS/2.  *Note
     MS-DOS and OS/2 Installation and Compilation: PC Installation, for
     details.

`vms/*'
     Files needed for building `gawk' under VMS.  *Note How to Compile
     and Install `gawk' on VMS: VMS Installation, for details.

`test/*'
     A test suite for `gawk'.  You can use `make check' from the top
     level `gawk' directory to run your version of `gawk' against the
     test suite.  If `gawk' successfully passes `make check' then you
     can be confident of a successful port.


File: gawk.info,  Node: Unix Installation,  Next: VMS Installation,  Prev: Gawk Distribution,  Up: Installation

Compiling and Installing `gawk' on Unix
=======================================

   Usually, you can compile and install `gawk' by typing only two
commands.  However, if you do use an unusual system, you may need to
configure `gawk' for your system yourself.

* Menu:

* Quick Installation::          Compiling `gawk' under Unix.
* Configuration Philosophy::    How it's all supposed to work.


File: gawk.info,  Node: Quick Installation,  Next: Configuration Philosophy,  Prev: Unix Installation,  Up: Unix Installation

Compiling `gawk' for Unix
-------------------------

   After you have extracted the `gawk' distribution, `cd' to
`gawk-3.0.2'.  Like most GNU software, `gawk' is configured
automatically for your Unix system by running the `configure' program.
This program is a Bourne shell script that was generated automatically
using GNU `autoconf'.  (The `autoconf' software is described fully
starting with *Note Introduction: (autoconf)Top.)

   To configure `gawk', simply run `configure':

     sh ./configure

   This produces a `Makefile' and `config.h' tailored to your system.
The `config.h' file describes various facts about your system.  You may
wish to edit the `Makefile' to change the `CFLAGS' variable, which
controls the command line options that are passed to the C compiler
(such as optimization levels, or compiling for debugging).

   Alternatively, you can add your own values for most `make'
variables, such as `CC' and `CFLAGS', on the command line when running
`configure':

     CC=cc CFLAGS=-g sh ./configure

See the file `INSTALL' in the `gawk' distribution for all the details.

   After you have run `configure', and possibly edited the `Makefile',
type:

     make

and shortly thereafter, you should have an executable version of `gawk'.
That's all there is to it!  (If these steps do not work, please send in
a bug report; *note Reporting Problems and Bugs: Bugs..)


File: gawk.info,  Node: Configuration Philosophy,  Prev: Quick Installation,  Up: Unix Installation

The Configuration Process
-------------------------

   (This section is of interest only if you know something about using
the C language and the Unix operating system.)

   The source code for `gawk' generally attempts to adhere to formal
standards wherever possible.  This means that `gawk' uses library
routines that are specified by the ANSI C standard and by the POSIX
operating system interface standard.  When using an ANSI C compiler,
function prototypes are used to help improve the compile-time checking.

   Many Unix systems do not support all of either the ANSI or the POSIX
standards.  The `missing' subdirectory in the `gawk' distribution
contains replacement versions of those subroutines that are most likely
to be missing.

   The `config.h' file that is created by the `configure' program
contains definitions that describe features of the particular operating
system where you are attempting to compile `gawk'.  The three things
described by this file are what header files are available, so that
they can be correctly included, what (supposedly) standard functions
are actually available in your C libraries, and other miscellaneous
facts about your variant of Unix.  For example, there may not be an
`st_blksize' element in the `stat' structure.  In this case
`HAVE_ST_BLKSIZE' would be undefined.

   It is possible for your C compiler to lie to `configure'. It may do
so by not exiting with an error when a library function is not
available.  To get around this, you can edit the file `custom.h'.  Use
an `#ifdef' that is appropriate for your system, and either `#define'
any constants that `configure' should have defined but didn't, or
`#undef' any constants that `configure' defined and should not have.
`custom.h' is automatically included by `config.h'.

   It is also possible that the `configure' program generated by
`autoconf' will not work on your system in some other fashion.  If you
do have a problem, the file `configure.in' is the input for `autoconf'.
You may be able to change this file, and generate a new version of
`configure' that will work on your system.  *Note Reporting Problems
and Bugs: Bugs, for information on how to report problems in
configuring `gawk'.  The same mechanism may be used to send in updates
to `configure.in' and/or `custom.h'.


File: gawk.info,  Node: VMS Installation,  Next: PC Installation,  Prev: Unix Installation,  Up: Installation

How to Compile and Install `gawk' on VMS
========================================

   This section describes how to compile and install `gawk' under VMS.

* Menu:

* VMS Compilation::             How to compile `gawk' under VMS.
* VMS Installation Details::    How to install `gawk' under VMS.
* VMS Running::                 How to run `gawk' under VMS.
* VMS POSIX::                   Alternate instructions for VMS POSIX.


File: gawk.info,  Node: VMS Compilation,  Next: VMS Installation Details,  Prev: VMS Installation,  Up: VMS Installation

Compiling `gawk' on VMS
-----------------------

   To compile `gawk' under VMS, there is a `DCL' command procedure that
will issue all the necessary `CC' and `LINK' commands, and there is
also a `Makefile' for use with the `MMS' utility.  From the source
directory, use either

     $ @[.VMS]VMSBUILD.COM

or

     $ MMS/DESCRIPTION=[.VMS]DESCRIP.MMS GAWK

   Depending upon which C compiler you are using, follow one of the sets
of instructions in this table:

VAX C V3.x
     Use either `vmsbuild.com' or `descrip.mms' as is.  These use
     `CC/OPTIMIZE=NOLINE', which is essential for Version 3.0.

VAX C V2.x
     You must have Version 2.3 or 2.4; older ones won't work.  Edit
     either `vmsbuild.com' or `descrip.mms' according to the comments
     in them.  For `vmsbuild.com', this just entails removing two `!'
     delimiters.  Also edit `config.h' (which is a copy of file
     `[.config]vms-conf.h') and comment out or delete the two lines
     `#define __STDC__ 0' and `#define VAXC_BUILTINS' near the end.

GNU C
     Edit `vmsbuild.com' or `descrip.mms'; the changes are different
     from those for VAX C V2.x, but equally straightforward.  No
     changes to `config.h' should be needed.

DEC C
     Edit `vmsbuild.com' or `descrip.mms' according to their comments.
     No changes to `config.h' should be needed.

   `gawk' has been tested under VAX/VMS 5.5-1 using VAX C V3.2, GNU C
1.40 and 2.3.  It should work without modifications for VMS V4.6 and up.


File: gawk.info,  Node: VMS Installation Details,  Next: VMS Running,  Prev: VMS Compilation,  Up: VMS Installation

Installing `gawk' on VMS
------------------------

   To install `gawk', all you need is a "foreign" command, which is a
`DCL' symbol whose value begins with a dollar sign. For example:

     $ GAWK :== $disk1:[gnubin]GAWK

(Substitute the actual location of `gawk.exe' for `$disk1:[gnubin]'.)
The symbol should be placed in the `login.com' of any user who wishes
to run `gawk', so that it will be defined every time the user logs on.
Alternatively, the symbol may be placed in the system-wide
`sylogin.com' procedure, which will allow all users to run `gawk'.

   Optionally, the help entry can be loaded into a VMS help library:

     $ LIBRARY/HELP SYS$HELP:HELPLIB [.VMS]GAWK.HLP

(You may want to substitute a site-specific help library rather than
the standard VMS library `HELPLIB'.)  After loading the help text,

     $ HELP GAWK

will provide information about both the `gawk' implementation and the
`awk' programming language.

   The logical name `AWK_LIBRARY' can designate a default location for
`awk' program files.  For the `-f' option, if the specified filename
has no device or directory path information in it, `gawk' will look in
the current directory first, then in the directory specified by the
translation of `AWK_LIBRARY' if the file was not found.  If after
searching in both directories, the file still is not found, then `gawk'
appends the suffix `.awk' to the filename and the file search will be
re-tried.  If `AWK_LIBRARY' is not defined, that portion of the file
search will fail benignly.


File: gawk.info,  Node: VMS Running,  Next: VMS POSIX,  Prev: VMS Installation Details,  Up: VMS Installation

Running `gawk' on VMS
---------------------

   Command line parsing and quoting conventions are significantly
different on VMS, so examples in this Info file or from other sources
often need minor changes.  They *are* minor though, and all `awk'
programs should run correctly.

   Here are a couple of trivial tests:

     $ gawk -- "BEGIN {print ""Hello, World!""}"
     $ gawk -"W" version
     ! could also be -"W version" or "-W version"

Note that upper-case and mixed-case text must be quoted.

   The VMS port of `gawk' includes a `DCL'-style interface in addition
to the original shell-style interface (see the help entry for details).
One side-effect of dual command line parsing is that if there is only a
single parameter (as in the quoted string program above), the command
becomes ambiguous.  To work around this, the normally optional `--'
flag is required to force Unix style rather than `DCL' parsing.  If any
other dash-type options (or multiple parameters such as data files to be
processed) are present, there is no ambiguity and `--' can be omitted.

   The default search path when looking for `awk' program files
specified by the `-f' option is `"SYS$DISK:[],AWK_LIBRARY:"'.  The
logical name `AWKPATH' can be used to override this default.  The format
of `AWKPATH' is a comma-separated list of directory specifications.
When defining it, the value should be quoted so that it retains a single
translation, and not a multi-translation `RMS' searchlist.


File: gawk.info,  Node: VMS POSIX,  Prev: VMS Running,  Up: VMS Installation

Building and Using `gawk' on VMS POSIX
--------------------------------------

   Ignore the instructions above, although `vms/gawk.hlp' should still
be made available in a help library.  The source tree should be unpacked
into a container file subsystem rather than into the ordinary VMS file
system.  Make sure that the two scripts, `configure' and
`vms/posix-cc.sh', are executable; use `chmod +x' on them if necessary.
Then execute the following two commands:

     psx> CC=vms/posix-cc.sh configure
     psx> make CC=c89 gawk

The first command will construct files `config.h' and `Makefile' out of
templates, using a script to make the C compiler fit `configure''s
expectations.  The second command will compile and link `gawk' using
the C compiler directly; ignore any warnings from `make' about being
unable to redefine `CC'.  `configure' will take a very long time to
execute, but at least it provides incremental feedback as it runs.

   This has been tested with VAX/VMS V6.2, VMS POSIX V2.0, and DEC C
V5.2.

   Once built, `gawk' will work like any other shell utility.  Unlike
the normal VMS port of `gawk', no special command line manipulation is
needed in the VMS POSIX environment.


File: gawk.info,  Node: PC Installation,  Next: Atari Installation,  Prev: VMS Installation,  Up: Installation

MS-DOS and OS/2 Installation and Compilation
============================================

   If you have received a binary distribution prepared by the DOS
maintainers, then `gawk' and the necessary support files will appear
under the `gnu' directory, with executables in `gnu/bin', libraries in
`gnu/lib/awk', and manual pages under `gnu/man'.  This is designed for
easy installation to a `/gnu' directory on your drive, but the files
can be installed anywhere provided `AWKPATH' is set properly.
Regardless of the installation directory, the first line of `igawk.cmd'
and `igawk.bat' (in `gnu/bin') may need to be edited.

   The binary distribution will contain a separate file describing the
contents. In particular, it may include more than one version of the
`gawk' executable. OS/2 binary distributions may have a different
arrangement, but installation is similar.

   The OS/2 and MS-DOS versions of `gawk' search for program files as
described in *Note The `AWKPATH' Environment Variable: AWKPATH Variable.
However, semicolons (rather than colons) separate elements in the
`AWKPATH' variable. If `AWKPATH' is not set or is empty, then the
default search path is `".;c:/lib/awk;c:/gnu/lib/awk"'.

   An `sh'-like shell (as opposed to `command.com' under MS-DOS or
`cmd.exe' under OS/2) may be useful for `awk' programming.  Ian
Stewartson has written an excellent shell for MS-DOS and OS/2, and a
`ksh' clone and GNU Bash are available for OS/2. The file
`README_d/README.pc' in the `gawk' distribution contains information on
these shells. Users of Stewartson's shell on DOS should examine its
documentation on handling of command-lines. In particular, the setting
for `gawk' in the shell configuration may need to be changed, and the
`ignoretype' option may also be of interest.

   `gawk' can be compiled for MS-DOS and OS/2 using the GNU development
tools from DJ Delorie (DJGPP, MS-DOS-only) or Eberhard Mattes (EMX,
MS-DOS and OS/2).  Microsoft C can be used to build 16-bit versions for
MS-DOS and OS/2.  The file `README_d/README.pc' in the `gawk'
distribution contains additional notes, and `pc/Makefile' contains
important notes on compilation options.

   To build `gawk', copy the files in the `pc' directory (*except* for
`ChangeLog') to the directory with the rest of the `gawk' sources. The
`Makefile' contains a configuration section with comments, and may need
to be edited in order to work with your `make' utility.

