EXPR(1) General Commands Manual EXPR(1) NAME expr -- evaluate expression 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 intmax_t data type (the largest integral type available), and expr will detect arithmetic overflow and re- turn 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 in- stead. If -e is not specified, arithmetic operations and parsing of inte- ger arguments will overflow silently according to the rules of the C stan- dard, using the long data type. Operators are listed below in order of increasing precedence; all are left- associative. Operators with equal precedence are grouped within { } sym- bols. 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 in- tegers; otherwise, returns the results of string comparison using the locale-specific collation sequence. The result of each compar- ison 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 ar- guments. expr1 {*, /, %} expr2 Return the results of multiplication, integer division, or remain- der 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 be- ginning 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 num- ber of characters matched. If the match fails and the pattern con- tains a regular expression subexpression the null string is re- turned; 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 lex- ically 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 FreeBSD 4.x compatibility is enabled, this version of expr adheres to the POSIX Utility Syntax Guidelines, which require that a leading argu- ment beginning with a minus sign be considered an option to the program. The standard -- syntax may be used to prevent this interpretation. How- ever, many historic implementations of expr, including the one in previous versions of FreeBSD, will not permit this syntax. See the examples below for portable ways to guarantee the correct interpretation. The check_util- ity_compat(3) function (with a utility argument of "expr") is used to de- termine whether compatibility mode should be enabled. This feature is in- tended for use as a transition and debugging aid, when expr is used in com- plex 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 FreeBSD. For historical rea- sons, defining the environment variable EXPR_COMPAT also enables compati- bility mode. ENVIRONMENT EXPR_COMPAT If set, enables compatibility mode. EXAMPLES * The following example (in sh(1) syntax) adds one to the variable a: a=$(expr $a + 1) * This will fail if the value of a is a negative number. To protect neg- ative values of a from being interpreted as options to the expr com- mand, one might rearrange the expression: a=$(expr 1 + $a) * More generally, parenthesize possibly-negative values: a=$(expr \( $a \) + 1) * This example prints the filename portion of a pathname stored in vari- able 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 IEEE Std 1003.1-2001 ("POSIX.1"), this is simple: expr -- "$a" : ".*" * For portability to older systems, however, a more complicated command is required: expr \( "X$a" : ".*" \) - 1 DIAGNOSTICS 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. SEE ALSO sh(1), test(1), check_utility_compat(3) STANDARDS The expr utility conforms to IEEE Std 1003.1-2001 ("POSIX.1"), provided that compatibility mode is not enabled. The -e flag is an extension. FreeBSD 5.0 May 10, 2002 EXPR(1)
NAME | SYNOPSIS | DESCRIPTION | ENVIRONMENT | EXAMPLES | DIAGNOSTICS | SEE ALSO | STANDARDS
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