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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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EXPR(1P) POSIX Programmer's Manual 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.
┌────────────────┬──────────────────────────────────────────┐
│ 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.
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:
┌──────────────────┬───────────────────────┐
│ 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$a" = X=
Also note that this volume of POSIX.1‐2017 permits implementations
to extend utilities. The expr utility permits the integer
arguments to be preceded with a unary minus. This means that an
integer argument could look like an option. Therefore, the
conforming application must employ the "--" construct of Guideline
10 of the Base Definitions volume of POSIX.1‐2017, Section 12.2,
Utility Syntax Guidelines to protect its operands if there is any
chance the first operand might be a negative integer (or any
string with a leading minus).
For testing string equality the test utility is preferred over
expr, as it is usually implemented as a shell built-in. However,
the functionality is not quite the same because the expr = and !=
operators check whether strings collate equally, whereas test
checks whether they are identical. Therefore, they can produce
different results in locales where the collation sequence does not
have a total ordering of all characters (see the Base Definitions
volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE).
The following command:
a=$(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 .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 EXPR(1P)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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