EXPR(1) | General Commands Manual | EXPR(1) |
expr
— evaluate
expression
expr |
operand ... |
The expr
utility evaluates the expression
consisting of the operand arguments and writes the
result on standard output.
Each operand is a separate argument 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:
expr2If 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 empty string is returned; otherwise 0.
Additionally, the following keywords are recognized:
Operator precedence (from highest to lowest):
The expr
utility exits with one of the
following values:
a=`expr $a + 1`
expr /$a :
'.*/\(.*\)'
expr $a :
'.*'
This implementation of expr
internally
uses 64 bit representation of integers and checks for over- and underflows.
It also treats “/” (the division mark) and option
“--” correctly depending upon context.
expr
on other systems (including
NetBSD up to and including NetBSD
1.5) might not be so graceful. Arithmetic results might be
arbitrarily limited on such systems, most commonly to 32 bit quantities.
This means such expr
can only process values between
-2147483648 and +2147483647.
On other systems, expr
might also not work
correctly for regular expressions where either side contains
“/” (a single forward slash), like this:
expr / : '.*/\(.*\)'
If this is the case, you might use “//” (a double forward slash) to avoid confusion with the division operator:
expr "//$a" : '.*/\(.*\)'
According to IEEE Std 1003.2
(“POSIX.2”), expr
has to
recognize the special option “--”, treat it as a delimiter to
mark the end of command line options, and ignore it. Some
expr
implementations do not recognize it at all;
others might ignore it even in cases where doing so results in syntax error.
There should be the same result for both following examples, but it might
not always be:
expr
handles
both cases correctly, you should not depend on this behavior for portability
reasons and avoid passing a bare “--” as the first argument.
The expr
utility conforms to
IEEE Std 1003.2 (“POSIX.2”). The
length keyword is an extension for compatibility with
GNU expr
.
An expr
utility first appeared in the
Programmer's Workbench (PWB/UNIX). A public domain version of
expr
written by Pace
Willisson ⟨pace@blitz.com⟩ appeared in
386BSD-0.1.
Initial implementation by Pace Willisson <pace@blitz.com> was largely rewritten by J.T. Conklin <jtc@NetBSD.org>. It was rewritten again for NetBSD 1.6 by Jaromir Dolecek <jdolecek@NetBSD.org>.
The empty string “” cannot be matched with the intuitive:
expr '' : '$'
The reason is that the returned number of matched characters (zero) is indistinguishable from a failed match, so this returns failure. To match the empty string, use something like:
expr x'' : 'x$'
June 28, 2025 | NetBSD 10.99 |