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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 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 IEEE Std 1003.1-2001, 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
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:
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 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:
* 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>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 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 .