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expr(P) expr(P)
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 operands, as described in the EXTENDED DESCRIP-
TION section. The application shall ensure that each of
the expression operator symbols:
( ) | & = > >= < <= != + - * / % :
and the symbols integer and string in the table are pro-
vided 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 internationaliza-
tion 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 cate-
gories.)
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 collat-
ing 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 con-
structs 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 opera-
tor 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 identi-
fied 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 $a = '='
looks like:
expr = = =
as the arguments are passed to expr (and they all may be
taken as the '=' operator). The following works reli-
ably:
expr X$a = X=
Also note that this volume of IEEE Std 1003.1-2001 per-
mits implementations to extend utilities. The expr util-
ity 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 applica-
tion 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:
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 sepa-
rate 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 + \( 2 \* 3 \)
In many cases, the arithmetic and string features pro-
vided 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 charac-
ters 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 unspeci-
fied 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 anchor-
ing condition is left unspecified to avoid breaking his-
torical 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 Operat-
ing System Interface (POSIX), The Open Group Base Speci-
fications Issue 6, Copyright (C) 2001-2003 by the Insti-
tute 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.open-
group.org/unix/online.html .
POSIX 2003 expr(P)
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