NAME
expr —
evaluate expression
SYNOPSIS
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 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
integer-valued arguments.
-
-
- expr1
{*, /, %}
expr2
- Returns the results of multiplication, integer division, or
remainder of integer-valued arguments.
-
-
- expr1
:
expr2
- The “:” operator matches
expr1 against expr2, which
must be a 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 0.
-
-
- ( expr
)
- Parentheses are used for grouping in the usual manner.
Additionally, the following keywords are recognized:
-
-
- length
expr
- Returns the length of the specified string in bytes.
Operator precedence (from highest to lowest):
- parentheses
- length
- “:”
- “*”,
“/”, and “%”
- “+” and
“-”
- compare operators
- “&”
- “|”
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
- The following example adds one to variable
“a”:
a=`expr $a + 1`
- The following example returns zero, due to subtraction
having higher precedence than the “&” operator:
expr 1 '&' 1 - 1
- The following example returns the filename portion of a
pathname stored in variable “a”:
expr /$a :
'.*/\(.*\)'
- The following example returns the number of characters in
variable “a”:
expr $a :
'.*'
COMPATIBILITY
This implementation of
expr internally uses 64 bit
representation of integers and checks for over- and underflows. It also treats
“/” (the division mark) and option “--” correctly
depending upon context.
expr on other systems (including
NetBSD up to and including
NetBSD
1.5) might not be so graceful. Arithmetic results might be arbitrarily
limited on such systems, most commonly to 32 bit quantities. This means such
expr can only process values between -2147483648 and
+2147483647.
On other systems,
expr might also not work correctly for
regular expressions where either side contains “/” (a single
forward slash), like this:
If this is the case, you might use “//” (a double forward slash) to
avoid confusion with the division operator:
expr "//$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 -- : .
- expr -- -- : .
Although
NetBSD expr handles both
cases correctly, you should not depend on this behavior for portability
reasons and avoid passing a bare “--” as the first argument.
STANDARDS
The
expr utility conforms to
IEEE Std 1003.2
(“POSIX.2”). The
length keyword is an
extension for compatibility with GNU
expr.
HISTORY
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.
AUTHORS
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>.
NOTES
The empty string “” cannot be matched with the intuitive:
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: