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 XBD 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

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

Expression	Description
expr1 | expr2	Returns the evaluation of expr1 if it is neither null nor zero; otherwise, returns the evaluation of expr2 if it is not null; otherwise, zero.
expr1 & expr2	Returns the evaluation of expr1 if neither expression evaluates to null or zero; otherwise, returns zero.
expr1 = expr2 expr1 > expr2 expr1 >= expr2 expr1 < expr2 expr1 <= expr2 expr1 != expr2	Returns the result of a decimal integer comparison if both arguments are integers; otherwise, returns the result of a string comparison using the locale-specific collation sequence. The result of each comparison is 1 if the specified relationship is true, or 0 if the relationship is false. Equal. Greater than. Greater than or equal. Less than. Less than or equal. Not equal.
expr1 + expr2 expr1 - expr2	Addition of decimal integer-valued arguments. Subtraction of decimal integer-valued arguments.
expr1 * expr2 expr1 / expr2 expr1 % expr2	Multiplication of decimal integer-valued arguments. Integer division of decimal integer-valued arguments, producing an integer result. Remainder of integer division of decimal integer-valued arguments.
expr1 : expr2	Matching expression; see below.
( expr )	Grouping symbols. Any expression can be placed within parentheses. Parentheses can be nested to a depth of {EXPR_NEST_MAX}.
integer string	An argument consisting only of an (optional) unary minus followed by digits. A string argument; see below.

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 XBD 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 "[\(...\)]", the string matched by the back-reference expression "\1" shall be returned. If the back-reference expression "\1" does not match, then the null string shall be returned.

Identification as Integer or String

An argument or the value of a subexpression that consists only of an optional unary minus followed by digits is a candidate for treatment as an integer if it is used as the left argument to the | operator or as either argument to any of the following operators: & = > >= < <= != + - * / %. Otherwise, the argument or subexpression value shall be treated as a string.

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.

APPLICATION USAGE

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:

  Invalid	Valid
  expr 1+2	expr 1 + 2
  expr "1 + 2"	expr 1 + 2
  expr 1 + (2 * 3)	expr 1 + \( 2 \* 3 \)

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 Parameters and Variables and 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 XBD 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 XBD LC_COLLATE).

EXAMPLES

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.

RATIONALE

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.

FUTURE DIRECTIONS

None.

SEE ALSO

Parameters and Variables, Arithmetic Expansion

XBD LC_COLLATE, Environment Variables, Basic Regular Expressions, Utility Syntax Guidelines

CHANGE HISTORY

First released in Issue 2.

Issue 5

The FUTURE DIRECTIONS section is added.

Issue 6

The expr utility is aligned with the IEEE P1003.2b draft standard, to include resolution of IEEE PASC Interpretation 1003.2 #104.

The normative text is reworded to avoid use of the term "must" for application requirements.

Issue 7

Austin Group Interpretation 1003.1-2001 #036 is applied, clarifying the behavior for BREs.

The SYNOPSIS and OPERANDS sections are revised to explicitly state that the name of each of the operands is operand.

POSIX.1-2008, Technical Corrigendum 2, XCU/TC2-2008/0094 [942], XCU/TC2-2008/0095 [709], XCU/TC2-2008/0096 [942], XCU/TC2-2008/0097 [963], and XCU/TC2-2008/0098 [942] are applied.