EXPR(1) | General Commands Manual | EXPR(1) |
expr
— evaluate
expression
expr |
expression |
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
‘}
’.
|
expr2&
expr2:
expr2:
” 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
.
The expr
utility exits with one of the
following values:
a=$(expr $a + 1)
expr
command, one
might rearrange the expression:
a=$(expr 1 + $a)
a=$(expr \( $a \) +
1)
a=$((a + 1))
//
characters resolve this ambiguity.
expr "//$a" :
'.*/\(.*\)'
"${a##*/}"
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 \( "X$a" :
".*" \) - 1
${#a}
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.
An expr
utility first appeared in the
Programmer's Workbench (PWB/UNIX). A public domain version of
expr
written by Pace
Willisson
<pace@blitz.com>
appeared in 386BSD-0.1.
Initial implementation by Pace Willisson <pace@blitz.com> was largely rewritten by J.T. Conklin <jtc@FreeBSD.org>.
October 5, 2016 | macOS 14.6 |