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



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

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 prece-
              dence 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 ele-
              ments 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  exam-
              ple, 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  writ-
              ten 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 out-
       put.

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 nei-
                                                   ther expression evaluates to null or
                                                   zero; otherwise, returns zero.
                                                   Returns the result of a decimal integer
                                                   comparison if both arguments are inte-
                                                   gers; otherwise, returns the result of a
                                                   string comparison using the locale-spe-
                                                   cific collation sequence. The result of
                                                   each comparison is 1 if the specified
                                                   relationship is true, or 0 if the rela-
                                                   tionship 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 argu-
                                                   ments.
                                  expr1 - expr2    Subtraction of decimal integer-valued
                                                   arguments.
                                  expr1 * expr2    Multiplication of decimal integer-valued
                                                   arguments.
                                  expr1 / expr2    Integer division of decimal integer-val-
                                                   ued 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 dig-
                                                   its.
                                  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 oper-
       ator 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 permits 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 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 ambigu-
       ity about the division operator and simplifies the whole expression. Also note that pathnames may contain char-
       acters  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  anchor-
       ing 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  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                  2003                             EXPR(1P)