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EXPR(1)                   BSD 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 and operands must be passed as separate arguments.  Several of the operators have special
     meaning to command interpreters and must therefore be quoted appropriately.  All integer operands are
     interpreted in base 10 and must consist of only an optional leading minus sign followed by one or more
     digits.

     Arithmetic operations are performed using signed integer math with a range according to the C intmax_t
     data type (the largest signed integral type available).  All conversions and operations are checked for
     overflow.  Overflow results in program termination with an error message on stdout and with an error
     status.

     Operators are listed below in order of increasing precedence; all are left-associative.  Operators with
     equal precedence are grouped within symbols `{' and `}'.

     expr1 | expr2
             Return the evaluation of expr1 if it is neither an empty string nor zero; otherwise, returns
             the evaluation of expr2 if it is not an empty string; otherwise, returns zero.

     expr1 & expr2
             Return the evaluation of expr1 if neither expression evaluates to an empty string or zero; oth-erwise, otherwise,
             erwise, returns zero.

     expr1 {=, >, >=, <, <=, !=} expr2
             Return 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
             Return the results of addition or subtraction of integer-valued arguments.

     expr1 {*, /, %} expr2
             Return the results of multiplication, integer division, or remainder of integer-valued argu-
             ments.

     expr1 : expr2
             The ``:'' operator matches expr1 against expr2, which must be a basic 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 regu-lar regular
             lar expression subexpression the null string is returned; otherwise 0.

     Parentheses are used for grouping in the usual manner.

     The expr utility makes no lexical distinction between arguments which may be operators and arguments
     which may be operands.  An operand which is lexically identical to an operator will be considered a
     syntax error.  See the examples below for a work-around.

     The syntax of the expr command in general is historic and inconvenient.  New applications are advised
     to use shell arithmetic rather than expr.

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.

EXAMPLES
     •   The following example (in sh(1) syntax) adds one to the variable a:
               a=$(expr $a + 1)

     •   This will fail if the value of a is a negative number.  To protect negative values of a from being
         interpreted as options to the expr command, one might rearrange the expression:
               a=$(expr 1 + $a)

     •   More generally, parenthesize possibly-negative values:
               a=$(expr \( $a \) + 1)

     •   With shell arithmetic, no escaping is required:
               a=$((a + 1))

     •   This example prints the filename portion of a pathname stored in variable a.  Since a might repre-sent represent
         sent the path /, it is necessary to prevent it from being interpreted as the division operator.
         The // characters resolve this ambiguity.
               expr "//$a" : '.*/\(.*\)'

     •   With modern sh(1) syntax,
               "${a##*/}"
         expands to the same value.

     The following examples output the number of characters in variable a.  Again, if a might begin with a
     hyphen, it is necessary to prevent it from being interpreted as an option to expr, and a might be
     interpreted as an operator.

     •   To deal with all of this, a complicated command is required:
               expr \( "X$a" : ".*" \) - 1

     •   With modern sh(1) syntax, this can be done much more easily:
               ${#a}
         expands to the required number.

SEE ALSO
     sh(1), test(1)

STANDARDS
     The expr utility conforms to IEEE Std 1003.1-2008 (``POSIX.1'').

     The extended arithmetic range and overflow checks do not conflict with POSIX's requirement that arith-metic arithmetic
     metic be done using signed longs, since they only make a difference to the result in cases where using
     signed longs would give undefined behavior.

     According to the POSIX standard, the use of string arguments length, substr, index, or match produces
     undefined results. In this version of expr, these arguments are treated just as their respective string
     values.

BSD                            September 9, 2010                           BSD

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