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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
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.
EXPR_COMPAT | |
If set, enables backwards compatibility mode. | |
0 | the expression is neither an empty string nor 0. |
1 | the expression is an empty string or 0. |
2 | the expression is invalid. |
a=$(expr $a + 1)
a=$(expr 1 + $a)
a=$(expr \( $a \) + 1)
a=$((a + 1))
expr q//$aq : '.*/\(.*\)'
q${a##*/}q
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.
expr \( qX$aq : q.*q \) - 1
${#a}
expands to the required number.
Backwards compatibility mode performs less strict checks of numeric arguments:
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
EXPR (1) | October 5, 2016 |
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Please direct any comments about this manual page service to Ben Bullock.
“ | Unix is the answer, but only if you phrase the question very carefully. | ” |
— Belinda Asbell |