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 fol-
     lowed 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 over-
     flow.  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; otherwise, 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 arguments.

     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 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.

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

     o   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)

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

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

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

     o   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.

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

     o   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 arithmetic 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