   The `Makefile' contains a number of targets for building various
MS-DOS and OS/2 versions. A list of targets will be printed if the
`make' command is given without a target. As an example, to build `gawk'
using the DJGPP tools, enter `make djgpp'.

   Using `make' to run the standard tests and to install `gawk'
requires additional Unix-like tools, including `sh', `sed', and `cp'.
In order to run the tests, the `test/*.ok' files may need to be
converted so that they have the usual DOS-style end-of-line markers.
Most of the tests will work properly with Stewartson's shell along with
the companion utilities or appropriate GNU utilities.  However, some
editing of `test/Makefile' is required. It is recommended that the file
`pc/Makefile.tst' be copied to `test/Makefile' as a replacement.
Details can be found in `README_d/README.pc'.


File: gawk.info,  Node: Atari Installation,  Next: Amiga Installation,  Prev: PC Installation,  Up: Installation

Installing `gawk' on the Atari ST
=================================

   There are no substantial differences when installing `gawk' on
various Atari models.  Compiled `gawk' executables do not require a
large amount of memory with most `awk' programs and should run on all
Motorola processor based models (called further ST, even if that is not
exactly right).

   In order to use `gawk', you need to have a shell, either text or
graphics, that does not map all the characters of a command line to
upper-case.  Maintaining case distinction in option flags is very
important (*note Command Line Options: Options.).  These days this is
the default, and it may only be a problem for some very old machines.
If your system does not preserve the case of option flags, you will
need to upgrade your tools.  Support for I/O redirection is necessary
to make it easy to import `awk' programs from other environments.
Pipes are nice to have, but not vital.

* Menu:

* Atari Compiling::           Compiling `gawk' on Atari
* Atari Using::               Running `gawk' on Atari


File: gawk.info,  Node: Atari Compiling,  Next: Atari Using,  Prev: Atari Installation,  Up: Atari Installation

Compiling `gawk' on the Atari ST
--------------------------------

   A proper compilation of `gawk' sources when `sizeof(int)' differs
from `sizeof(void *)' requires an ANSI C compiler. An initial port was
done with `gcc'.  You may actually prefer executables where `int's are
four bytes wide, but the other variant works as well.

   You may need quite a bit of memory when trying to recompile the
`gawk' sources, as some source files (`regex.c' in particular) are quite
big.  If you run out of memory compiling such a file, try reducing the
optimization level for this particular file; this may help.

   With a reasonable shell (Bash will do), and in particular if you run
Linux, MiNT or a similar operating system, you have a pretty good
chance that the `configure' utility will succeed.  Otherwise sample
versions of `config.h' and `Makefile.st' are given in the `atari'
subdirectory and can be edited and copied to the corresponding files in
the main source directory.  Even if `configure' produced something, it
might be advisable to compare its results with the sample versions and
possibly make adjustments.

   Some `gawk' source code fragments depend on a preprocessor define
`atarist'.  This basically assumes the TOS environment with `gcc'.
Modify these sections as appropriate if they are not right for your
environment.  Also see the remarks about `AWKPATH' and `envsep' in
*Note Running `gawk' on the Atari ST: Atari Using.

   As shipped, the sample `config.h' claims that the `system' function
is missing from the libraries, which is not true, and an alternative
implementation of this function is provided in `atari/system.c'.
Depending upon your particular combination of shell and operating
system, you may wish to change the file to indicate that `system' is
available.


File: gawk.info,  Node: Atari Using,  Prev: Atari Compiling,  Up: Atari Installation

Running `gawk' on the Atari ST
------------------------------

   An executable version of `gawk' should be placed, as usual, anywhere
in your `PATH' where your shell can find it.

   While executing, `gawk' creates a number of temporary files.  When
using `gcc' libraries for TOS, `gawk' looks for either of the
environment variables `TEMP' or `TMPDIR', in that order.  If either one
is found, its value is assumed to be a directory for temporary files.
This directory must exist, and if you can spare the memory, it is a
good idea to put it on a RAM drive.  If neither `TEMP' nor `TMPDIR' are
found, then `gawk' uses the current directory for its temporary files.

   The ST version of `gawk' searches for its program files as described
in *Note The `AWKPATH' Environment Variable: AWKPATH Variable.  The
default value for the `AWKPATH' variable is taken from `DEFPATH'
defined in `Makefile'. The sample `gcc'/TOS `Makefile' for the ST in
the distribution sets `DEFPATH' to `".,c:\lib\awk,c:\gnu\lib\awk"'.
The search path can be modified by explicitly setting `AWKPATH' to
whatever you wish.  Note that colons cannot be used on the ST to
separate elements in the `AWKPATH' variable, since they have another,
reserved, meaning.  Instead, you must use a comma to separate elements
in the path.  When recompiling, the separating character can be
modified by initializing the `envsep' variable in `atari/gawkmisc.atr'
to another value.

   Although `awk' allows great flexibility in doing I/O redirections
from within a program, this facility should be used with care on the ST
running under TOS.  In some circumstances the OS routines for file
handle pool processing lose track of certain events, causing the
computer to crash, and requiring a reboot.  Often a warm reboot is
sufficient.  Fortunately, this happens infrequently, and in rather
esoteric situations.  In particular, avoid having one part of an `awk'
program using `print' statements explicitly redirected to
`"/dev/stdout"', while other `print' statements use the default
standard output, and a calling shell has redirected standard output to
a file.

   When `gawk' is compiled with the ST version of `gcc' and its usual
libraries, it will accept both `/' and `\' as path separators.  While
this is convenient, it should be remembered that this removes one,
technically valid, character (`/') from your file names, and that it
may create problems for external programs, called via the `system'
function, which may not support this convention.  Whenever it is
possible that a file created by `gawk' will be used by some other
program, use only backslashes.  Also remember that in `awk',
backslashes in strings have to be doubled in order to get literal
backslashes (*note Escape Sequences::.).


File: gawk.info,  Node: Amiga Installation,  Next: Bugs,  Prev: Atari Installation,  Up: Installation

Installing `gawk' on an Amiga
=============================

   You can install `gawk' on an Amiga system using a Unix emulation
environment available via anonymous `ftp' from `wuarchive.wustl.edu' in
the directory `pub/aminet/dev/gcc'.  This includes a shell based on
`pdksh'.  The primary component of this environment is a Unix emulation
library, `ixemul.lib'.

   A more complete distribution for the Amiga is available on the
FreshFish CD-ROM from:

     CRONUS
     1840 E. Warner Road #105-265
     Tempe, AZ 85284  USA
     US Toll Free: (800) 804-0833
     Phone: +1-602-491-0442
     FAX: +1-602-491-0048
     Email:  `info@ninemoons.com'
     WWW: `http://www.ninemoons.com'
     Anonymous `ftp' site: `ftp.ninemoons.com'

   Once you have the distribution, you can configure `gawk' simply by
running `configure':

     configure -v m68k-cbm-amigados

   Then run `make', and you should be all set!  (If these steps do not
work, please send in a bug report; *note Reporting Problems and Bugs:
Bugs..)


File: gawk.info,  Node: Bugs,  Next: Other Versions,  Prev: Amiga Installation,  Up: Installation

Reporting Problems and Bugs
===========================

   If you have problems with `gawk' or think that you have found a bug,
please report it to the developers; we cannot promise to do anything
but we might well want to fix it.

   Before reporting a bug, make sure you have actually found a real bug.
Carefully reread the documentation and see if it really says you can do
what you're trying to do.  If it's not clear whether you should be able
to do something or not, report that too; it's a bug in the
documentation!

   Before reporting a bug or trying to fix it yourself, try to isolate
it to the smallest possible `awk' program and input data file that
reproduces the problem.  Then send us the program and data file, some
idea of what kind of Unix system you're using, and the exact results
`gawk' gave you.  Also say what you expected to occur; this will help
us decide whether the problem was really in the documentation.

   Once you have a precise problem, there are two e-mail addresses you
can send mail to.

Internet:
     `bug-gnu-utils@prep.ai.mit.edu'

UUCP:
     `uunet!prep.ai.mit.edu!bug-gnu-utils'

   Please include the version number of `gawk' you are using.  You can
get this information with the command `gawk --version'.  You should
send a carbon copy of your mail to Arnold Robbins, who can be reached
at `arnold@gnu.ai.mit.edu'.

   *Important!* Do *not* try to report bugs in `gawk' by posting to the
Usenet/Internet newsgroup `comp.lang.awk'.  While the `gawk' developers
do occasionally read this newsgroup, there is no guarantee that we will
see your posting.  The steps described above are the official,
recognized ways for reporting bugs.

   Non-bug suggestions are always welcome as well.  If you have
questions about things that are unclear in the documentation or are
just obscure features, ask Arnold Robbins; he will try to help you out,
although he may not have the time to fix the problem.  You can send him
electronic mail at the Internet address above.

   If you find bugs in one of the non-Unix ports of `gawk', please send
an electronic mail message to the person who maintains that port.  They
are listed below, and also in the `README' file in the `gawk'
distribution.  Information in the `README' file should be considered
authoritative if it conflicts with this Info file.

   The people maintaining the non-Unix ports of `gawk' are:

MS-DOS
     Scott Deifik, `scottd@amgen.com', and Darrel Hankerson,
     `hankedr@mail.auburn.edu'.

OS/2
     Kai Uwe Rommel, `rommel@ars.de'.

VMS
     Pat Rankin, `rankin@eql.caltech.edu'.

Atari ST
     Michal Jaegermann, `michal@gortel.phys.ualberta.ca'.

Amiga
     Fred Fish, `fnf@ninemoons.com'.

   If your bug is also reproducible under Unix, please send copies of
your report to the general GNU bug list, as well as to Arnold Robbins,
at the addresses listed above.


File: gawk.info,  Node: Other Versions,  Prev: Bugs,  Up: Installation

Other Freely Available `awk' Implementations
============================================

     It's kind of fun to put comments like this in your awk code.
           `// Do C++ comments work? answer: yes! of course'
     Michael Brennan

   There are two other freely available `awk' implementations.  This
section briefly describes where to get them.

Unix `awk'
     Brian Kernighan has been able to make his implementation of `awk'
     freely available.  You can get it via anonymous `ftp' to the host
     `netlib.att.com'.  Change directory to `/netlib/research'. Use
     "binary" or "image" mode, and retrieve `awk.bundle.Z'.

     This is a shell archive that has been compressed with the
     `compress' utility. It can be uncompressed with either
     `uncompress' or the GNU `gunzip' utility.

     This version requires an ANSI C compiler; GCC (the GNU C compiler)
     works quite nicely.

`mawk'
     Michael Brennan has written an independent implementation of `awk',
     called `mawk'.  It is available under the GPL (*note GNU GENERAL
     PUBLIC LICENSE: Copying.), just as `gawk' is.

     You can get it via anonymous `ftp' to the host `ftp.whidbey.net'.
     Change directory to `/pub/brennan'.  Use "binary" or "image" mode,
     and retrieve `mawk1.3.3.tar.gz' (or the latest version that is
     there).

     `gunzip' may be used to decompress this file. Installation is
     similar to `gawk''s (*note Compiling and Installing `gawk' on
     Unix: Unix Installation.).


File: gawk.info,  Node: Notes,  Next: Glossary,  Prev: Installation,  Up: Top

Implementation Notes
********************

   This appendix contains information mainly of interest to
implementors and maintainers of `gawk'.  Everything in it applies
specifically to `gawk', and not to other implementations.

* Menu:

* Compatibility Mode::          How to disable certain `gawk' extensions.
* Additions::                   Making Additions To `gawk'.
* Future Extensions::           New features that may be implemented one day.
* Improvements::                Suggestions for improvements by volunteers.


File: gawk.info,  Node: Compatibility Mode,  Next: Additions,  Prev: Notes,  Up: Notes

Downward Compatibility and Debugging
====================================

   *Note Extensions in `gawk' Not in POSIX `awk': POSIX/GNU, for a
summary of the GNU extensions to the `awk' language and program.  All
of these features can be turned off by invoking `gawk' with the
`--traditional' option, or with the `--posix' option.

   If `gawk' is compiled for debugging with `-DDEBUG', then there is
one more option available on the command line:

`-W parsedebug'
`--parsedebug'
     Print out the parse stack information as the program is being
     parsed.

   This option is intended only for serious `gawk' developers, and not
for the casual user.  It probably has not even been compiled into your
version of `gawk', since it slows down execution.


File: gawk.info,  Node: Additions,  Next: Future Extensions,  Prev: Compatibility Mode,  Up: Notes

Making Additions to `gawk'
==========================

   If you should find that you wish to enhance `gawk' in a significant
fashion, you are perfectly free to do so.  That is the point of having
free software; the source code is available, and you are free to change
it as you wish (*note GNU GENERAL PUBLIC LICENSE: Copying.).

   This section discusses the ways you might wish to change `gawk', and
any considerations you should bear in mind.

* Menu:

* Adding Code::             Adding code to the main body of `gawk'.
* New Ports::               Porting `gawk' to a new operating system.


