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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.

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):

  1. parentheses
  2. length
  3. “:”
  4. “*”, “/”, and “%”
  5. “+” and “-”
  6. compare operators
  7. “&”
  8. “|”

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).

  1. The following example adds one to variable “a”:
    a=`expr $a + 1`
  2. The following example returns zero, due to subtraction having higher precedence than the “&” operator:
    expr 1 '&' 1 - 1
  3. The following example returns the filename portion of a pathname stored in variable “a”:
    expr /$a : '.*/\(.*\)'
  4. The following example returns the number of characters in variable “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 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:

  1. expr -- : .
  2. 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.

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$'
August 23, 2016 NetBSD-9.2