EXPR(C) UNIX System V
Name
expr - evaluates arguments as an expression
Syntax
expr arguments
Description
The arguments are taken as an expression. After evaluation,
the result is written on the standard output. Terms of the
expression must be separated by blanks. Characters special
to the shell must be escaped. Note that zero is returned to
indicate a zero value, rather than the null string. Strings
containing blanks or other special characters should be
quoted. Integer-valued arguments may be preceded by a unary
minus sign. Internally, integers are treated as 32-bit, 2's
complement numbers.
The operators and keywords are listed below. Expressions
should be quoted, since many of the characters that have
special meaning in the shell also have special meaning in
expr. The list is in order of increasing precedence, with
equal precedence operators grouped within braces ({ and }).
expr | expr
Returns the first expr if it is neither null nor 0,
otherwise returns the second expr.
expr & expr
Returns the first expr if neither expr is null nor 0,
otherwise returns 0.
expr { =, >, >=, <, <=, != } expr
Returns the result of an integer comparison if both
arguments are integers, otherwise returns the result of
a lexical comparison, as defined by the locale.
expr { +, - } expr
Addition or subtraction of integer-valued arguments.
expr { *, /, % } expr
Multiplication, division, or remainder of the integer-
valued arguments.
expr : expr
The matching operator : compares the first argument
with the second argument which must be a regular
expression; regular expression syntax is the same as
that of ed(C), except that all patterns are
``anchored'' (i.e., begin with a caret (^)) and
therefore the caret is not a special character in that
context. (Note that in the shell, the caret has the
same meaning as the pipe symbol (|).) Normally the
matching operator returns the number of characters
matched (zero on failure). Alternatively, the \(...\)
pattern symbols can be used to return a portion of the
first argument.
Examples
1. a=`expr $a + 1`
Adds 1 to the shell variable a.
2.
# 'For $a ending in "/file"
expr $a : '.*/\(.*\)'
Returns the last segment of a pathname (i.e.,
file). Watch out for the slash alone as an
argument: expr will take it as the division
operator (see Notes on the next page).
3.
expr $VAR : '.*'
Returns the number of characters in
$VAR.
See Also
coltbl(M), ed(C), locale(M), sh(C)
Diagnostics
As a side effect of expression evaluation, expr returns the
following exit values:
0 If the expression is neither null nor zero
1 If the expression is null or zero
2 For invalid expressions
Other diagnostics include:
syntax error For operator/operand errors, including unset
variables
nonnumeric argument
If arithmetic is attempted on a nonnumeric
string
Notes
After argument processing by the shell, expr cannot tell the
difference between an operator and an operand except by the
value. If $a is an equals sign (=), the command:
expr $a = '='
looks like:
expr = = =
The arguments are passed to expr and will all be taken as
the = operator. The following permits comparing equals
signs:
expr X$a = X=
Standards Conformance
expr is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 2/15/90) EXPR(C)