File: gawk.info,  Node: Adding Code,  Next: New Ports,  Prev: Additions,  Up: Additions

Adding New Features
-------------------

   You are free to add any new features you like to `gawk'.  However,
if you want your changes to be incorporated into the `gawk'
distribution, there are several steps that you need to take in order to
make it possible for me to include to your changes.

  1. Get the latest version.  It is much easier for me to integrate
     changes if they are relative to the most recent distributed
     version of `gawk'.  If your version of `gawk' is very old, I may
     not be able to integrate them at all.  *Note Getting the `gawk'
     Distribution: Getting, for information on getting the latest
     version of `gawk'.

  2. See *note : (Version)Top standards, GNU Coding Standards.  This
     document describes how GNU software should be written. If you
     haven't read it, please do so, preferably *before* starting to
     modify `gawk'.  (The `GNU Coding Standards' are available as part
     of the Autoconf distribution, from the FSF.)

  3. Use the `gawk' coding style.  The C code for `gawk' follows the
     instructions in the `GNU Coding Standards', with minor exceptions.
     The code is formatted using the traditional "K&R" style,
     particularly as regards the placement of braces and the use of
     tabs.  In brief, the coding rules for `gawk' are:

        * Use old style (non-prototype) function headers when defining
          functions.

        * Put the name of the function at the beginning of its own line.

        * Put the return type of the function, even if it is `int', on
          the line above the line with the name and arguments of the
          function.

        * The declarations for the function arguments should not be
          indented.

        * Put spaces around parentheses used in control structures
          (`if', `while', `for', `do', `switch' and `return').

        * Do not put spaces in front of parentheses used in function
          calls.

        * Put spaces around all C operators, and after commas in
          function calls.

        * Do not use the comma operator to produce multiple
          side-effects, except in `for' loop initialization and
          increment parts, and in macro bodies.

        * Use real tabs for indenting, not spaces.

        * Use the "K&R" brace layout style.

        * Use comparisons against `NULL' and `'\0'' in the conditions of
          `if', `while' and `for' statements, and in the `case's of
          `switch' statements, instead of just the plain pointer or
          character value.

        * Use the `TRUE', `FALSE', and `NULL' symbolic constants, and
          the character constant `'\0'' where appropriate, instead of
          `1' and `0'.

        * Provide one-line descriptive comments for each function.

        * Do not use `#elif'. Many older Unix C compilers cannot handle
          it.

        * Do not use the `alloca' function for allocating memory off
          the stack.  Its use causes more portability trouble than the
          minor benefit of not having to free the storage. Instead, use
          `malloc' and `free'.

     If I have to reformat your code to follow the coding style used in
     `gawk', I may not bother.

  4. Be prepared to sign the appropriate paperwork.  In order for the
     FSF to distribute your changes, you must either place those
     changes in the public domain, and submit a signed statement to that
     effect, or assign the copyright in your changes to the FSF.  Both
     of these actions are easy to do, and *many* people have done so
     already. If you have questions, please contact me (*note Reporting
     Problems and Bugs: Bugs.), or `gnu@prep.ai.mit.edu'.

  5. Update the documentation.  Along with your new code, please supply
     new sections and or chapters for this Info file.  If at all
     possible, please use real Texinfo, instead of just supplying
     unformatted ASCII text (although even that is better than no
     documentation at all).  Conventions to be followed in `The GNU Awk
     User's Guide' are provided after the `@bye' at the end of the
     Texinfo source file.  If possible, please update the man page as
     well.

     You will also have to sign paperwork for your documentation
     changes.

  6. Submit changes as context diffs or unified diffs.  Use `diff -c -r
     -N' or `diff -u -r -N' to compare the original `gawk' source tree
     with your version.  (I find context diffs to be more readable, but
     unified diffs are more compact.) I recommend using the GNU version
     of `diff'.  Send the output produced by either run of `diff' to me
     when you submit your changes.  *Note Reporting Problems and Bugs:
     Bugs, for the electronic mail information.

     Using this format makes it easy for me to apply your changes to the
     master version of the `gawk' source code (using `patch').  If I
     have to apply the changes manually, using a text editor, I may not
     do so, particularly if there are lots of changes.

   Although this sounds like a lot of work, please remember that while
you may write the new code, I have to maintain it and support it, and
if it isn't possible for me to do that with a minimum of extra work,
then I probably will not.


File: gawk.info,  Node: New Ports,  Prev: Adding Code,  Up: Additions

Porting `gawk' to a New Operating System
----------------------------------------

   If you wish to port `gawk' to a new operating system, there are
several steps to follow.

  1. Follow the guidelines in *Note Adding New Features: Adding Code,
     concerning coding style, submission of diffs, and so on.

  2. When doing a port, bear in mind that your code must co-exist
     peacefully with the rest of `gawk', and the other ports. Avoid
     gratuitous changes to the system-independent parts of the code. If
     at all possible, avoid sprinkling `#ifdef's just for your port
     throughout the code.

     If the changes needed for a particular system affect too much of
     the code, I probably will not accept them.  In such a case, you
     will, of course, be able to distribute your changes on your own,
     as long as you comply with the GPL (*note GNU GENERAL PUBLIC
     LICENSE: Copying.).

  3. A number of the files that come with `gawk' are maintained by other
     people at the Free Software Foundation.  Thus, you should not
     change them unless it is for a very good reason. I.e. changes are
     not out of the question, but changes to these files will be
     scrutinized extra carefully.  The files are `alloca.c',
     `getopt.h', `getopt.c', `getopt1.c', `regex.h', `regex.c', `dfa.h',
     `dfa.c', `install-sh', and `mkinstalldirs'.

  4. Be willing to continue to maintain the port.  Non-Unix operating
     systems are supported by volunteers who maintain the code needed
     to compile and run `gawk' on their systems. If no-one volunteers
     to maintain a port, that port becomes unsupported, and it may be
     necessary to remove it from the distribution.

  5. Supply an appropriate `gawkmisc.???' file.  Each port has its own
     `gawkmisc.???' that implements certain operating system specific
     functions. This is cleaner than a plethora of `#ifdef's scattered
     throughout the code.  The `gawkmisc.c' in the main source
     directory includes the appropriate `gawkmisc.???' file from each
     subdirectory.  Be sure to update it as well.

     Each port's `gawkmisc.???' file has a suffix reminiscent of the
     machine or operating system for the port. For example,
     `pc/gawkmisc.pc' and `vms/gawkmisc.vms'. The use of separate
     suffixes, instead of plain `gawkmisc.c', makes it possible to move
     files from a port's subdirectory into the main subdirectory,
     without accidentally destroying the real `gawkmisc.c' file.
     (Currently, this is only an issue for the MS-DOS and OS/2 ports.)

  6. Supply a `Makefile' and any other C source and header files that
     are necessary for your operating system.  All your code should be
     in a separate subdirectory, with a name that is the same as, or
     reminiscent of, either your operating system or the computer
     system.  If possible, try to structure things so that it is not
     necessary to move files out of the subdirectory into the main
     source directory.  If that is not possible, then be sure to avoid
     using names for your files that duplicate the names of files in
     the main source directory.

  7. Update the documentation.  Please write a section (or sections)
     for this Info file describing the installation and compilation
     steps needed to install and/or compile `gawk' for your system.

  8. Be prepared to sign the appropriate paperwork.  In order for the
     FSF to distribute your code, you must either place your code in
     the public domain, and submit a signed statement to that effect,
     or assign the copyright in your code to the FSF.  Both of these
     actions are easy to do, and *many* people have done so already. If
     you have questions, please contact me, or `gnu@prep.ai.mit.edu'.

   Following these steps will make it much easier to integrate your
changes into `gawk', and have them co-exist happily with the code for
other operating systems that is already there.

   In the code that you supply, and that you maintain, feel free to use
a coding style and brace layout that suits your taste.


File: gawk.info,  Node: Future Extensions,  Next: Improvements,  Prev: Additions,  Up: Notes

Probable Future Extensions
==========================

     AWK is a language similar to PERL, only considerably more elegant.
     Arnold Robbins
     
     Hey!
     Larry Wall

   This section briefly lists extensions and possible improvements that
indicate the directions we are currently considering for `gawk'.  The
file `FUTURES' in the `gawk' distributions lists these extensions as
well.

   This is a list of probable future changes that will be usable by the
`awk' language programmer.

Localization
     The GNU project is starting to support multiple languages.  It
     will at least be possible to make `gawk' print its warnings and
     error messages in languages other than English.  It may be
     possible for `awk' programs to also use the multiple language
     facilities, separate from `gawk' itself.

Databases
     It may be possible to map a GDBM/NDBM/SDBM file into an `awk'
     array.

A `PROCINFO' Array
     The special files that provide process-related information (*note
     Special File Names in `gawk': Special Files.) may be superseded by
     a `PROCINFO' array that would provide the same information, in an
     easier to access fashion.

More `lint' warnings
     There are more things that could be checked for portability.

Control of subprocess environment
     Changes made in `gawk' to the array `ENVIRON' may be propagated to
     subprocesses run by `gawk'.

   This is a list of probable improvements that will make `gawk'
perform better.

An Improved Version of `dfa'
     The `dfa' pattern matcher from GNU `grep' has some problems.
     Either a new version or a fixed one will deal with some important
     regexp matching issues.

Use of GNU `malloc'
     The GNU version of `malloc' could potentially speed up `gawk',
     since it relies heavily on the use of dynamic memory allocation.

Use of the `rx' regexp library
     The `rx' regular expression library could potentially speed up all
     regexp operations that require knowing the exact location of
     matches.  This includes record termination, field and array
     splitting, and the `sub', `gsub', `gensub' and `match' functions.


File: gawk.info,  Node: Improvements,  Prev: Future Extensions,  Up: Notes

Suggestions for Improvements
============================

   Here are some projects that would-be `gawk' hackers might like to
take on.  They vary in size from a few days to a few weeks of
programming, depending on which one you choose and how fast a
programmer you are.  Please send any improvements you write to the
maintainers at the GNU project.  *Note Adding New Features: Adding Code,
for guidelines to follow when adding new features to `gawk'.  *Note
Reporting Problems and Bugs: Bugs, for information on contacting the
maintainers.

  1. Compilation of `awk' programs: `gawk' uses a Bison (YACC-like)
     parser to convert the script given it into a syntax tree; the
     syntax tree is then executed by a simple recursive evaluator.
     This method incurs a lot of overhead, since the recursive
     evaluator performs many procedure calls to do even the simplest
     things.

     It should be possible for `gawk' to convert the script's parse tree
     into a C program which the user would then compile, using the
     normal C compiler and a special `gawk' library to provide all the
     needed functions (regexps, fields, associative arrays, type
     coercion, and so on).

     An easier possibility might be for an intermediate phase of `awk'
     to convert the parse tree into a linear byte code form like the
     one used in GNU Emacs Lisp.  The recursive evaluator would then be
     replaced by a straight line byte code interpreter that would be
     intermediate in speed between running a compiled program and doing
     what `gawk' does now.

  2. The programs in the test suite could use documenting in this
     Info file.

  3. See the `FUTURES' file for more ideas.  Contact us if you would
     seriously like to tackle any of the items listed there.


File: gawk.info,  Node: Glossary,  Next: Copying,  Prev: Notes,  Up: Top

Glossary
********

Action
     A series of `awk' statements attached to a rule.  If the rule's
     pattern matches an input record, `awk' executes the rule's action.
     Actions are always enclosed in curly braces.  *Note Overview of
     Actions: Action Overview.

Amazing `awk' Assembler
     Henry Spencer at the University of Toronto wrote a retargetable
     assembler completely as `awk' scripts.  It is thousands of lines
     long, including machine descriptions for several eight-bit
     microcomputers.  It is a good example of a program that would have
     been better written in another language.

Amazingly Workable Formatter (`awf')
     Henry Spencer at the University of Toronto wrote a formatter that
     accepts a large subset of the `nroff -ms' and `nroff -man'
     formatting commands, using `awk' and `sh'.

ANSI
     The American National Standards Institute.  This organization
     produces many standards, among them the standards for the C and
     C++ programming languages.

Assignment
     An `awk' expression that changes the value of some `awk' variable
     or data object.  An object that you can assign to is called an
     "lvalue".  The assigned values are called "rvalues".  *Note
     Assignment Expressions: Assignment Ops.

`awk' Language
     The language in which `awk' programs are written.

`awk' Program
     An `awk' program consists of a series of "patterns" and "actions",
     collectively known as "rules".  For each input record given to the
     program, the program's rules are all processed in turn.  `awk'
     programs may also contain function definitions.

`awk' Script
     Another name for an `awk' program.

Bash
     The GNU version of the standard shell (the Bourne-Again shell).
     See "Bourne Shell."

BBS
     See "Bulletin Board System."

Boolean Expression
     Named after the English mathematician Boole. See "Logical
     Expression."

