NAME

expr —

evaluate expression

SYNOPSIS

expr expression

DESCRIPTION

The expr utility evaluates expression and writes the result on standard output. All operators are separate arguments to the expr utility. Characters special to the command interpreter must be escaped.

Operators are listed below in order of increasing precedence. Operators with equal precedence are grouped within { } symbols.

expr1 | expr2

Returns the evaluation of expr1 if it is neither an empty string nor zero; otherwise, returns the evaluation of expr2.

expr1 & expr2

Returns the evaluation of expr1 if neither expression evaluates to an empty string or zero; otherwise, returns zero.

expr1 {=, >, >=, <, <=, !=} expr2

Returns the results of integer comparison if both arguments are decimal integers; otherwise, returns the results of string comparison using the locale-specific collation sequence. The result of each comparison is 1 if the specified relation is true, or 0 if the relation is false.

expr1 {+, -} expr2

Returns the results of addition or subtraction of decimal integer-valued arguments.

expr1 {*, /, %} expr2

Returns the results of multiplication, integer division, or remainder of decimal integer-valued arguments.

expr1 : expr2

The ‘:’ operator matches expr1 against expr2, which must be a basic regular expression. The regular expression is anchored to the beginning of the string with an implicit ‘^’.

If the match succeeds and the pattern contains at least one regular expression subexpression “\(...\)”, the string corresponding to “\1” is returned; otherwise, the matching operator returns the number of characters matched. If the match fails and the pattern contains a regular expression subexpression the null string is returned; otherwise, returns 0.

Note: the empty string cannot be matched using

expr '' : '$'

This is because the returned number of matched characters (zero) is indistinguishable from a failed match, so expr returns failure (0). To match the empty string, use a structure such as:

expr X'' : 'X$'

Parentheses are used for grouping in the usual manner.

EXIT STATUS

The expr utility exits with one of the following values:

0: The expression is neither an empty string nor 0.
1: The expression is an empty string or 0.
2: The expression is invalid.
>2: An error occurred (such as memory allocation failure).

EXAMPLES

Add 1 to the variable a:

$ a=`expr $a + 1`

Return the filename portion of a pathname stored in variable a. The ‘//’ characters act to eliminate ambiguity with the division operator:

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

Return the number of characters in variable a:

$ expr $a : '.*'

STANDARDS

The expr utility is compliant with the IEEE Std 1003.1-2008 (“POSIX.1”) specification.

HISTORY

The expr utility first appeared in the Programmer's Workbench (PWB/UNIX) and has supported regular expressions since Version 7 AT&T UNIX. It was rewritten from scratch for 386BSD-0.1 and again for NetBSD 1.1.

AUTHORS

The first free version was written by Pace Willisson in 1992. This version was written by John T. Conklin in 1994.