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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
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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.1‐2017, 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.
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 │
├───────────────┼──────────────────────────────────────────┤
│integer │ An argument consisting only of an │
│ │ (optional) unary minus followed by │
│ │ digits. │
│string │ A string argument; see below. │
├───────────────┼──────────────────────────────────────────┤
│( expr ) │ Grouping symbols. Any expression can be │
│ │ placed within parentheses. Parentheses │
│ │ can be nested to a depth of │
│ │ {EXPR_NEST_MAX}. │
├───────────────┼──────────────────────────────────────────┤
│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 expr1 if │
│ │ neither expression evaluates to null or │
│ │ zero; otherwise, returns zero. │
├───────────────┼──────────────────────────────────────────┤
│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. │
└───────────────┴──────────────────────────────────────────┘
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.1‐2017,
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 "\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.
The following sections are informative.
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 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$a" = X=
Also note that this volume of POSIX.1‐2017 permits implementations to extend utilities.
The expr utility permits the integer arguments to be preceded with a unary minus. This
means that an integer argument could look like an option. Therefore, the conforming
application must employ the "--" construct of Guideline 10 of the Base Definitions volume
of POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines to protect its operands if there
is any chance the first operand might be a negative integer (or any string with a leading
minus).
For testing string equality the test utility is preferred over expr, as it is usually
implemented as a shell built-in. However, the functionality is not quite the same because
the expr = and != operators check whether strings collate equally, whereas test checks
whether they are identical. Therefore, they can produce different results in locales where
the collation sequence does not have a total ordering of all characters (see the Base
Definitions volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE).
EXAMPLES
The following command:
a=$(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
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
COPYRIGHT
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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