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EXPR (1) | General commands | Unix Manual Pages | :man

NAME

expr - evaluate expression

CONTENTS

Synopsis
Description
Compatibility with previous implementations
Environment
Exit Status
Examples
See Also
Standards

SYNOPSIS

expr [-e] expression

DESCRIPTION

The expr utility evaluates expression and writes the result on standard output.

All operators and operands must be passed as separate arguments. Several of the operators have special meaning to command interpreters and must therefore be quoted appropriately. All integer operands are interpreted in base 10.

Arithmetic operations are performed using signed integer math. If the -e flag is specified, arithmetic uses the C
.Vt intmax_t data type (the largest integral type available), and expr will detect arithmetic overflow and return an error indication. If a numeric operand is specified which is so large as to overflow conversion to an integer, it is parsed as a string instead. If -e is not specified, arithmetic operations and parsing of integer arguments will overflow silently according to the rules of the C standard, using the
.Vt long data type.

Operators are listed below in order of increasing precedence; all are left-associative. Operators with equal precedence are grouped within { } symbols.

expr1 | expr2
Return the evaluation of expr1 if it is neither an empty string nor zero; otherwise, returns the evaluation of expr2.
expr1 & expr2
Return the evaluation of expr1 if neither expression evaluates to an empty string or zero; otherwise, returns zero.
expr1 "{=, >, >=, <, <=, !=}" expr2
Return the results of integer comparison if both arguments are integers; otherwise, returns the results of string comparison using the locale-specific collation sequence. The result of each comparison is 1 if the specified relation is true, or 0 if the relation is false.
expr1 "{+, -}" expr2
Return the results of addition or subtraction of integer-valued arguments.
expr1 "{*, /, %}" expr2
Return the results of multiplication, integer division, or remainder of integer-valued arguments.
expr1: expr2
The ":" operator matches expr1 against expr2, which must be a basic regular expression. The regular expression is anchored to the beginning of the string with an implicit "^".

If the match succeeds and the pattern contains at least one regular expression subexpression ""\(...\)"", the string corresponding to ""\1"" is returned; otherwise the matching operator returns the number of characters matched. If the match fails and the pattern contains a regular expression subexpression the null string is returned; otherwise 0.

Parentheses are used for grouping in the usual manner.

The expr utility makes no lexical distinction between arguments which may be operators and arguments which may be operands. An operand which is lexically identical to an operator will be considered a syntax error. See the examples below for a work-around.

The syntax of the expr command in general is historic and inconvenient. New applications are advised to use shell arithmetic rather than expr.

Compatibility with previous implementations

Unless
.Fx 4.x compatibility is enabled, this version of expr adheres to the POSIX Utility Syntax Guidelines, which require that a leading argument beginning with a minus sign be considered an option to the program. The standard
syntax may be used to prevent this interpretation. However, many historic implementations of expr, including the one in previous versions of
.Fx , will not permit this syntax. See the examples below for portable ways to guarantee the correct interpretation. The check_utility_compat(3) function (with a utility argument of "expr") is used to determine whether compatibility mode should be enabled. This feature is intended for use as a transition and debugging aid, when expr is used in complex scripts which cannot easily be recast to avoid the non-portable usage. Enabling compatibility mode also implicitly enables the -e option, since this matches the historic behavior of expr in
.Fx . For historical reasons, defining the environment variable EXPR_COMPAT also enables compatibility mode.

ENVIRONMENT

EXPR_COMPAT If set, enables compatibility mode.

EXIT STATUS

The expr utility exits with one of the following values:
0 the expression is neither an empty string nor 0.
1 the expression is an empty string or 0.
2 the expression is invalid.

EXAMPLES

  • The following example (in sh(1) syntax) adds one to the variable a:

    "a=$(expr $a + 1)"

  • This will fail if the value of a is a negative number. To protect negative values of a from being interpreted as options to the expr command, one might rearrange the expression:

    "a=$(expr 1 + $a)"

  • More generally, parenthesize possibly-negative values:

    "a=$(expr \( $a \) + 1)"

  • This example prints the filename portion of a pathname stored in variable a. Since a might represent the path /, it is necessary to prevent it from being interpreted as the division operator. The // characters resolve this ambiguity.

    "expr "//$a" : ’.*/\(.*\)’"

The following examples output the number of characters in variable a. Again, if a might begin with a hyphen, it is necessary to prevent it from being interpreted as an option to expr.

  • If the expr command conforms to -p1003.1-2001, this is simple:

    "expr -- "$a" : ".*""

  • For portability to older systems, however, a more complicated command is required:

    "expr \( "X$a" : ".*" \) - 1"

SEE ALSO

sh(1), test(1), check_utility_compat(3)

STANDARDS


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