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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 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
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.
HISTORY
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.
AUTHORS
Initial implementation by Pace Willisson <pace@blitz.com> was largely
rewritten by J.T. Conklin <jtc@FreeBSD.org>.
FreeBSD 14.2-RELEASE October 5, 2016 FreeBSD 14.2-RELEASE