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       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (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 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.1‐2008,  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.

                      ┌───────────────┬──────────────────────────────────────────┐
                      │  ExpressionDescription                │
                      ├───────────────┼──────────────────────────────────────────┤
                      │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.                               │
                      │expr1expr2  │ 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  POSIX.1‐2008,
       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
       matched  by  the  back-reference  expression "\1" shall be returned. If the back-reference
       expression "\1" does not match, then the null string 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 POSIX.1‐2008 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.1‐2008, 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> characters is required. For example:

                                 ┌─────────────────┬───────────────────────┐
                                 │    InvalidValid         │
                                 ├─────────────────┼───────────────────────┤
                                 │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.

       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

       Section 2.5, Parameters and Variables, Section 2.6.4, Arithmetic Expansion

       The Base Definitions volume of POSIX.1‐2008, 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, 2013 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX),  The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical  Corrigendum  1  applied.)  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.unix.org/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 .