Section: POSIX Programmer's Manual (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.
--- evaluate arguments as an expression
utility shall evaluate an expression and write the result to standard
The single expression evaluated by
shall be formed from the
operands, as described in the EXTENDED DESCRIPTION section. The
application shall ensure that each of the expression operator symbols:
( ) | & = > >= < <= != + - * / % :
and the symbols
in the table are provided as separate arguments to
The following environment variables shall affect the execution of
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.)
If set to a non-empty string value, override the values of all the
other internationalization variables.
Determine the locale for the behavior of ranges, equivalence classes,
and multi-character collating elements within regular expressions and
by the string comparison operators.
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.
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard error.
Determine the location of message catalogs for the processing of
utility shall evaluate the expression and write the result, followed by
to standard output.
The standard error shall be used only for diagnostic messages.
The formation of the expression to be evaluated is shown in the
following table. The symbols
represent expressions formed from
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.
An argument consisting only of an (optional) unary minus followed
A string argument; see below.
|( expr )||
Grouping symbols. Any expression can be placed within parentheses.
Parentheses can be nested to a depth of
|expr1 : expr2||
Matching expression; see below.
|expr1 * expr2||
Multiplication of decimal integer-valued arguments.
|expr1 / expr2||
Integer division of decimal integer-valued arguments, producing
an integer result.
|expr1 % expr2||
Remainder of integer division of decimal integer-valued arguments.
|expr1 + expr2||
Addition of decimal integer-valued arguments.
|expr1 - expr2||
Subtraction of decimal integer-valued arguments.
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.
|expr1 = expr2||Equal.|
|expr1 > expr2||Greater than.|
|expr1 >= expr2||Greater than or equal.|
|expr1 < expr2||Less than.|
|expr1 <= expr2||Less than or equal.|
|expr1 != expr2||Not equal.|
|expr1 & expr2||
Returns the evaluation of
if neither expression evaluates to null or zero; otherwise, returns zero.
|expr1 | expr2||
Returns the evaluation of
if it is neither null nor zero; otherwise, returns the evaluation of
if it is not null; otherwise, zero.
matching operator shall compare the string resulting from the
with the regular expression pattern resulting from the evaluation of
Regular expression syntax shall be that defined in the Base Definitions volume of POSIX.1-2017,
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
the string matched by the back-reference expression
shall be returned. If the back-reference expression
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
produces unspecified results.
The following exit values shall be returned:
evaluates to neither null nor zero.
evaluates to null or zero.
An error occurred.
CONSEQUENCES OF ERRORS
The following sections are informative.
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
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 || |
|After argument processing by the shell, || |
|is not required to be able to tell the difference between an operator || |
|and an operand except by the value. If|
|expr "$a" = '='|
|expr = = =|
|as the arguments are passed to|
|(and they all may be taken as the|
|operator). The following works reliably:|
|expr "X$a" = X=|
|Also note that this volume of POSIX.1-2017 permits implementations to extend utilities. The|
|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,|
|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|
|utility is preferred over|
|as it is usually implemented as a shell built-in. However, the|
|functionality is not quite the same because the|
|operators check whether strings collate equally, whereas|
|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,|
|The following command:|
|a=$(expr "$a" + 1)|
|adds 1 to the variable|
|The following command, for|
|equal to either|
|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|
|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 "X$VAR" : '.*' - 1|
|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|
|in the BRE is unspecified because many historical implementations have|
|treated it as a special character, despite their system documentation. For|
|expr foo : hafoo expr hafoo : hafoo|
|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|
|The Base Definitions volume of POSIX.1-2017,|
|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|
|Any typographical or formatting errors that appear|
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