expr(1) BSD General Commands Manual expr(1)

## NAME

expr-- evaluate expression

## SYNOPSIS

exprexpression

## DESCRIPTION

Theexprutility evaluatesexpressionand 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 Cintmax_tdata 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|expr2Return the evaluation ofexpr1if it is neither an empty string nor zero; otherwise, returns the evaluation ofexpr2if it is not an empty string; otherwise, returns zero.expr1&expr2Return the evaluation ofexpr1if neither expression evaluates to an empty string or zero; otherwise, returns zero.expr1{=, >, >=, <, <=, !=}expr2Return 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{+, -}expr2Return the results of addition or subtraction of integer-valued arguments.expr1{*, /, %}expr2Return the results of multiplication, integer division, or remainder of integer-valued arguments.expr1:expr2The ``:'' operator matchesexpr1againstexpr2, 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. Theexprutility 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 theexprcommand in general is historic and inconvenient. New applications are advised to use shell arithmetic rather thanexpr.

## EXIT STATUS

Theexprutility 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

oThe following example (insh(1)syntax) adds one to the variablea: a=$(expr $a + 1)oThis will fail if the value ofais a negative number. To protect negative values ofafrom being interpreted as options to theexprcommand, one might rearrange the expression: a=$(expr 1 + $a)oMore generally, parenthesize possibly-negative values: a=$(expr \( $a \) + 1)oWith shell arithmetic, no escaping is required: a=$((a + 1))oThis example prints the filename portion of a pathname stored in variablea. Sinceamight represent the path/, it is necessary to prevent it from being interpreted as the division operator. The // characters resolve this ambiguity. expr "//$a" : '.*/\(.*\)'oWith modernsh(1)syntax, "${a##*/}" expands to the same value. The following examples output the number of characters in variablea. Again, ifamight begin with a hyphen, it is necessary to prevent it from being interpreted as an option toexpr, andamight be interpreted as an operator.oTo deal with all of this, a complicated command is required: expr \( "X$a" : ".*" \) - 1oWith modernsh(1)syntax, this can be done much more easily: ${#a} expands to the required number.

## SEE ALSO

sh(1),test(1)

## STANDARDS

Theexprutility conforms to . 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 argumentslength,substr,index, ormatchproduces undefined results. In this version ofexpr, these arguments are treated just as their respective string values. BSD September 9, 2010 BSD

Mac OS X 10.8 - Generated Tue Aug 21 09:09:57 CDT 2012