expr(1p) — Linux manual page


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

PROLOG         top

       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         top

       expr — evaluate arguments as an expression

SYNOPSIS         top

       expr operand...

DESCRIPTION         top

       The expr utility shall evaluate an expression and write the
       result to standard output.

OPTIONS         top


OPERANDS         top

       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         top

       Not used.

INPUT FILES         top



       The following environment variables shall affect the execution of

       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.

                 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.

                 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.



STDOUT         top

       The expr utility shall evaluate the expression and write the
       result, followed by a <newline>, to standard output.

STDERR         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top



       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 ('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.

   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         top

       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.



       The following sections are informative.


       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

        *  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         top

       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         top

       In an early proposal, EREs were used in the matching expression
       syntax.  This was changed to BREs to avoid breaking historical

       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

           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.



SEE ALSO         top

       Section 2.5, Parameters and Variables, Section 2.6.4, Arithmetic

       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         top

       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(1P)