EXPR(1) | General Commands Manual | EXPR(1) |
expr
— evaluate
expression
expr |
expression |
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.
&
expr2{=, >, >=, <, <=,
!=}
expr2{+, -}
expr2{*, /, %}
expr2:
expr2:
’ 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.
The expr
utility exits with one of the
following values:
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 : '.*'
The expr
utility is compliant with the
IEEE Std 1003.1-2008 (“POSIX.1”)
specification.
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.
The first free version was written by Pace Willisson in 1992. This version was written by John T. Conklin in 1994.
August 16, 2017 | OpenBSD-current |