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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.1‐2008, 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.
┌───────────────┬──────────────────────────────────────────┐
│ 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. │
├───────────────┼──────────────────────────────────────────┤
│ │ 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 │ Addition of decimal integer-valued │
│ │ arguments. │
│expr1 − expr2 │ Subtraction of decimal integer-valued │
│ │ arguments. │
├───────────────┼──────────────────────────────────────────┤
│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 │ 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 │ An argument consisting only of an │
│ │ (optional) unary minus followed by │
│ │ digits. │
│string │ 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 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 "[\(...\)]", 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.
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 "$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.1‐2008 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 "−−" 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:
┌─────────────────┬───────────────────────┐
│ 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.
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 "$a" expand into
multiple arguments.
The following command:
expr "$VAR" : '.*'
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.1‐2008, 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, 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
http://www.unix.org/online.html .
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