Bourne Shell
     The standard shell (`/bin/sh') on Unix and Unix-like systems,
     originally written by Steven R. Bourne.  Many shells (Bash, `ksh',
     `pdksh', `zsh') are generally upwardly compatible with the Bourne
     shell.

Built-in Function
     The `awk' language provides built-in functions that perform various
     numerical, time stamp related, and string computations.  Examples
     are `sqrt' (for the square root of a number) and `substr' (for a
     substring of a string).  *Note Built-in Functions: Built-in.

Built-in Variable
     `ARGC', `ARGIND', `ARGV', `CONVFMT', `ENVIRON', `ERRNO',
     `FIELDWIDTHS', `FILENAME', `FNR', `FS', `IGNORECASE', `NF', `NR',
     `OFMT', `OFS', `ORS', `RLENGTH', `RSTART', `RS', `RT', and
     `SUBSEP', are the variables that have special meaning to `awk'.
     Changing some of them affects `awk''s running environment.
     Several of these variables are specific to `gawk'.  *Note Built-in
     Variables::.

Braces
     See "Curly Braces."

Bulletin Board System
     A computer system allowing users to log in and read and/or leave
     messages for other users of the system, much like leaving paper
     notes on a bulletin board.

C
     The system programming language that most GNU software is written
     in.  The `awk' programming language has C-like syntax, and this
     Info file points out similarities between `awk' and C when
     appropriate.

Character Set
     The set of numeric codes used by a computer system to represent the
     characters (letters, numbers, punctuation, etc.) of a particular
     country or place. The most common character set in use today is
     ASCII (American Standard Code for Information Interchange).  Many
     European countries use an extension of ASCII known as ISO-8859-1
     (ISO Latin-1).

CHEM
     A preprocessor for `pic' that reads descriptions of molecules and
     produces `pic' input for drawing them.  It was written in `awk' by
     Brian Kernighan and Jon Bentley, and is available from
     `netlib@research.att.com'.

Compound Statement
     A series of `awk' statements, enclosed in curly braces.  Compound
     statements may be nested.  *Note Control Statements in Actions:
     Statements.

Concatenation
     Concatenating two strings means sticking them together, one after
     another, giving a new string.  For example, the string `foo'
     concatenated with the string `bar' gives the string `foobar'.
     *Note String Concatenation: Concatenation.

Conditional Expression
     An expression using the `?:' ternary operator, such as `EXPR1 ?
     EXPR2 : EXPR3'.  The expression EXPR1 is evaluated; if the result
     is true, the value of the whole expression is the value of EXPR2,
     otherwise the value is EXPR3.  In either case, only one of EXPR2
     and EXPR3 is evaluated.  *Note Conditional Expressions:
     Conditional Exp.

Comparison Expression
     A relation that is either true or false, such as `(a < b)'.
     Comparison expressions are used in `if', `while', `do', and `for'
     statements, and in patterns to select which input records to
     process.  *Note Variable Typing and Comparison Expressions: Typing
     and Comparison.

Curly Braces
     The characters `{' and `}'.  Curly braces are used in `awk' for
     delimiting actions, compound statements, and function bodies.

Dark Corner
     An area in the language where specifications often were (or still
     are) not clear, leading to unexpected or undesirable behavior.
     Such areas are marked in this Info file with "(d.c.)" in the text,
     and are indexed under the heading "dark corner."

Data Objects
     These are numbers and strings of characters.  Numbers are
     converted into strings and vice versa, as needed.  *Note
     Conversion of Strings and Numbers: Conversion.

Double Precision
     An internal representation of numbers that can have fractional
     parts.  Double precision numbers keep track of more digits than do
     single precision numbers, but operations on them are more
     expensive.  This is the way `awk' stores numeric values.  It is
     the C type `double'.

Dynamic Regular Expression
     A dynamic regular expression is a regular expression written as an
     ordinary expression.  It could be a string constant, such as
     `"foo"', but it may also be an expression whose value can vary.
     *Note Using Dynamic Regexps: Computed Regexps.

Environment
     A collection of strings, of the form NAME`='VAL, that each program
     has available to it. Users generally place values into the
     environment in order to provide information to various programs.
     Typical examples are the environment variables `HOME' and `PATH'.

Empty String
     See "Null String."

Escape Sequences
     A special sequence of characters used for describing non-printing
     characters, such as `\n' for newline, or `\033' for the ASCII ESC
     (escape) character.  *Note Escape Sequences::.

Field
     When `awk' reads an input record, it splits the record into pieces
     separated by whitespace (or by a separator regexp which you can
     change by setting the built-in variable `FS').  Such pieces are
     called fields.  If the pieces are of fixed length, you can use the
     built-in variable `FIELDWIDTHS' to describe their lengths.  *Note
     Specifying How Fields are Separated: Field Separators, and also see
     *Note Reading Fixed-width Data: Constant Size.

Floating Point Number
     Often referred to in mathematical terms as a "rational" number,
     this is just a number that can have a fractional part.  See
     "Double Precision" and "Single Precision."

Format
     Format strings are used to control the appearance of output in the
     `printf' statement.  Also, data conversions from numbers to strings
     are controlled by the format string contained in the built-in
     variable `CONVFMT'.  *Note Format-Control Letters: Control Letters.

Function
     A specialized group of statements used to encapsulate general or
     program-specific tasks.  `awk' has a number of built-in functions,
     and also allows you to define your own.  *Note Built-in Functions:
     Built-in, and *Note User-defined Functions: User-defined.

FSF
     See "Free Software Foundation."

Free Software Foundation
     A non-profit organization dedicated to the production and
     distribution of freely distributable software.  It was founded by
     Richard M. Stallman, the author of the original Emacs editor.  GNU
     Emacs is the most widely used version of Emacs today.

`gawk'
     The GNU implementation of `awk'.

General Public License
     This document describes the terms under which `gawk' and its source
     code may be distributed. (*note GNU GENERAL PUBLIC LICENSE:
     Copying.)

GNU
     "GNU's not Unix".  An on-going project of the Free Software
     Foundation to create a complete, freely distributable,
     POSIX-compliant computing environment.

GPL
     See "General Public License."

Hexadecimal
     Base 16 notation, where the digits are `0'-`9' and `A'-`F', with
     `A' representing 10, `B' representing 11, and so on up to `F' for
     15.  Hexadecimal numbers are written in C using a leading `0x', to
     indicate their base.  Thus, `0x12' is 18 (one times 16 plus 2).

I/O
     Abbreviation for "Input/Output," the act of moving data into and/or
     out of a running program.

Input Record
     A single chunk of data read in by `awk'.  Usually, an `awk' input
     record consists of one line of text.  *Note How Input is Split
     into Records: Records.

Integer
     A whole number, i.e. a number that does not have a fractional part.

Keyword
     In the `awk' language, a keyword is a word that has special
     meaning.  Keywords are reserved and may not be used as variable
     names.

     `gawk''s keywords are: `BEGIN', `END', `if', `else', `while',
     `do...while', `for', `for...in', `break', `continue', `delete',
     `next', `nextfile', `function', `func', and `exit'.

Logical Expression
     An expression using the operators for logic, AND, OR, and NOT,
     written `&&', `||', and `!' in `awk'. Often called Boolean
     expressions, after the mathematician who pioneered this kind of
     mathematical logic.

Lvalue
     An expression that can appear on the left side of an assignment
     operator.  In most languages, lvalues can be variables or array
     elements.  In `awk', a field designator can also be used as an
     lvalue.

Null String
     A string with no characters in it.  It is represented explicitly in
     `awk' programs by placing two double-quote characters next to each
     other (`""').  It can appear in input data by having two successive
     occurrences of the field separator appear next to each other.

Number
     A numeric valued data object.  The `gawk' implementation uses
     double precision floating point to represent numbers.  Very old
     `awk' implementations use single precision floating point.

Octal
     Base-eight notation, where the digits are `0'-`7'.  Octal numbers
     are written in C using a leading `0', to indicate their base.
     Thus, `013' is 11 (one times 8 plus 3).

Pattern
     Patterns tell `awk' which input records are interesting to which
     rules.

     A pattern is an arbitrary conditional expression against which
     input is tested.  If the condition is satisfied, the pattern is
     said to "match" the input record.  A typical pattern might compare
     the input record against a regular expression.  *Note Pattern
     Elements: Pattern Overview.

POSIX
     The name for a series of standards being developed by the IEEE
     that specify a Portable Operating System interface.  The "IX"
     denotes the Unix heritage of these standards.  The main standard
     of interest for `awk' users is `IEEE Standard for Information
     Technology, Standard 1003.2-1992, Portable Operating System
     Interface (POSIX) Part 2: Shell and Utilities'.  Informally, this
     standard is often referred to as simply "P1003.2."

Private
     Variables and/or functions that are meant for use exclusively by
     library functions, and not for the main `awk' program. Special
     care must be taken when naming such variables and functions.
     *Note Naming Library Function Global Variables: Library Names.

Range (of input lines)
     A sequence of consecutive lines from the input file.  A pattern
     can specify ranges of input lines for `awk' to process, or it can
     specify single lines.  *Note Pattern Elements: Pattern Overview.

Recursion
     When a function calls itself, either directly or indirectly.  If
     this isn't clear, refer to the entry for "recursion."

Redirection
     Redirection means performing input from other than the standard
     input stream, or output to other than the standard output stream.

     You can redirect the output of the `print' and `printf' statements
     to a file or a system command, using the `>', `>>', and `|'
     operators.  You can redirect input to the `getline' statement using
     the `<' and `|' operators.  *Note Redirecting Output of `print'
     and `printf': Redirection, and *Note Explicit Input with
     `getline': Getline.

Regexp
     Short for "regular expression".  A regexp is a pattern that
     denotes a set of strings, possibly an infinite set.  For example,
     the regexp `R.*xp' matches any string starting with the letter `R'
     and ending with the letters `xp'.  In `awk', regexps are used in
     patterns and in conditional expressions.  Regexps may contain
     escape sequences.  *Note Regular Expressions: Regexp.

Regular Expression
     See "regexp."

Regular Expression Constant
     A regular expression constant is a regular expression written
     within slashes, such as `/foo/'.  This regular expression is chosen
     when you write the `awk' program, and cannot be changed doing its
     execution.  *Note How to Use Regular Expressions: Regexp Usage.

Rule
     A segment of an `awk' program that specifies how to process single
     input records.  A rule consists of a "pattern" and an "action".
     `awk' reads an input record; then, for each rule, if the input
     record satisfies the rule's pattern, `awk' executes the rule's
     action.  Otherwise, the rule does nothing for that input record.

Rvalue
     A value that can appear on the right side of an assignment
     operator.  In `awk', essentially every expression has a value.
     These values are rvalues.

`sed'
     See "Stream Editor."

Short-Circuit
     The nature of the `awk' logical operators `&&' and `||'.  If the
     value of the entire expression can be deduced from evaluating just
     the left-hand side of these operators, the right-hand side will not
     be evaluated (*note Boolean Expressions: Boolean Ops.).

Side Effect
     A side effect occurs when an expression has an effect aside from
     merely producing a value.  Assignment expressions, increment and
     decrement expressions and function calls have side effects.  *Note
     Assignment Expressions: Assignment Ops.

Single Precision
     An internal representation of numbers that can have fractional
     parts.  Single precision numbers keep track of fewer digits than
     do double precision numbers, but operations on them are less
     expensive in terms of CPU time.  This is the type used by some
     very old versions of `awk' to store numeric values.  It is the C
     type `float'.

Space
     The character generated by hitting the space bar on the keyboard.

Special File
     A file name interpreted internally by `gawk', instead of being
     handed directly to the underlying operating system.  For example,
     `/dev/stderr'.  *Note Special File Names in `gawk': Special Files.

Stream Editor
     A program that reads records from an input stream and processes
     them one or more at a time.  This is in contrast with batch
     programs, which may expect to read their input files in entirety
     before starting to do anything, and with interactive programs,
     which require input from the user.

String
     A datum consisting of a sequence of characters, such as `I am a
     string'.  Constant strings are written with double-quotes in the
     `awk' language, and may contain escape sequences.  *Note Escape
     Sequences::.

Tab
     The character generated by hitting the `TAB' key on the keyboard.
     It usually expands to up to eight spaces upon output.

Unix
     A computer operating system originally developed in the early
     1970's at AT&T Bell Laboratories.  It initially became popular in
     universities around the world, and later moved into commercial
     evnironments as a software development system and network server
     system. There are many commercial versions of Unix, as well as
     several work-alike systems whose source code is freely available
     (such as Linux, NetBSD, and FreeBSD).

Whitespace
     A sequence of space, tab, or newline characters occurring inside
     an input record or a string.


File: gawk.info,  Node: Copying,  Next: Index,  Prev: Glossary,  Up: Top

GNU GENERAL PUBLIC LICENSE
**************************

                         Version 2, June 1991

     Copyright (C) 1989, 1991 Free Software Foundation, Inc.
     59 Temple Place --- Suite 330, Boston, MA 02111-1307, USA
     
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

Preamble
========

   The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software--to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.

