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EXPR(1)                 FreeBSD General Commands Manual                EXPR(1)

NAME
     expr - evaluate expression

SYNOPSIS
     expr [-e] 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 (unless less strict parsing has been
     enabled for backwards compatibility with prior versions of expr in
     FreeBSD).

     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.

     The -e option enables backwards compatible behaviour as detailed below.

     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
             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.

     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.

   Compatibility with previous implementations
     Unless FreeBSD 4.x compatibility is enabled, this version of expr adheres
     to the POSIX Utility Syntax Guidelines, which require that a leading
     argument beginning with a minus sign be considered an option to the
     program.  The standard -- syntax may be used to prevent this
     interpretation.  However, many historic implementations of expr,
     including the one in previous versions of FreeBSD, will not permit this
     syntax.  See the examples below for portable ways to guarantee the
     correct interpretation.  The check_utility_compat(3) function (with a
     utility argument of ``expr'') is used to determine whether backwards
     compatibility mode should be enabled.  This feature is intended for use
     as a transition and debugging aid, when expr is used in complex scripts
     which cannot easily be recast to avoid the non-portable usage.  Enabling
     backwards compatibility mode also implicitly enables the -e option, since
     this matches the historic behavior of expr in FreeBSD. This option makes
     number parsing less strict and permits leading white space and an
     optional leading plus sign.  In addition, empty operands have an implied
     value of zero in numeric context.  For historical reasons, defining the
     environment variable EXPR_COMPAT also enables backwards compatibility
     mode.

ENVIRONMENT
     EXPR_COMPAT      If set, enables backwards compatibility mode.

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), check_utility_compat(3)

STANDARDS
     The expr utility conforms to IEEE Std 1003.1-2008 (``POSIX.1''), provided
     that backwards compatibility mode is not enabled.

     Backwards compatibility mode performs less strict checks of numeric
     arguments:

     +o   An empty operand string is interpreted as 0.

     +o   Leading white space and/or a plus sign before an otherwise valid
         positive numeric operand are allowed and will be ignored.

     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.

     The -e flag is an extension.

FreeBSD 11.0-PRERELEASE        February 25, 2012       FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | ENVIRONMENT | EXIT STATUS | EXAMPLES | SEE ALSO | STANDARDS

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