expr (1)

PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

expr --- evaluate arguments as an expression

SYNOPSIS

expr operand...

DESCRIPTION

The expr utility shall evaluate an expression and write the result to standard output.

OPTIONS

None.

OPERANDS

The single expression evaluated by expr shall be formed from the operand operands, as described in the EXTENDED DESCRIPTION section. The application shall ensure that each of the expression operator symbols:

(  )  |  &  =  >  >=  <  <=  !=  +  -  *  /  %  :

and the symbols integer and string in the table are provided as separate arguments to expr.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of expr:

LANG

Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1-2008, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)

LC_ALL

If set to a non-empty string value, override the values of all the other internationalization variables.

LC_COLLATE

Determine the locale for the behavior of ranges, equivalence classes, and multi-character collating elements within regular expressions and by the string comparison operators.

LC_CTYPE

Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments) and the behavior of character classes within regular expressions.

LC_MESSAGES

Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.

NLSPATH

Determine the location of message catalogs for the processing of LC_MESSAGES.

ASYNCHRONOUS EVENTS

Default.

STDOUT

The expr utility shall evaluate the expression and write the result, followed by a <newline>, to standard output.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

The formation of the expression to be evaluated is shown in the following table. The symbols expr, expr1, and expr2 represent expressions formed from integer and string symbols and the expression operator symbols (all separate arguments) by recursive application of the constructs described in the table. The expressions are listed in order of increasing precedence, with equal-precedence operators grouped between horizontal lines. All of the operators shall be left-associative.

Matching Expression

The ':' matching operator shall compare the string resulting from the evaluation of expr1 with the regular expression pattern resulting from the evaluation of expr2. Regular expression syntax shall be that defined in the Base Definitions volume of POSIX.1-2008, Section 9.3, Basic Regular Expressions, except that all patterns are anchored to the beginning of the string (that is, only sequences starting at the first character of a string are matched by the regular expression) and, therefore, it is unspecified whether '^' is a special character in that context. Usually, the matching operator shall return a string representing the number of characters matched ('0' on failure). Alternatively, if the pattern contains at least one regular expression subexpression dq[\(...\)]dq, the string matched by the back-reference expression dq\1dq shall be returned. If the back-reference expression dq\1dq does not match, then the null string shall be returned.

String Operand

A string argument is an argument that cannot be identified as an integer argument or as one of the expression operator symbols shown in the OPERANDS section. The use of string arguments length, substr, index, or match produces unspecified results.

EXIT STATUS

The following exit values shall be returned:

 0

The expression evaluates to neither null nor zero.

 1

The expression evaluates to null or zero.

 2

Invalid expression.

>2

An error occurred.

CONSEQUENCES OF ERRORS

Default.

The following sections are informative.

APPLICATION USAGE

After argument processing by the shell, expr is not required to be able to tell the difference between an operator and an operand except by the value. If dq$adq is '=', the command:

expr $a = '='

looks like:

expr = = =

as the arguments are passed to expr (and they all may be taken as the '=' operator). The following works reliably:

expr X$a = X=

Also note that this volume of POSIX.1-2008 permits implementations to extend utilities. The expr utility permits the integer arguments to be preceded with a unary minus. This means that an integer argument could look like an option. Therefore, the conforming application must employ the dq--dq construct of Guideline 10 of the Base Definitions volume of POSIX.1-2008, Section 12.2, Utility Syntax Guidelines to protect its operands if there is any chance the first operand might be a negative integer (or any string with a leading minus).

EXAMPLES

The expr utility has a rather difficult syntax:

*

Many of the operators are also shell control operators or reserved words, so they have to be escaped on the command line.

*

Each part of the expression is composed of separate arguments, so liberal usage of <blank> characters is required. For example:

In many cases, the arithmetic and string features provided as part of
the shell command language are easier to use than their equivalents in
Newly written scripts should avoid
and
The following command:
a=$(expr $a + 1)
adds 1 to the variable
The following command, for
equal to either
or just
expr $a : '.*/\(.*\)' \| $a
returns the last segment of a pathname (that is,
Applications should avoid the character
used alone as an argument;
may interpret it as the division operator.
The following command:
expr "//$a" : '.*/\(.*\)'
is a better representation of the previous example. The addition of
the
characters eliminates any ambiguity about the division operator and
simplifies the whole expression. Also note that pathnames may contain
characters contained in the
variable and should be quoted to avoid having
expand into multiple arguments.
The following command:
expr "$VAR" : '.*'
returns the number of characters in
In an early proposal, EREs were used in the matching expression syntax.
This was changed to BREs to avoid breaking historical applications.
The use of a leading
<circumflex>
in the BRE is unspecified because many historical implementations have
treated it as a special character, despite their system documentation. For
example:
expr foo : ^foo expr ^foo : ^foo
return 3 and 0, respectively, on those systems; their documentation
would imply the reverse. Thus, the anchoring condition is left
unspecified to avoid breaking historical scripts relying on this
undocumented feature.
None.
The Base Definitions volume of POSIX.1-2008,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
www.unix.org/online.html .
Any typographical or formatting errors that appear
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to have been introduced during the conversion of the source files to
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www.kernel.org/doc/man-pages/reporting_bugs.html .