   When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it in
new free programs; and that you know you can do these things.

   To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

   For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.

   We protect your rights with two steps: (1) copyright the software,
and (2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

   Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

   Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

   The precise terms and conditions for copying, distribution and
modification follow.

    TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. This License applies to any program or other work which contains a
     notice placed by the copyright holder saying it may be distributed
     under the terms of this General Public License.  The "Program",
     below, refers to any such program or work, and a "work based on
     the Program" means either the Program or any derivative work under
     copyright law: that is to say, a work containing the Program or a
     portion of it, either verbatim or with modifications and/or
     translated into another language.  (Hereinafter, translation is
     included without limitation in the term "modification".)  Each
     licensee is addressed as "you".

     Activities other than copying, distribution and modification are
     not covered by this License; they are outside its scope.  The act
     of running the Program is not restricted, and the output from the
     Program is covered only if its contents constitute a work based on
     the Program (independent of having been made by running the
     Program).  Whether that is true depends on what the Program does.

  1. You may copy and distribute verbatim copies of the Program's
     source code as you receive it, in any medium, provided that you
     conspicuously and appropriately publish on each copy an appropriate
     copyright notice and disclaimer of warranty; keep intact all the
     notices that refer to this License and to the absence of any
     warranty; and give any other recipients of the Program a copy of
     this License along with the Program.

     You may charge a fee for the physical act of transferring a copy,
     and you may at your option offer warranty protection in exchange
     for a fee.

  2. You may modify your copy or copies of the Program or any portion
     of it, thus forming a work based on the Program, and copy and
     distribute such modifications or work under the terms of Section 1
     above, provided that you also meet all of these conditions:

       a. You must cause the modified files to carry prominent notices
          stating that you changed the files and the date of any change.

       b. You must cause any work that you distribute or publish, that
          in whole or in part contains or is derived from the Program
          or any part thereof, to be licensed as a whole at no charge
          to all third parties under the terms of this License.

       c. If the modified program normally reads commands interactively
          when run, you must cause it, when started running for such
          interactive use in the most ordinary way, to print or display
          an announcement including an appropriate copyright notice and
          a notice that there is no warranty (or else, saying that you
          provide a warranty) and that users may redistribute the
          program under these conditions, and telling the user how to
          view a copy of this License.  (Exception: if the Program
          itself is interactive but does not normally print such an
          announcement, your work based on the Program is not required
          to print an announcement.)

     These requirements apply to the modified work as a whole.  If
     identifiable sections of that work are not derived from the
     Program, and can be reasonably considered independent and separate
     works in themselves, then this License, and its terms, do not
     apply to those sections when you distribute them as separate
     works.  But when you distribute the same sections as part of a
     whole which is a work based on the Program, the distribution of
     the whole must be on the terms of this License, whose permissions
     for other licensees extend to the entire whole, and thus to each
     and every part regardless of who wrote it.

     Thus, it is not the intent of this section to claim rights or
     contest your rights to work written entirely by you; rather, the
     intent is to exercise the right to control the distribution of
     derivative or collective works based on the Program.

     In addition, mere aggregation of another work not based on the
     Program with the Program (or with a work based on the Program) on
     a volume of a storage or distribution medium does not bring the
     other work under the scope of this License.

  3. You may copy and distribute the Program (or a work based on it,
     under Section 2) in object code or executable form under the terms
     of Sections 1 and 2 above provided that you also do one of the
     following:

       a. Accompany it with the complete corresponding machine-readable
          source code, which must be distributed under the terms of
          Sections 1 and 2 above on a medium customarily used for
          software interchange; or,

       b. Accompany it with a written offer, valid for at least three
          years, to give any third party, for a charge no more than your
          cost of physically performing source distribution, a complete
          machine-readable copy of the corresponding source code, to be
          distributed under the terms of Sections 1 and 2 above on a
          medium customarily used for software interchange; or,

       c. Accompany it with the information you received as to the offer
          to distribute corresponding source code.  (This alternative is
          allowed only for non-commercial distribution and only if you
          received the program in object code or executable form with
          such an offer, in accord with Subsection b above.)

     The source code for a work means the preferred form of the work for
     making modifications to it.  For an executable work, complete
     source code means all the source code for all modules it contains,
     plus any associated interface definition files, plus the scripts
     used to control compilation and installation of the executable.
     However, as a special exception, the source code distributed need
     not include anything that is normally distributed (in either
     source or binary form) with the major components (compiler,
     kernel, and so on) of the operating system on which the executable
     runs, unless that component itself accompanies the executable.

     If distribution of executable or object code is made by offering
     access to copy from a designated place, then offering equivalent
     access to copy the source code from the same place counts as
     distribution of the source code, even though third parties are not
     compelled to copy the source along with the object code.

  4. You may not copy, modify, sublicense, or distribute the Program
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense or distribute the Program is
     void, and will automatically terminate your rights under this
     License.  However, parties who have received copies, or rights,
     from you under this License will not have their licenses
     terminated so long as such parties remain in full compliance.

  5. You are not required to accept this License, since you have not
     signed it.  However, nothing else grants you permission to modify
     or distribute the Program or its derivative works.  These actions
     are prohibited by law if you do not accept this License.
     Therefore, by modifying or distributing the Program (or any work
     based on the Program), you indicate your acceptance of this
     License to do so, and all its terms and conditions for copying,
     distributing or modifying the Program or works based on it.

  6. Each time you redistribute the Program (or any work based on the
     Program), the recipient automatically receives a license from the
     original licensor to copy, distribute or modify the Program
     subject to these terms and conditions.  You may not impose any
     further restrictions on the recipients' exercise of the rights
     granted herein.  You are not responsible for enforcing compliance
     by third parties to this License.

  7. If, as a consequence of a court judgment or allegation of patent
     infringement or for any other reason (not limited to patent
     issues), conditions are imposed on you (whether by court order,
     agreement or otherwise) that contradict the conditions of this
     License, they do not excuse you from the conditions of this
     License.  If you cannot distribute so as to satisfy simultaneously
     your obligations under this License and any other pertinent
     obligations, then as a consequence you may not distribute the
     Program at all.  For example, if a patent license would not permit
     royalty-free redistribution of the Program by all those who
     receive copies directly or indirectly through you, then the only
     way you could satisfy both it and this License would be to refrain
     entirely from distribution of the Program.

     If any portion of this section is held invalid or unenforceable
     under any particular circumstance, the balance of the section is
     intended to apply and the section as a whole is intended to apply
     in other circumstances.

     It is not the purpose of this section to induce you to infringe any
     patents or other property right claims or to contest validity of
     any such claims; this section has the sole purpose of protecting
     the integrity of the free software distribution system, which is
     implemented by public license practices.  Many people have made
     generous contributions to the wide range of software distributed
     through that system in reliance on consistent application of that
     system; it is up to the author/donor to decide if he or she is
     willing to distribute software through any other system and a
     licensee cannot impose that choice.

     This section is intended to make thoroughly clear what is believed
     to be a consequence of the rest of this License.

  8. If the distribution and/or use of the Program is restricted in
     certain countries either by patents or by copyrighted interfaces,
     the original copyright holder who places the Program under this
     License may add an explicit geographical distribution limitation
     excluding those countries, so that distribution is permitted only
     in or among countries not thus excluded.  In such case, this
     License incorporates the limitation as if written in the body of
     this License.

  9. The Free Software Foundation may publish revised and/or new
     versions of the General Public License from time to time.  Such
     new versions will be similar in spirit to the present version, but
     may differ in detail to address new problems or concerns.

     Each version is given a distinguishing version number.  If the
     Program specifies a version number of this License which applies
     to it and "any later version", you have the option of following
     the terms and conditions either of that version or of any later
     version published by the Free Software Foundation.  If the Program
     does not specify a version number of this License, you may choose
     any version ever published by the Free Software Foundation.

 10. If you wish to incorporate parts of the Program into other free
     programs whose distribution conditions are different, write to the
     author to ask for permission.  For software which is copyrighted
     by the Free Software Foundation, write to the Free Software
     Foundation; we sometimes make exceptions for this.  Our decision
     will be guided by the two goals of preserving the free status of
     all derivatives of our free software and of promoting the sharing
     and reuse of software generally.

                                NO WARRANTY

 11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO
     WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE
     LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
     HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT
     WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT
     NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
     FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS TO THE
     QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
     PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY
     SERVICING, REPAIR OR CORRECTION.

 12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
     WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY
     MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE
     LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL,
     INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR
     INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
     DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU
     OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY
     OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN
     ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

                      END OF TERMS AND CONDITIONS

How to Apply These Terms to Your New Programs
=============================================

   If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

   To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the "copyright" line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND AN IDEA OF WHAT IT DOES.
     Copyright (C) 19YY  NAME OF AUTHOR
     
     This program is free software; you can redistribute it and/or
     modify it under the terms of the GNU General Public License
     as published by the Free Software Foundation; either version 2
     of the License, or (at your option) any later version.
     
     This program is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     GNU General Public License for more details.
     
     You should have received a copy of the GNU General Public License
     along with this program; if not, write to the Free Software
     Foundation, Inc., 59 Temple Place --- Suite 330, Boston, MA 02111-1307, USA.

   Also add information on how to contact you by electronic and paper
mail.

   If the program is interactive, make it output a short notice like
this when it starts in an interactive mode:

     Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR
     Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
     type `show w'.  This is free software, and you are welcome
     to redistribute it under certain conditions; type `show c'
     for details.

   The hypothetical commands `show w' and `show c' should show the
appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than `show w' and `show
c'; they could even be mouse-clicks or menu items--whatever suits your
program.

   You should also get your employer (if you work as a programmer) or
your school, if any, to sign a "copyright disclaimer" for the program,
if necessary.  Here is a sample; alter the names:

     Yoyodyne, Inc., hereby disclaims all copyright
     interest in the program `Gnomovision'
     (which makes passes at compilers) written
     by James Hacker.
     
     SIGNATURE OF TY COON, 1 April 1989
     Ty Coon, President of Vice

   This General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use the
GNU Library General Public License instead of this License.


File: gawk.info,  Node: Index,  Prev: Copying,  Up: Top

Index
*****

* Menu:

