 |
[Home page] | [Basic HTML] | [Forms] | [CGI scripts] | [New HTML] | |
| |
[Home page] | [Basic HTML] | [Forms] | [CGI scripts] | [New HTML] | |
CGI scripts
Example of calling a CGI script file
Decoding data sent to a CGI script
Script to record users of web page
Post vs. get
Check list
|
CGI scripts
A CGI script file is written in a programming language which can be either:
Examples, of languages used include:
The CGI script is executed when an anchor tag <A ... > or an image tag <IMG ...> refers to the CGI script file rather than a normal file. The determination of whether this is a CGI script file or just an HTML file is made on the physical placement of the file on the server. Usually this placement is in the web servers cgi-bin directory. However the exact location of this directory on the server machine is determined by the web administrator. This placement and control of the cgi-bin directory is determined by the web administrator to prevent security problems, that could occur if arbitrary programs where allowed to be executed by anybody accessing the machine.
Call of a CGI script file
An anchor tag to execute the CGI script dynamic_page on the server www.mc.com is:
| <A HREF="http://www.mc.com/cgi-bin/dynamic_page">Dynamic page</A> |
When the web server process a request to fetch a file, if the requested file is in the servers nominated cgi-bin directory then as long as this file is marked as being executable the script will be run on the server. If the file is not executable then an error will be reported.
The script eventually returns an HTML page or image to be displayed as the result of its execution. When a CGI script file executes it may access environment variables to discover additional information about the process that it is to perform. The first line of the returned data must be:
| Type of returned data | Text |
|---|---|
| An HTML page | Content-type: text/html |
| A gif image | Content-type: image/gif |
A simple CGI script on a unix based system to return a list of the current users who are logged onto that system is:
#!/bin/sh echo Content-type: text/html echo echo echo "<HTML>" echo "<HEAD>" echo "</HEAD>" echo "<BODY>" echo "<H2>Users logged on the server are:</H2>" echo "<PRE>" who echo "</PRE>" echo "</BODY>" echo "</HTML>" |
Remember:
On a Unix system:
|
| Environment variable | Contains |
|---|---|
| QUERY_STRING | Data sent to the CGI script, by its caller. This may be the output from a form, or other dynamically or statically generated data. |
| REMOTE_ADDR | The Internet address of the host machine making the request. |
A C++ program mas_env.cpp when run prints many of the environment variables available to a CGI script.
CGI scripts can be written in any language. For example, a CGI script to return the contents of the environment variable QUERY_STRING can be written in Ada 95.
Decoding data sent to a CGI script
To pass information explicitly to the environment variable QUERY_STRING a modified form of an anchor tag is used. In this modified anchor tag, the data to be sent to the environment variable QUERY_STRING is appended after the URL which denotes the CGI script. The character ? is used to separate the URL denoting the CGI script and the data that is to be sent to the script. For example:
| <A HREF="/cgi-bin/script?name=Your+name&action=find"> Link </A> |
The data "name=Your+name&action=find" is placed in the environment variable QUERY_STRING and the cgi script script executed.
A class written in C++ composed of the specification parse.h and implementation parse.cpp is used to extract the individual components in the QUERY_STRING . The header file t99_type.h contains definitions for C++ features not implemented in some compilers. The members of this class are:
| Method | Responsibility |
|---|---|
| Parse | Set the string that will be parsed. |
| set | Set a different string to be parsed. |
| get_item | Return the string associated with the keyword passed as a parameter. If no data return NULL. |
| get_item_n | Return the string associated with the keyword passed as a parameter. If no data then return the null string. |
When using the member functions get_item and get_item_n the optional second parameter specifies which occurrence of the string associated with a keyword to return. This is to allow the recovery of information attached to identical keywords. In addition the returned string will have had the following substitutions made on it.
For example, if the QUERY_STRING contained:
| tag=one&name=mike&action=%2B10%25&tag=two&log=~mas/log&tag=three |
Then the following program when compiled and run:
enum bool { false, true };
#include <iostream.h>
#include <stdlib.h>
#include "parse.h"
#include "parse.cpp"
void main()
{
char *query_str = getenv("QUERY_STRING");
Parse list( query_str );
cout << "name = " << list.get_item_n( "name" ) << "\n";
cout << "action= " << list.get_item_n( "action" ) << "\n";
cout << "log = " << list.get_item_n( "log", 1, true ) << "\n";
for ( int i=1; i<=4; i++ )
{
cout << "tag (" << i << ") = ";
cout << list.get_item_n( "tag" , i ) << "\n";
}
}
|
would produce the following output:
name = mike action= +10% log = /usr/staff/mas/log tag (1) = one tag (2) = two tag (3) = three tag (4) = |
Script to record users of web page
| Formatted text | HTML markup required |
|---|---|
|
<IMG SRC="/cgi-bin/mas_rec?page=HTML&file=log&img=dot.gif"
ALT="Record not made">
|
The CGI script mas_rec written in C++ is sent the following information:
| Parameter name | Specifies |
|---|---|
| file | The name of the file in which the usage information will be appended. |
| page | A name for the page that will recorded in the log. |
| img | The image that will be loaded. |
Of course for this to work, the viewer of the page must be viewing and hence loading images. Several reasons why images may not be loaded include:
Post vs. Get
So far the method used to send information to the CGI script has been GET. When the method GET is used the data sent is placed in the environment variable QUERY_STRING for the CGI script to process.
An alternative method is to use POST. When the method POST is used the data is sent by a separate stream and becomes the standard input to the CGI script. The method used is specified on the <FORM ..> tag using the attribute METHOD="get" or METHOD="post". The default method is GET.
For example:
| Generated form | HTML markup required |
|---|---|
<FORM METHOD="get"
ACTION="http://host/cgi-bin/mas_form">
<INPUT TYPE="text" NAME="name"
SIZE=20 VALUE="Try it (get)">
</FORM>
| |
<FORM METHOD="post"
ACTION="http://host/cgi-bin/mas_form">
<INPUT TYPE="text" NAME="name"
SIZE=20 VALUE="Try it (post)">
</FORM>
|
When using the POST attribute, the following environment variables are set:
| Environment variable | Contains |
|---|---|
| CONTENT_LENGTH | The length of the data sent via the standard input to the CGI program. |
| CONTENT_TYPE | The MIME type of the data. |
A simple script to record in a log file data sent by a user is:
#!/bin/sh echo Content-type: text/html echo echo echo "<HTML>" echo "<HEAD>" echo "</HEAD>" echo "<BODY>" echo "<H2>Data recorded</H2>" echo Use the back arrow on the browser echo to return to the original web page echo "</BODY>" echo "</HTML>" cat >> /home/snowwhite/staff/mas/log echo >> /home/snowwhite/staff/mas/log |
Remember:
On a Unix system:
|
An example of its use is shown below:
| Generated form | HTML markup required |
|---|---|
<FORM METHOD="post"
ACTION="http:/machine/cgi-bin/mas_cgi1">
<INPUT TYPE="text" NAME="name"
SIZE=20 VALUE="fill in">
|
Check list
| To my global Home page | Comments, suggestions etc. to: M.A.Smith@brighton.ac.uk |
|
List of: CGI Environment variables |