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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 |
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.1‐2008, 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.
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 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.
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.
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).
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.
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.1‐2008, 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, 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 .
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 2013 EXPR(1P)