EXPR(1L)

NAME
expr - evaluate expressions

SYNOPSIS
expr expression...

DESCRIPTION
This manual page documents the GNU version of expr. expr evaluates an expression and writes the result on its standard output. Each token of the expression must be a separate argument. Operands are either numbers or strings. Strings are not quoted for expr, though you may need to quote them to protect them from the shell. expr coerces anything appearing in an operand position to an integer or a string depending on the operation being applied to it.

The operators (in order of increasing precedence) are:

|   Yields its first argument if it is neither null nor 0, otherwise its second argument. This is the usual `or' operation.

&   Yields its first argument if neither argument is null or 0, otherwise 0.

<  <=  =  ==  !=  >=  >
    Compare their arguments and return 1 if the relation is true, 0 otherwise. (== is a synonym for =.) expr tries to coerce both arguments to numbers and do a numeric comparison; if it fails when trying to coerce either argument it then does a lexicographic comparison.

+  -
    Perform arithmetic operations. Both arguments are coerced to numbers; an error occurs if this cannot be done.

*  /  %
    Perform arithmetic operations (`%' is the remainder operation, as in C). Both arguments are coerced to numbers; an error occurs if this cannot be done.

:   Perform pattern matching. Its arguments are coerced to strings and the second one is considered to be a regular expression, with a `^' implicitly added at the beginning. The first argument is then matched against this regular expression. If the match succeeds and part of the string is enclosed in `\(' and `\)', that part is the value of the : expression; otherwise an integer whose value is the number of characters matched is returned. If the match fails, the : operator returns the null string if `\(' and `\)' are used, otherwise 0. Only one `\(' and `\)' pair can be used.

In addition, the following keywords are recognized:

match string regex
    An alternative way to do pattern matching. This is the same as ``string : regex''.

substr string position length
    Return the substring of string beginning at position with length at most length. If either position or length is negative or non-numeric, return a null string.

index string character-class
    Return the first position in string where the first character in character-class was found. If no character in character-class is found in string, return 0.

length string
    Return the length of string.

Parentheses are used for grouping in the usual manner. The keywords cannot be used as strings.

EXAMPLES

To add 1 to the shell variable a:

    a=`expr $a + 1`

To find the filename part of the pathname stored in variable a, which may or may not contain `/':

    expr $a : '.*/\(.*\)' '|' $a

Note the quoted shell metacharacters.

expr returns the following exit status:

0 if the expression is neither null nor 0,
1 if the expression is null or 0,
2 for invalid expressions.