* ! operator:                            Boolean Ops.
* != operator:                           Typing and Comparison.
* !~ operator <1>:                       Regexp Constants.
* !~ operator <2>:                       Regexp Usage.
* !~ operator <3>:                       Typing and Comparison.
* !~ operator <4>:                       Case-sensitivity.
* !~ operator:                           Computed Regexps.
* # (comment):                           Comments.
* #! (executable scripts):               Executable Scripts.
* $ (field operator):                    Fields.
* && operator:                           Boolean Ops.
* --assign option:                       Options.
* --compat option:                       Options.
* --copyleft option:                     Options.
* --copyright option:                    Options.
* --field-separator option:              Options.
* --file option:                         Options.
* --help option:                         Options.
* --lint option:                         Options.
* --lint-old option:                     Options.
* --posix option:                        Options.
* --source option:                       Options.
* --traditional option:                  Options.
* --usage option:                        Options.
* --version option:                      Options.
* -f option <1>:                         Long.
* -f option:                             Options.
* -F option <1>:                         Command Line Field Separator.
* -F option:                             Options.
* -v option:                             Options.
* -W option:                             Options.
* /dev/fd:                               Special Files.
* /dev/pgrpid:                           Special Files.
* /dev/pid:                              Special Files.
* /dev/ppid:                             Special Files.
* /dev/stderr:                           Special Files.
* /dev/stdin:                            Special Files.
* /dev/stdout:                           Special Files.
* /dev/user <1>:                         Special Files.
* /dev/user:                             Passwd Functions.
* < operator:                            Typing and Comparison.
* <= operator:                           Typing and Comparison.
* == operator:                           Typing and Comparison.
* > operator:                            Typing and Comparison.
* >= operator:                           Typing and Comparison.
* \' regexp operator:                    GNU Regexp Operators.
* \< regexp operator:                    GNU Regexp Operators.
* \> regexp operator:                    GNU Regexp Operators.
* \` regexp operator:                    GNU Regexp Operators.
* \B regexp operator:                    GNU Regexp Operators.
* \W regexp operator:                    GNU Regexp Operators.
* \w regexp operator:                    GNU Regexp Operators.
* \y regexp operator:                    GNU Regexp Operators.
* _gr_init:                              Group Functions.
* _pw_init:                              Passwd Functions.
* _tm_addup:                             Mktime Function.
* _tm_isleap:                            Mktime Function.
* accessing fields:                      Fields.
* account information <1>:               Passwd Functions.
* account information:                   Group Functions.
* acronym:                               History.
* action, curly braces:                  Action Overview.
* action, default:                       Very Simple.
* action, definition of:                 Action Overview.
* action, empty:                         Very Simple.
* action, separating statements:         Action Overview.
* adding new features:                   Adding Code.
* addition:                              Arithmetic Ops.
* Aho, Alfred:                           History.
* AI programming, using gawk:            Distribution contents.
* alarm.awk:                             Alarm Program.
* amiga:                                 Amiga Installation.
* anchors in regexps:                    Regexp Operators.
* and operator:                          Boolean Ops.
* anonymous ftp <1>:                     Getting.
* anonymous ftp:                         Other Versions.
* applications of awk:                   When.
* ARGC:                                  Auto-set.
* ARGIND <1>:                            Auto-set.
* ARGIND:                                Other Arguments.
* argument processing:                   Getopt Function.
* arguments in function call:            Function Calls.
* arguments, command line:               Invoking Gawk.
* ARGV <1>:                              Other Arguments.
* ARGV:                                  Auto-set.
* arithmetic operators:                  Arithmetic Ops.
* array assignment:                      Assigning Elements.
* array reference:                       Reference to Elements.
* array subscripts, uninitialized variables: Uninitialized Subscripts.
* arrays:                                Array Intro.
* arrays, associative:                   Array Intro.
* arrays, definition of:                 Array Intro.
* arrays, deleting an element:           Delete.
* arrays, deleting entire contents:      Delete.
* arrays, multi-dimensional subscripts:  Multi-dimensional.
* arrays, presence of elements:          Reference to Elements.
* arrays, sparse:                        Array Intro.
* arrays, special for statement:         Scanning an Array.
* arrays, the in operator:               Reference to Elements.
* artificial intelligence, using gawk:   Distribution contents.
* ASCII:                                 Ordinal Functions.
* assert:                                Assert Function.
* assert, C version:                     Assert Function.
* assertions:                            Assert Function.
* assignment operators:                  Assignment Ops.
* assignment to fields:                  Changing Fields.
* associative arrays:                    Array Intro.
* atan2:                                 Numeric Functions.
* atari:                                 Atari Installation.
* automatic initialization:              More Complex.
* awk language, POSIX version <1>:       OFMT.
* awk language, POSIX version <2>:       Field Splitting Summary.
* awk language, POSIX version <3>:       Break Statement.
* awk language, POSIX version <4>:       Format Modifiers.
* awk language, POSIX version <5>:       String Functions.
* awk language, POSIX version <6>:       Continue Statement.
* awk language, POSIX version <7>:       Definition Syntax.
* awk language, POSIX version <8>:       Precedence.
* awk language, POSIX version <9>:       Next Statement.
* awk language, POSIX version <10>:      String Functions.
* awk language, POSIX version <11>:      User-modified.
* awk language, POSIX version <12>:      Precedence.
* awk language, POSIX version <13>:      Escape Sequences.
* awk language, POSIX version <14>:      Conversion.
* awk language, POSIX version <15>:      Arithmetic Ops.
* awk language, POSIX version <16>:      Regexp Operators.
* awk language, POSIX version <17>:      Assignment Ops.
* awk language, POSIX version:           Regexp Operators.
* awk language, V.4 version <1>:         Escape Sequences.
* awk language, V.4 version <2>:         SVR4.
* awk language, V.4 version:             Escape Sequences.
* AWKPATH environment variable:          AWKPATH Variable.
* awksed:                                Simple Sed.
* backslash continuation <1>:            Egrep Program.
* backslash continuation:                Statements/Lines.
* backslash continuation and comments:   Statements/Lines.
* backslash continuation in csh <1>:     More Complex.
* backslash continuation in csh:         Statements/Lines.
* basic function of awk:                 Getting Started.
* BBS-list file:                         Sample Data Files.
* BEGIN special pattern:                 BEGIN/END.
* beginfile:                             Filetrans Function.
* body of a loop:                        While Statement.
* book, using this:                      This Manual.
* boolean expressions:                   Boolean Ops.
* boolean operators:                     Boolean Ops.
* break statement:                       Break Statement.
* break, outside of loops:               Break Statement.
* Brennan, Michael <1>:                  Simple Sed.
* Brennan, Michael <2>:                  Delete.
* Brennan, Michael:                      Other Versions.
* buffer matching operators:             GNU Regexp Operators.
* buffering output:                      I/O Functions.
* buffering, interactive vs. non-interactive: I/O Functions.
* buffering, non-interactive vs. interactive: I/O Functions.
* buffers, flushing:                     I/O Functions.
* bugs, known in gawk:                   Known Bugs.
* built-in functions:                    Built-in.
* built-in variables:                    Built-in Variables.
* built-in variables, convey information: Auto-set.
* built-in variables, user modifiable:   User-modified.
* call by reference:                     Function Caveats.
* call by value:                         Function Caveats.
* calling a function <1>:                Function Caveats.
* calling a function:                    Function Calls.
* case conversion:                       String Functions.
* case sensitivity:                      Case-sensitivity.
* changing contents of a field:          Changing Fields.
* changing the record separator:         Records.
* character classes:                     Regexp Operators.
* character encodings:                   Ordinal Functions.
* character list:                        Regexp Operators.
* character list, complemented:          Regexp Operators.
* character sets:                        Ordinal Functions.
* chr:                                   Ordinal Functions.
* close <1>:                             Close Files And Pipes.
* close:                                 I/O Functions.
* closing input files and pipes:         Close Files And Pipes.
* closing output files and pipes:        Close Files And Pipes.
* coding style used in gawk:             Adding Code.
* collating elements:                    Regexp Operators.
* collating symbols:                     Regexp Operators.
* command line:                          Invoking Gawk.
* command line formats:                  Running gawk.
* command line, setting FS on:           Command Line Field Separator.
* comments:                              Comments.
* comments and backslash continuation:   Statements/Lines.
* common mistakes <1>:                   Typing and Comparison.
* common mistakes <2>:                   Computed Regexps.
* common mistakes <3>:                   Print Examples.
* common mistakes:                       Basic Field Splitting.
* comp.lang.awk:                         Bugs.
* comparison expressions:                Typing and Comparison.
* comparisons, string vs. regexp:        Typing and Comparison.
* compatibility mode <1>:                Options.
* compatibility mode:                    POSIX/GNU.
* complemented character list:           Regexp Operators.
* compound statement:                    Statements.
* computed regular expressions:          Computed Regexps.
* concatenation:                         Concatenation.
* conditional expression:                Conditional Exp.
* configuring gawk:                      Configuration Philosophy.
* constants, types of:                   Constants.
* continuation of lines:                 Statements/Lines.
* continue statement:                    Continue Statement.
* continue, outside of loops:            Continue Statement.
* control statement:                     Statements.
* conversion of case:                    String Functions.
* conversion of strings and numbers:     Conversion.
* conversions, during subscripting:      Numeric Array Subscripts.
* converting dates to timestamps:        Mktime Function.
* CONVFMT <1>:                           User-modified.
* CONVFMT <2>:                           Numeric Array Subscripts.
* CONVFMT:                               Conversion.
* cos:                                   Numeric Functions.
* csh, backslash continuation <1>:       Statements/Lines.
* csh, backslash continuation:           More Complex.
* curly braces:                          Action Overview.
* custom.h configuration file:           Configuration Philosophy.
* cut utility:                           Cut Program.
* cut.awk:                               Cut Program.
* d.c., see "dark corner":               This Manual.
* dark corner <1>:                       Assignment Options.
* dark corner <2>:                       Using Constant Regexps.
* dark corner <3>:                       Conversion.
* dark corner <4>:                       Using Constant Regexps.
* dark corner <5>:                       Truth Values.
* dark corner <6>:                       Auto-set.
* dark corner <7>:                       Plain Getline.
* dark corner <8>:                       Other Arguments.
* dark corner <9>:                       Records.
* dark corner <10>:                      Uninitialized Subscripts.
* dark corner <11>:                      Multiple Line.
* dark corner <12>:                      Auto-set.
* dark corner <13>:                      Break Statement.
* dark corner <14>:                      Single Character Fields.
* dark corner <15>:                      Getline Summary.
* dark corner <16>:                      OFMT.
* dark corner <17>:                      Escape Sequences.
* dark corner <18>:                      Exit Statement.
* dark corner <19>:                      Invoking Gawk.
* dark corner <20>:                      String Functions.
* dark corner <21>:                      This Manual.
* dark corner <22>:                      Continue Statement.
* dark corner <23>:                      Using BEGIN/END.
* dark corner <24>:                      Format Modifiers.
* dark corner <25>:                      Control Letters.
* dark corner:                           Field Splitting Summary.
* data-driven languages:                 Getting Started.
* dates, converting to timestamps:       Mktime Function.
* decrement operators:                   Increment Ops.
* default action:                        Very Simple.
* default pattern:                       Very Simple.
* defining functions:                    Definition Syntax.
* Deifik, Scott <1>:                     Bugs.
* Deifik, Scott:                         Acknowledgements.
* delete statement:                      Delete.
* deleting elements of arrays:           Delete.
* deleting entire arrays:                Delete.
* deprecated features:                   Obsolete.
* deprecated options:                    Obsolete.
* differences between gawk and awk <1>:  Records.
* differences between gawk and awk <2>:  Redirection.
* differences between gawk and awk <3>:  Close Files And Pipes.
* differences between gawk and awk <4>:  Case-sensitivity.
* differences between gawk and awk <5>:  Calling Built-in.
* differences between gawk and awk <6>:  AWKPATH Variable.
* differences between gawk and awk <7>:  Single Character Fields.
* differences between gawk and awk <8>:  Conditional Exp.
* differences between gawk and awk <9>:  String Functions.
* differences between gawk and awk <10>: Scalar Constants.
* differences between gawk and awk <11>: Special Files.
* differences between gawk and awk <12>: Close Files And Pipes.
* differences between gawk and awk <13>: I/O And BEGIN/END.
* differences between gawk and awk <14>: String Functions.
* differences between gawk and awk <15>: Arithmetic Ops.
* differences between gawk and awk <16>: Using Constant Regexps.
* differences between gawk and awk <17>: Getline Intro.
* differences between gawk and awk <18>: Delete.
* differences between gawk and awk <19>: Getline Summary.
* differences between gawk and awk:      Nextfile Statement.
* directory search:                      AWKPATH Variable.
* division:                              Arithmetic Ops.
* documenting awk programs <1>:          Library Names.
* documenting awk programs:              Comments.
* dupword.awk:                           Dupword Program.
* dynamic regular expressions:           Computed Regexps.
* EBCDIC:                                Ordinal Functions.
* egrep <1>:                             One-shot.
* egrep:                                 Regexp Operators.
* egrep utility:                         Egrep Program.
* egrep.awk:                             Egrep Program.
* element assignment:                    Assigning Elements.
* element of array:                      Reference to Elements.
* empty action:                          Very Simple.
* empty pattern:                         Empty.
* empty program:                         Invoking Gawk.
* empty string <1>:                      Regexp Field Splitting.
* empty string <2>:                      Records.
* empty string <3>:                      Truth Values.
* empty string:                          Conversion.
* END special pattern:                   BEGIN/END.
* endfile:                               Filetrans Function.
* endgrent:                              Group Functions.
* endpwent:                              Passwd Functions.
* ENVIRON:                               Auto-set.
* environment variable, AWKPATH:         AWKPATH Variable.
* environment variable, POSIXLY_CORRECT: Options.
* equivalence classes:                   Regexp Operators.
* ERRNO <1>:                             Auto-set.
* ERRNO <2>:                             Close Files And Pipes.
* ERRNO:                                 Getline Intro.
* errors, common <1>:                    Print Examples.
* errors, common <2>:                    Computed Regexps.
* errors, common <3>:                    Typing and Comparison.
* errors, common:                        Basic Field Splitting.
* escape processing, sub et. al.:        String Functions.
* escape sequence notation:              Escape Sequences.
* evaluation, order of:                  Calling Built-in.
* examining fields:                      Fields.
* executable scripts:                    Executable Scripts.
* exit statement:                        Exit Statement.
* exp:                                   Numeric Functions.
* explicit input:                        Getline.
* exponentiation:                        Arithmetic Ops.
* expression:                            Expressions.
* expression, assignment:                Assignment Ops.
* expression, boolean:                   Boolean Ops.
* expression, comparison:                Typing and Comparison.
* expression, conditional:               Conditional Exp.
* expression, matching:                  Typing and Comparison.
* extract.awk:                           Extract Program.
* features, adding:                      Adding Code.
* fflush:                                I/O Functions.
* field operator $:                      Fields.
* field separator, choice of:            Basic Field Splitting.
* field separator, FS:                   Basic Field Splitting.
* field separator, on command line:      Command Line Field Separator.
* field, changing contents of:           Changing Fields.
* fields:                                Fields.
* fields, separating:                    Basic Field Splitting.
* FIELDWIDTHS:                           User-modified.
* file descriptors:                      Special Files.
* file, awk program:                     Long.
* FILENAME <1>:                          Auto-set.
* FILENAME <2>:                          Getline Summary.
* FILENAME:                              Reading Files.
* FILENAME, being set by getline:        Getline Summary.
* Fish, Fred:                            Bugs.
* flushing buffers:                      I/O Functions.
* FNR <1>:                               Auto-set.
* FNR:                                   Records.
* for (x in ...):                        Scanning an Array.
* for statement:                         For Statement.
* format specifier:                      Control Letters.
* format string:                         Basic Printf.
* format, numeric output:                OFMT.
* formatted output:                      Printf.
* formatted timestamps:                  Gettimeofday Function.
* Free Software Foundation <1>:          Manual History.
* Free Software Foundation:              Getting.
* FreeBSD:                               Manual History.
* Friedl, Jeffrey:                       Acknowledgements.
* FS <1>:                                User-modified.
* FS:                                    Basic Field Splitting.
* ftp, anonymous <1>:                    Getting.
* ftp, anonymous:                        Other Versions.
* function call <1>:                     Function Caveats.
* function call:                         Function Calls.
* function definition:                   Definition Syntax.
* function, recursive:                   Definition Syntax.
* functions, undefined:                  Function Caveats.
* functions, user-defined:               User-defined.
* gawk coding style:                     Adding Code.
* gensub:                                String Functions.
* getgrent:                              Group Functions.
* getgrent, C version:                   Group Functions.
* getgrgid:                              Group Functions.
* getgrnam:                              Group Functions.
* getgruser:                             Group Functions.
* getline:                               Getline.
* getline, return values:                Getline Intro.
* getline, setting FILENAME:             Getline Summary.
* getopt:                                Getopt Function.
* getopt, C version:                     Getopt Function.
* getpwent:                              Passwd Functions.
* getpwent, C version:                   Passwd Functions.
* getpwnam:                              Passwd Functions.
* getpwuid:                              Passwd Functions.
* gettimeofday:                          Gettimeofday Function.
* getting gawk:                          Getting.
* GNU Project:                           Manual History.
* grcat program:                         Group Functions.
* grcat.c:                               Group Functions.
* group file:                            Group Functions.
* group information:                     Group Functions.
