EXPR(1) (Essential Utilities) EXPR(1)
NAME
expr - evaluate arguments as an expression
SYNOPSIS
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 0 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, 2s
complement numbers.
The operators and keywords are listed below. Characters
that need to be escaped are preceded by \. The list is in
order of increasing precedence, with equal precedence
operators grouped within {} symbols.
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 or 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.
expr { +, - } expr
addition or subtraction of integer-valued arguments.
expr { \*, /, % } expr
multiplication, division, or remainder of the integer-
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EXPR(1) (Essential Utilities) EXPR(1)
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(1), except that all patterns are
``anchored'' (i.e., begin with ^) and, therefore, ^ is
not a special character, in that context. Normally,
the matching operator returns the number of characters
matched (0 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 equal to either "/usr/abc/file" or just
"file"'
expr $a : '.*/\(.*\)' \| $a
returns the last segment of a path name (i.e.,
file). Watch out for / alone as an argument:
expr will take it as the division operator (see
BUGS below).
3. # A better representation of example 2.
expr //$a : '.*/\(.*\)'
The addition of the // characters eliminates any
ambiguity about the division operator and
simplifies the whole expression.
4. expr $VAR : '.*'
returns the number of characters in $VAR.
SEE ALSO
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EXPR(1) (Essential Utilities) EXPR(1)
ed(1), sh(1).
DIAGNOSTICS
As a side effect of expression evaluation, expr returns the
following exit values:
0 if the expression is neither null nor 0
1 if the expression is null or 0
2 for invalid expressions.
syntax error for operator/operand errors
non-numeric argument
if arithmetic is attempted on such a string
BUGS
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 =, the command:
expr $a = '='
looks like:
expr = = =
as the arguments are passed to expr (and they will all be
taken as the = operator). The following works:
expr X$a = X=
Page 3 May 1989