EXPR(1) | General Commands Manual | EXPR(1) |
expr
—
expr |
expression |
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:
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 null string is returned; otherwise 0.
Additionally, the following keywords are recognized:
Operator precedence (from highest to lowest):
expr
utility exits with one of the following values:
a=`expr $a + 1`
expr 1 '&' 1 - 1
expr /$a :
'.*/\(.*\)'
expr $a :
'.*'
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 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 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.
expr
utility conforms to IEEE Std
1003.2 (“POSIX.2”). The length
keyword is an extension for compatibility with GNU
expr
.
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.
expr '' : '$'
The reason is that the returned number of matched characters (zero) is indistinguishable from a failed match, so this returns failure. To match the empty string, use something like:
expr x'' : 'x$'
August 23, 2016 | NetBSD 9.4 |