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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 .