* gsub:                                  String Functions.
* gsub, third argument of:               String Functions.
* Hankerson, Darrel <1>:                 Bugs.
* Hankerson, Darrel:                     Acknowledgements.
* historical features <1>:               Break Statement.
* historical features <2>:               Continue Statement.
* historical features <3>:               Historical Features.
* historical features <4>:               String Functions.
* historical features:                   Command Line Field Separator.
* history of awk:                        History.
* histsort.awk:                          History Sorting.
* how awk works:                         Two Rules.
* Hughes, Phil:                          Acknowledgements.
* I/O from BEGIN and END:                I/O And BEGIN/END.
* id utility:                            Id Program.
* id.awk:                                Id Program.
* if-else statement:                     If Statement.
* igawk.sh:                              Igawk Program.
* IGNORECASE <1>:                        Case-sensitivity.
* IGNORECASE:                            User-modified.
* ignoring case:                         Case-sensitivity.
* implementation limits <1>:             Getline Summary.
* implementation limits:                 Redirection.
* in operator:                           Typing and Comparison.
* increment operators:                   Increment Ops.
* index:                                 String Functions.
* initialization, automatic:             More Complex.
* input:                                 Reading Files.
* input file, sample:                    Sample Data Files.
* input files, skipping:                 Nextfile Function.
* input pipeline:                        Getline/Pipe.
* input redirection:                     Getline/File.
* input, explicit:                       Getline.
* input, getline command:                Getline.
* input, multiple line records:          Multiple Line.
* input, standard:                       Read Terminal.
* installation, amiga:                   Amiga Installation.
* installation, atari:                   Atari Installation.
* installation, MS-DOS and OS/2:         PC Installation.
* installation, unix:                    Quick Installation.
* installation, vms:                     VMS Installation.
* int:                                   Numeric Functions.
* interaction, awk and other programs:   I/O Functions.
* interactive buffering vs. non-interactive: I/O Functions.
* interval expressions:                  Regexp Operators.
* inventory-shipped file:                Sample Data Files.
* invocation of gawk:                    Invoking Gawk.
* ISO 8601:                              Time Functions.
* ISO 8859-1 <1>:                        Case-sensitivity.
* ISO 8859-1:                            Glossary.
* ISO Latin-1 <1>:                       Glossary.
* ISO Latin-1:                           Case-sensitivity.
* Jaegermann, Michal <1>:                Acknowledgements.
* Jaegermann, Michal:                    Bugs.
* join:                                  Join Function.
* Kernighan, Brian <1>:                  Acknowledgements.
* Kernighan, Brian <2>:                  History.
* Kernighan, Brian <3>:                  Other Versions.
* Kernighan, Brian:                      BTL.
* known bugs:                            Known Bugs.
* labels.awk:                            Labels Program.
* language, awk:                         This Manual.
* language, data-driven:                 Getting Started.
* language, procedural:                  Getting Started.
* leftmost longest match <1>:            Leftmost Longest.
* leftmost longest match:                Multiple Line.
* length:                                String Functions.
* limitations <1>:                       Getline Summary.
* limitations:                           Redirection.
* line break:                            Statements/Lines.
* line continuation <1>:                 Conditional Exp.
* line continuation <2>:                 Boolean Ops.
* line continuation <3>:                 Print Examples.
* line continuation:                     Statements/Lines.
* Linux <1>:                             Manual History.
* Linux:                                 Atari Compiling.
* locale, definition of:                 Time Functions.
* log:                                   Numeric Functions.
* logical false:                         Truth Values.
* logical operations:                    Boolean Ops.
* logical true:                          Truth Values.
* login information:                     Passwd Functions.
* long options:                          Invoking Gawk.
* loop:                                  While Statement.
* loops, exiting:                        Break Statement.
* lvalue:                                Assignment Ops.
* mark parity:                           Ordinal Functions.
* match:                                 String Functions.
* matching ranges of lines:              Ranges.
* matching, leftmost longest <1>:        Leftmost Longest.
* matching, leftmost longest:            Multiple Line.
* mawk:                                  Other Versions.
* merging strings:                       Join Function.
* metacharacters:                        Regexp Operators.
* mistakes, common <1>:                  Computed Regexps.
* mistakes, common <2>:                  Basic Field Splitting.
* mistakes, common <3>:                  Typing and Comparison.
* mistakes, common:                      Print Examples.
* mktime:                                Mktime Function.
* modifiers (in format specifiers):      Format Modifiers.
* multi-dimensional subscripts:          Multi-dimensional.
* multiple line records:                 Multiple Line.
* multiple passes over data:             Other Arguments.
* multiple statements on one line:       Statements/Lines.
* multiplication:                        Arithmetic Ops.
* names, use of:                         Definition Syntax.
* namespace issues in awk:               Library Names.
* namespaces:                            Definition Syntax.
* NetBSD:                                Manual History.
* new awk:                               History.
* new awk vs. old awk:                   Names.
* newline:                               Statements/Lines.
* next file statement:                   Nextfile Statement.
* next statement:                        Next Statement.
* next, inside a user-defined function:  Next Statement.
* nextfile function:                     Nextfile Function.
* nextfile statement:                    Nextfile Statement.
* NF <1>:                                Auto-set.
* NF:                                    Fields.
* non-interactive buffering vs. interactive: I/O Functions.
* not operator:                          Boolean Ops.
* NR <1>:                                Records.
* NR:                                    Auto-set.
* null string <1>:                       Truth Values.
* null string <2>:                       Conversion.
* null string:                           Regexp Field Splitting.
* null string, as array subscript:       Uninitialized Subscripts.
* number of fields, NF:                  Fields.
* number of records, NR, FNR:            Records.
* numbers, used as subscripts:           Numeric Array Subscripts.
* numeric character values:              Ordinal Functions.
* numeric constant:                      Scalar Constants.
* numeric output format:                 OFMT.
* numeric string:                        Typing and Comparison.
* numeric value:                         Scalar Constants.
* obsolete features:                     Obsolete.
* obsolete options:                      Obsolete.
* OFMT <1>:                              OFMT.
* OFMT <2>:                              Conversion.
* OFMT:                                  User-modified.
* OFS <1>:                               Output Separators.
* OFS:                                   User-modified.
* old awk:                               History.
* old awk vs. new awk:                   Names.
* one-liners:                            One-liners.
* operations, logical:                   Boolean Ops.
* operator precedence:                   Precedence.
* operators, arithmetic:                 Arithmetic Ops.
* operators, assignment:                 Assignment Ops.
* operators, boolean:                    Boolean Ops.
* operators, decrement:                  Increment Ops.
* operators, increment:                  Increment Ops.
* operators, regexp matching:            Regexp Usage.
* operators, relational:                 Typing and Comparison.
* operators, short-circuit:              Boolean Ops.
* operators, string:                     Concatenation.
* operators, string-matching:            Regexp Usage.
* options, command line:                 Invoking Gawk.
* options, long:                         Invoking Gawk.
* or operator:                           Boolean Ops.
* ord:                                   Ordinal Functions.
* order of evaluation:                   Calling Built-in.
* ORS <1>:                               User-modified.
* ORS:                                   Output Separators.
* output:                                Printing.
* output field separator, OFS:           Output Separators.
* output format specifier, OFMT:         OFMT.
* output record separator, ORS:          Output Separators.
* output redirection:                    Redirection.
* output, buffering:                     I/O Functions.
* output, formatted:                     Printf.
* output, piping:                        Redirection.
* passes, multiple:                      Other Arguments.
* password file:                         Passwd Functions.
* path, search:                          AWKPATH Variable.
* pattern, BEGIN:                        BEGIN/END.
* pattern, default:                      Very Simple.
* pattern, definition of:                Patterns and Actions.
* pattern, empty:                        Empty.
* pattern, END:                          BEGIN/END.
* pattern, range:                        Ranges.
* pattern, regular expressions:          Regexp.
* patterns, types of:                    Kinds of Patterns.
* per file initialization and clean-up:  Filetrans Function.
* PERL:                                  Future Extensions.
* pipeline, input:                       Getline/Pipe.
* pipes for output:                      Redirection.
* portability issues <1>:                Portability Notes.
* portability issues <2>:                Delete.
* portability issues <3>:                Escape Sequences.
* portability issues <4>:                Statements/Lines.
* portability issues <5>:                Definition Syntax.
* portability issues <6>:                String Functions.
* portability issues <7>:                Close Files And Pipes.
* portability issues:                    I/O Functions.
* porting gawk:                          New Ports.
* POSIX awk <1>:                         Field Splitting Summary.
* POSIX awk <2>:                         User-modified.
* POSIX awk <3>:                         Assignment Ops.
* POSIX awk <4>:                         Precedence.
* POSIX awk <5>:                         Arithmetic Ops.
* POSIX awk <6>:                         Break Statement.
* POSIX awk <7>:                         Precedence.
* POSIX awk <8>:                         Next Statement.
* POSIX awk <9>:                         OFMT.
* POSIX awk <10>:                        Continue Statement.
* POSIX awk <11>:                        String Functions.
* POSIX awk <12>:                        Format Modifiers.
* POSIX awk <13>:                        Conversion.
* POSIX awk <14>:                        String Functions.
* POSIX awk <15>:                        Regexp Operators.
* POSIX awk <16>:                        Escape Sequences.
* POSIX awk <17>:                        Definition Syntax.
* POSIX awk:                             Regexp Operators.
* POSIX mode:                            Options.
* POSIXLY_CORRECT environment variable:  Options.
* precedence:                            Precedence.
* precedence, regexp operators:          Regexp Operators.
* print statement:                       Print.
* printf statement, syntax of:           Basic Printf.
* printf, format-control characters:     Control Letters.
* printf, modifiers:                     Format Modifiers.
* printing:                              Printing.
* procedural languages:                  Getting Started.
* process information:                   Special Files.
* processing arguments:                  Getopt Function.
* program file:                          Long.
* program, awk:                          This Manual.
* program, definition of:                Getting Started.
* program, self contained:               Executable Scripts.
* programs, documenting <1>:             Comments.
* programs, documenting:                 Library Names.
* pwcat program:                         Passwd Functions.
* pwcat.c:                               Passwd Functions.
* quotient:                              Arithmetic Ops.
* quoting, shell <1>:                    Read Terminal.
* quoting, shell:                        Long.
* Rakitzis, Byron:                       History Sorting.
* rand:                                  Numeric Functions.
* random numbers, seed of:               Numeric Functions.
* range pattern:                         Ranges.
* Rankin, Pat <1>:                       Acknowledgements.
* Rankin, Pat <2>:                       Bugs.
* Rankin, Pat:                           Assignment Ops.
* reading files:                         Reading Files.
* reading files, getline command:        Getline.
* reading files, multiple line records:  Multiple Line.
* record separator, RS:                  Records.
* record terminator, RT:                 Records.
* record, definition of:                 Records.
* records, multiple line:                Multiple Line.
* recursive function:                    Definition Syntax.
* redirection of input:                  Getline/File.
* redirection of output:                 Redirection.
* reference to array:                    Reference to Elements.
* regexp:                                Regexp.
* regexp as expression:                  Typing and Comparison.
* regexp comparison vs. string comparison: Typing and Comparison.
* regexp constant:                       Regexp Usage.
* regexp constants, difference between slashes and quotes: Computed Regexps.
* regexp match/non-match operators <1>:  Typing and Comparison.
* regexp match/non-match operators:      Regexp Usage.
* regexp matching operators:             Regexp Usage.
* regexp operators:                      Regexp Operators.
* regexp operators, GNU specific:        GNU Regexp Operators.
* regexp operators, precedence of:       Regexp Operators.
* regexp, anchors:                       Regexp Operators.
* regexp, dynamic:                       Computed Regexps.
* regexp, effect of command line options: GNU Regexp Operators.
* regular expression:                    Regexp.
* regular expression metacharacters:     Regexp Operators.
* regular expressions as field separators: Basic Field Splitting.
* regular expressions as patterns:       Regexp.
* regular expressions as record separators: Records.
* regular expressions, computed:         Computed Regexps.
* relational operators:                  Typing and Comparison.
* remainder:                             Arithmetic Ops.
* removing elements of arrays:           Delete.
* return statement:                      Return Statement.
* RFC-1036:                              Time Functions.
* RFC-822:                               Time Functions.
* RLENGTH <1>:                           Auto-set.
* RLENGTH:                               String Functions.
* Robbins, Miriam:                       Acknowledgements.
* Rommel, Kai Uwe <1>:                   Acknowledgements.
* Rommel, Kai Uwe:                       Bugs.
* round:                                 Round Function.
* rounding:                              Round Function.
* RS <1>:                                User-modified.
* RS:                                    Records.
* RSTART <1>:                            String Functions.
* RSTART:                                Auto-set.
* RT <1>:                                Records.
* RT <2>:                                Multiple Line.
* RT:                                    Auto-set.
* rule, definition of:                   Getting Started.
* running awk programs:                  Running gawk.
* running long programs:                 Long.
* rvalue:                                Assignment Ops.
* sample input file:                     Sample Data Files.
* scanning an array:                     Scanning an Array.
* script, definition of:                 Getting Started.
* scripts, executable:                   Executable Scripts.
* scripts, shell:                        Executable Scripts.
* search path:                           AWKPATH Variable.
* search path, for source files:         AWKPATH Variable.
* sed utility <1>:                       Simple Sed.
* sed utility <2>:                       Igawk Program.
* sed utility:                           Field Splitting Summary.
* seed for random numbers:               Numeric Functions.
* self contained programs:               Executable Scripts.
* shell quoting <1>:                     Read Terminal.
* shell quoting:                         Long.
* shell scripts:                         Executable Scripts.
* short-circuit operators:               Boolean Ops.
* side effect:                           Assignment Ops.
* simple stream editor:                  Simple Sed.
* sin:                                   Numeric Functions.
* single character fields:               Single Character Fields.
* single quotes, why needed:             One-shot.
* skipping input files:                  Nextfile Function.
* skipping lines between markers:        Ranges.
* sparse arrays:                         Array Intro.
* split:                                 String Functions.
* split utility:                         Split Program.
* split.awk:                             Split Program.
* sprintf:                               String Functions.
* sqrt:                                  Numeric Functions.
* srand:                                 Numeric Functions.
* Stallman, Richard <1>:                 Acknowledgements.
* Stallman, Richard:                     Manual History.
* standard error output:                 Special Files.
* standard input <1>:                    Reading Files.
* standard input <2>:                    Read Terminal.
* standard input:                        Special Files.
* standard output:                       Special Files.
* statement, compound:                   Statements.
* stream editor:                         Field Splitting Summary.
* stream editor, simple:                 Simple Sed.
* strftime:                              Time Functions.
* string comparison vs. regexp comparison: Typing and Comparison.
* string constants:                      Constants.
* string operators:                      Concatenation.
* string-matching operators:             Regexp Usage.
* sub:                                   String Functions.
* sub, third argument of:                String Functions.
* subscripts in arrays:                  Multi-dimensional.
* SUBSEP <1>:                            User-modified.
* SUBSEP:                                Multi-dimensional.
* substr:                                String Functions.
* subtraction:                           Arithmetic Ops.
* system:                                I/O Functions.
* systime:                               Time Functions.
* Tcl:                                   Library Names.
* tee utility:                           Tee Program.
* tee.awk:                               Tee Program.
* terminator, record:                    Records.
* time of day:                           Time Functions.
* timestamps:                            Time Functions.
* timestamps, converting from dates:     Mktime Function.
* timestamps, formatted:                 Gettimeofday Function.
* tolower:                               String Functions.
* toupper:                               String Functions.
* translate.awk:                         Translate Program.
* Trueman, David:                        Acknowledgements.
* truth values:                          Truth Values.
* type conversion:                       Conversion.
* types of variables <1>:                Typing and Comparison.
* types of variables:                    Assignment Ops.
* undefined functions:                   Function Caveats.
* undocumented features:                 Undocumented.
* uninitialized variables, as array subscripts: Uninitialized Subscripts.
* uniq utility:                          Uniq Program.
* uniq.awk:                              Uniq Program.
* use of comments:                       Comments.
* user information:                      Passwd Functions.
* user-defined functions:                User-defined.
* user-defined variables:                Using Variables.
* uses of awk:                           What Is Awk.
* using this book:                       This Manual.
* values of characters as numbers:       Ordinal Functions.
* variable shadowing:                    Definition Syntax.
* variable typing:                       Typing and Comparison.
* variables, user-defined:               Using Variables.
* Wall, Larry:                           Future Extensions.
* wc utility:                            Wc Program.
* wc.awk:                                Wc Program.
* Weinberger, Peter:                     History.
* when to use awk:                       When.
* while statement:                       While Statement.
* word boundaries, matching:             GNU Regexp Operators.
* word, regexp definition of:            GNU Regexp Operators.
* wordfreq.sh:                           Word Sorting.
* || operator:                           Boolean Ops.
* ~ operator <1>:                        Typing and Comparison.
* ~ operator <2>:                        Regexp Usage.
* ~ operator <3>:                        Case-sensitivity.
* ~ operator <4>:                        Computed Regexps.
* ~ operator:                            Regexp Constants.



