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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.
Arithmetic operations are performed using signed integer math. If the -e
flag is specified, arithmetic uses the C intmax_t data type (the largest
integral type available), and expr will detect arithmetic overflow and
return an error indication. If a numeric operand is specified which is
so large as to overflow conversion to an integer, it is parsed as a
string instead. If -e is not specified, arithmetic operations and pars-
ing of integer arguments will overflow silently according to the rules of
the C standard, using the long data type.
Operators are listed below in order of increasing precedence; all are
left-associative. Operators with equal precedence are grouped within { }
symbols.
expr1 | expr2
Return the evaluation of expr1 if it is neither an empty string
nor zero; otherwise, returns the evaluation of expr2.
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 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.
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 pro-
gram. The standard -- syntax may be used to prevent this interpretation.
However, many historic implementations of expr, including the one in pre-
vious 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 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 compatibility mode also implicitly enables
the -e option, since this matches the historic behavior of expr in
FreeBSD. For historical reasons, defining the environment variable
EXPR_COMPAT also enables compatibility mode.
ENVIRONMENT
EXPR_COMPAT If set, enables 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 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" : '.*/\(.*\)'
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.
o If the expr command conforms to IEEE Std 1003.1-2001 (``POSIX.1''),
this is simple:
expr -- "$a" : ".*"
o For portability to older systems, however, a more complicated command
is required:
expr \( "X$a" : ".*" \) - 1
SEE ALSO
sh(1), test(1), check_utility_compat(3)
STANDARDS
The expr utility conforms to IEEE Std 1003.1-2001 (``POSIX.1''), provided
that compatibility mode is not enabled. The -e flag is an extension.
FreeBSD 6.1 July 12, 2004 FreeBSD 6.1
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