NAME
expr
- evaluate expression
SYNOPSIS
expr
expression
DESCRIPTION
The
expr
utility evaluates
expression
and writes the result on standard output.
All operators are separate arguments to the
expr
utility.
Characters special to the command interpreter must be escaped.
Operators are listed below in order of increasing precedence.
Operators with equal precedence are grouped within { } symbols.
- expr1
|
expr2 -
Returns the evaluation of
expr1
if it is neither an empty string nor zero;
otherwise, returns the evaluation of
expr2.
- expr1
&
expr2 -
Returns the evaluation of
expr1
if neither expression evaluates to an empty string or zero;
otherwise, returns zero.
- expr1
{=, >, , <, , !=}
expr2 -
Returns 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 -
Returns the results of addition or subtraction of integer-valued arguments.
- expr1
{*, /, %}
expr2 -
Returns the results of multiplication, integer division, or remainder of integer-valued arguments.
- expr1
: expr2 -
The
``:''
operator matches
expr1
against
expr2,
which must be a 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.
- ( expr
- )
Parentheses are used for grouping in the usual manner.
Additionally, the following keywords are recognized:
- length expr
-
Returns the length of the specified string in bytes.
Operator precedence (from highest to lowest):
-
parentheses
-
length
-
``:''
-
``*'',
``/'',
and
``%''
-
``+''
and
``-''
-
compare operators
-
``&''
-
``|''
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.
- >2
-
an error occurred (such as memory allocation failure).
EXAMPLES
-
The following example adds one to the variable a.
a=`expr
$a
+
1`
-
The following example returns zero, due to deduction having higher precedence
than '&' operator.
expr
1
'&'
1
-
1
-
The following example returns the filename portion of a pathname stored
in variable a.
expr
/$a
: '.*/\(.*\)'
-
The following example returns the number of characters in variable a.
expr
$a
: '.*'
STANDARDS
The
expr
utility conforms to
IEEE Std 1003.2 (``POSIX.2'') .
The
length
keyword is an extension for compatibility with GNU
.
AUTHORS
Original implementation was written by
J.T. Conklin
<jtc@NetBSD.org>.
It was rewritten for
NetBSD1.6
by
Jaromir Dolecek
<jdolecek@NetBSD.org>.
NOTES
The empty string
``''
cannot be matched with the intuitive:
-
expr '' : '$'
The reason is that the returned number of matched characters (zero)
is indistinguishable from a failed match, so this returns failure.
To match the empty string, use something like:
-
expr x'' : 'x$'
COMPATIBILITY
This implementation of
expr
internally uses 64 bit representation of integers and checks for
over- and underflows.
It also treats / (division mark) and
option '--' correctly depending upon context.
expr
on other systems (including
NetBSD
up to and including
NetBSD1.5)
might not be so graceful.
Arithmetic results might be arbitrarily
limited on such systems, most commonly to 32 bit quantities.
This means such
expr
can only process values between -2147483648 and +2147483647.
On other systems,
expr
might also not work correctly for regular expressions where
either side contains single forward slash, like this:
-
expr / : '.*/\(.*\)'
If this is the case, you might use // (double forward slash)
to avoid confusion with the division operator:
-
expr "//$a" : '.*/\(.*\)'
According to
IEEE Std 1003.2 (``POSIX.2'') ,
expr
has to recognize special option '--', treat it as an end of command
line options and ignore it.
Some
expr
implementations don't recognize it at all, others
might ignore it even in cases where doing so results in syntax
error.
There should be same result for both following examples,
but it might not always be:
-
expr -- : .
-
expr -- -- : .
Although
NetBSD
expr
handles both cases correctly, you should not depend on this behavior
for portability reasons and avoid passing bare '--' as first
argument.