expr(1) - NetBSD Manual Pages

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EXPR(1)                 NetBSD General Commands Manual                 EXPR(1)


NAME

     expr -- evaluate expression


SYNOPSIS

     expr expression


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 regu-
             lar expression subexpression ``\(...\)'', the string correspond-
             ing 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.   ``|''


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

     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 : '.*'


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 lim-
     ited 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 expres-
     sions 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 syn-
     tax 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.


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:

           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$'

NetBSD 9.0                      August 23, 2016                     NetBSD 9.0