EXPR(1)                    POSIX Programmer's Manual                   EXPR(1)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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
       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.

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 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.

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 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 (''00'' on failure).
       Alternatively, if the pattern contains at least one regular expression
       subexpression ""[[\\((......\\))]]"", the string matched by the back-reference
       expression ""\\11"" shall be returned. If the back-reference expression
       ""\\11"" 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.

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
       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).

EXAMPLES
       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.

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
       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

COPYRIGHT
       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(1)
