Manual Page - expr(1p)
Manual Reference Pages - EXPR (P)
NAME
expr - evaluate arguments as an expression
CONTENTS
Synopsis
Description
Options
Operands
Stdin
Input Files
Environment Variables
Asynchronous Events
Stdout
Stderr
Output Files
Extended Description
Matching Expression
String Operand
Exit Status
Consequences Of Errors
Application Usage
Examples
Rationale
Future Directions
See Also
Copyright
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 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:
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LANG
|
Provide a default value for the internationalization variables that
are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables
for
the precedence of internationalization variables used to determine
the values of locale categories.)
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LC_ALL
|
If set to a non-empty string value, override the values of all the
other internationalization variables.
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LC_COLLATE
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Determine the locale for the behavior of ranges, equivalence classes,
and multi-character collating elements within regular
expressions and by the string comparison operators.
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LC_CTYPE
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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.
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LC_MESSAGES
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Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard
error.
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NLSPATH
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Determine the location of message catalogs for the processing of LC_MESSAGES
.
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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 IEEE Std 1003.1-2001, 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 corresponding to "\1" 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:
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0
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The expression evaluates to neither null nor zero.
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1
|
The expression evaluates to null or zero.
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2
|
Invalid expression.
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>2
|
An error occurred.
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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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001, 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:
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*
|
Many of the operators are also shell control operators or reserved
words, so they have to be escaped on the command line.
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*
|
Each part of the expression is composed of separate arguments, so
liberal usage of <blank>s is required. For example:
| Invalid | Valid | | | | |
| expr 1+2 | expr 1 + 2 | | | | |
| expr "1 + 2" | expr 1 + 2 | | | | |
| expr 1 + (2 * 3) | expr 1 + \( 2 \* 3 \) | | | | |
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|
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 Parameters and
Variables and 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
Parameters and Variables , Arithmetic
Expansion
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
| IEEE/The Open Group | EXPR (P) | 2003 |
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