Manpages - expr.1p

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.

expr — evaluate arguments as an expression

  expr operand...

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

None.

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.

Not used.

None.

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.1‐2017, 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.

Default.

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

The standard error shall be used only for diagnostic messages.

None.

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 decreasing precedence, with equal-precedence operators grouped between horizontal lines. All of the operators shall be left-associative.

TABLE

as a string.

The use of string arguments length, substr, index, or match produces unspecified results.

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.

Default.

The following sections are informative.

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:

TABLE

In many cases, the arithmetic and string features provided as part of the shell command language are easier to use than their equivalents in expr. Newly written scripts should avoid expr in favor of the new features within the shell; see Section 2.5, Parameters and Variables and Section 2.6.4, Arithmetic Expansion.

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 “$a” 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.1‐2017 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 “–” construct of Guideline 10 of the Base Definitions volume of POSIX.1‐2017, 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).

For testing string equality the test utility is preferred over expr, as it is usually implemented as a shell built-in. However, the functionality is not quite the same because the expr = and != operators check whether strings collate equally, whereas test checks whether they are identical. Therefore, they can produce different results in locales where the collation sequence does not have a total ordering of all characters (see the Base Definitions volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE).

The following command:

    a=$(expr "$a" + 1)

adds 1 to the variable a.

The following command, for “$a” equal to either /usr/abc/file or just file:

    expr $a : '.*/\(.*\)' \| $a

returns the last segment of a pathname (that is, file). Applications should avoid the character ’/’ used alone as an argument; expr 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 IFS variable and should be quoted to avoid having “$a” expand into multiple arguments.

The following command:

    expr "X$VAR" : '.*' - 1

returns the number of characters in VAR.

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.

Section 2.5, Parameters and Variables, Section 2.6.4, Arithmetic Expansion

The Base Definitions volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE, Chapter 8, Environment Variables, Section 9.3, Basic Regular Expressions, Section 12.2, Utility Syntax Guidelines

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology – Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 http://www.opengroup.org/unix/online.html .

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