Tag Table:
Node: Top1225
Node: Preface20713
Node: History22063
Node: Manual History23421
Node: Acknowledgements27011
Node: What Is Awk30636
Node: This Manual32290
Node: Conventions34932
Node: Sample Data Files36224
Node: Getting Started39307
Node: Names41615
Node: Running gawk43185
Node: One-shot44346
Node: Read Terminal45733
Node: Long47345
Node: Executable Scripts48738
Node: Comments50993
Node: Very Simple52153
Node: Two Rules54200
Node: More Complex56379
Node: Statements/Lines59495
Node: Other Features63768
Node: When64494
Node: One-liners66429
Node: Regexp69316
Node: Regexp Usage70642
Node: Escape Sequences72792
Node: Regexp Operators78244
Node: GNU Regexp Operators89277
Node: Case-sensitivity92981
Node: Leftmost Longest96097
Node: Computed Regexps97632
Node: Reading Files100289
Node: Records102057
Node: Fields108552
Node: Non-Constant Fields111621
Node: Changing Fields113908
Node: Field Separators118315
Node: Basic Field Splitting119017
Node: Regexp Field Splitting122246
Node: Single Character Fields124813
Node: Command Line Field Separator125882
Node: Field Splitting Summary129123
Node: Constant Size131142
Node: Multiple Line135179
Node: Getline140586
Node: Getline Intro141660
Node: Plain Getline142623
Node: Getline/Variable144887
Node: Getline/File146029
Node: Getline/Variable/File147339
Node: Getline/Pipe149313
Node: Getline/Variable/Pipe151403
Node: Getline Summary152521
Node: Printing154115
Node: Print155183
Node: Print Examples157283
Node: Output Separators159893
Node: OFMT161791
Node: Printf163193
Node: Basic Printf164097
Node: Control Letters165631
Node: Format Modifiers168319
Node: Printf Examples172468
Node: Redirection175246
Node: Special Files179885
Node: Close Files And Pipes185122
Node: Expressions189182
Node: Constants191378
Node: Scalar Constants191857
Node: Regexp Constants192862
Node: Using Constant Regexps193324
Node: Variables196525
Node: Using Variables197179
Node: Assignment Options198614
Node: Conversion200559
Node: Arithmetic Ops203741
Node: Concatenation205875
Node: Assignment Ops207230
Node: Increment Ops212826
Node: Truth Values215354
Node: Typing and Comparison216402
Node: Boolean Ops222302
Node: Conditional Exp225995
Node: Function Calls227672
Node: Precedence230552
Node: Patterns and Actions233940
Node: Pattern Overview234366
Node: Kinds of Patterns235141
Node: Regexp Patterns236278
Node: Expression Patterns236832
Node: Ranges240483
Node: BEGIN/END243202
Node: Using BEGIN/END243671
Node: I/O And BEGIN/END246634
Node: Empty248650
Node: Action Overview248949
Node: Statements251521
Node: If Statement253227
Node: While Statement254730
Node: Do Statement256760
Node: For Statement257862
Node: Break Statement261119
Node: Continue Statement263390
Node: Next Statement265386
Node: Nextfile Statement267883
Node: Exit Statement269797
Node: Built-in Variables271807
Node: User-modified272903
Node: Auto-set277756
Node: ARGC and ARGV284286
Node: Arrays286988
Node: Array Intro288451
Node: Reference to Elements292327
Node: Assigning Elements294277
Node: Array Example294779
Node: Scanning an Array296498
Node: Delete298828
Node: Numeric Array Subscripts300887
Node: Uninitialized Subscripts302793
Node: Multi-dimensional304433
Node: Multi-scanning307528
Node: Built-in309171
Node: Calling Built-in310160
Node: Numeric Functions312131
Node: String Functions315950
Node: I/O Functions334882
Node: Time Functions340467
Node: User-defined349320
Node: Definition Syntax350033
Node: Function Example354282
Node: Function Caveats356611
Node: Return Statement360481
Node: Invoking Gawk363136
Node: Options364371
Node: Other Arguments373201
Node: AWKPATH Variable375849
Node: Obsolete378598
Node: Undocumented379264
Node: Known Bugs379472
Node: Library Functions380604
Node: Portability Notes383022
Node: Nextfile Function384306
Node: Assert Function389182
Node: Round Function392521
Node: Ordinal Functions394165
Node: Join Function397617
Node: Mktime Function399669
Node: Gettimeofday Function411242
Node: Filetrans Function415254
Node: Getopt Function418931
Node: Passwd Functions430287
Node: Group Functions438620
Node: Library Names446517
Node: Sample Programs450442
Node: Clones450933
Node: Cut Program452027
Node: Egrep Program462056
Node: Id Program469719
Node: Split Program472990
Node: Tee Program476358
Node: Uniq Program479154
Node: Wc Program486699
Node: Miscellaneous Programs491117
Node: Dupword Program492027
Node: Alarm Program493698
Node: Translate Program498242
Node: Labels Program503054
Node: Word Sorting506598
Node: History Sorting510943
Node: Extract Program512912
Node: Simple Sed519869
Node: Igawk Program523213
Node: Language History536356
Node: V7/SVR3.1537589
Node: SVR4540244
Node: POSIX541766
Node: BTL543386
Node: POSIX/GNU544149
Node: Gawk Summary548581
Node: Command Line Summary549403
Node: Language Summary552379
Node: Variables/Fields554760
Node: Fields Summary555494
Node: Built-in Summary557280
Node: Arrays Summary560925
Node: Data Type Summary562218
Node: Rules Summary564044
Node: Pattern Summary565572
Node: Regexp Summary567757
Node: Actions Summary571140
Node: Operator Summary572972
Node: Control Flow Summary574199
Node: I/O Summary574756
Node: Printf Summary577745
Node: Special File Summary581083
Node: Built-in Functions Summary582761
Node: Time Functions Summary586761
Node: String Constants Summary587652
Node: Functions Summary588972
Node: Historical Features590033
Node: Installation591531
Node: Gawk Distribution592746
Node: Getting593249
Node: Extracting596195
Node: Distribution contents597582
Node: Unix Installation602496
Node: Quick Installation603005
Node: Configuration Philosophy604523
Node: VMS Installation606925
Node: VMS Compilation607464
Node: VMS Installation Details609068
Node: VMS Running610710
Node: VMS POSIX612300
Node: PC Installation613580
Node: Atari Installation616983
Node: Atari Compiling618167
Node: Atari Using620076
Node: Amiga Installation622923
Node: Bugs624041
Node: Other Versions627010
Node: Notes628584
Node: Compatibility Mode629191
Node: Additions630034
Node: Adding Code630732
Node: New Ports636071
Node: Future Extensions640239
Node: Improvements642487
Node: Glossary644355
Node: Copying661420
Node: Index680612

End Tag Table
