EXPR(1) General Commands Manual EXPR(1)

expr — evaluate arguments as an expression

expr operand ...

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

None.

The single expression evaluated by expr shall be formed from the operands, as described in EXTENDED DESCRIPTION . Each of the expression operator symbols: ( ) | & = > >= < <= != + − ∗ / % : and the symbols integer and string in the table shall be provided by the application as separate arguments to expr.

None.

None.

The following environment variables shall affect the execution of expr:

This variable shall determine the locale to use for the locale categories when both LC_ALL and the corresponding environment variable (beginning with LC_ ) do not specify a locale. See 2.6.

This variable shall determine the locale to be used to override any values for locale categories specified by the settings of LANG or any environment variables beginning with LC_.

This variable shall determine the locale for the behavior of ranges, equivalence classes, and multicharacter collating elements within regular expressions and by the string comparison operators.

This variable shall determine the locale for the interpretation of sequences of bytes of text data as characters (e.g., single- versus multibyte characters in arguments) and the behavior of character classes within regular expressions.

This variable shall determine the language in which messages should be written.

Default.

The expr utility shall evaluate the expression and write the result to standard output. The character ’0’ shall be written to indicate a zero value and nothing shall be written to indicate a null string.

Used only for diagnostic messages.

None.

The formation of the expression to be evaluated is shown in Table 4-7. 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 in Table 4-7 are listed in order of increasing precedence, with equal-precedence operators grouped between horizontal lines. All of the operators shall be leftassociative.

Table 4-7 − expr Expressions

Expression Description

expr1 | expr2 Returns the evaluation of expr1 if it is neither null nor zero; otherwise,

5766

returns the evaluation of expr2. 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

5771

the locale-specific collation sequence. The result of each comparison shall be 1 if the specified relation is true, or 0 if the relation is false.

expr1 = expr2 Equal. expr1 > expr2 Greater than. expr1 >= expr2 Greater than or equal. 5776 expr1 < expr2 Less than.

expr1 <= expr2 Less than or equal. expr1 != expr2 Not equal. expr1 + expr2 Addition of decimal integer-valued arguments.

expr1 − expr2

Subtraction of decimal integer-valued arguments. 5781 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 MATCHING EXPRESSION .

( expr ) Grouping symbols. Any expression can be placed within parentheses. Parentheses can be nested to a depth of .

integer An argument consisting only of an (optional) unary minus followed by digits. 5790 string A string argument. See STRING OPERAND .

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 2.8.3 (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 shall be matched by the regular expression) and, therefore, it is unspecified whether ˆ 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 corresponding to \1 shall be returned (see 2.8.3.3).

A string argument is an argument that cannot be identified as an integer argument or as one of the expression operator symbols shown in OPERANDS . The use of string arguments length, substr, index, or match produces unspecified results.

The expr utility shall exit with one of the following values:

If the expression evaluates to neither null nor zero.

If the expression evaluates to null or zero.

For invalid expressions.

An error occurred.

Default.

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>s is required. For example:

Invalid Valid expr 1+2 expr 1 + expr "1 + 2" expr 1 + 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; the utility was retained by . 2 as acknowledgment of the many historical shell scripts that use it. Newly written scripts should avoid expr in favor of the new features within the shell. The following command a=$( expr $a + 1) adds 1 to the variable a. A new application should use

a=$(($a+1))

The following command, for $a equal to either /usr/abc/file or just file: expr $a : ’.∗/\(.∗\)’ \| $a returns the last segment of a pathname (i.e., file). Applications should avoid the character / used alone as an argument: expr may interpret it as the division operator. The following command: expr "//$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 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. Usage Warning: 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 standard 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 portable application must employ the "--" construct of Guideline 10 (see 2.10.2) to protect its operands if there is any chance the first operand might be a negative integer (or any string with a leading minus).

In an earlier draft, Extended Regular Expressions were used in the matching expression syntax. This was changed to the Basic variety to avoid breaking historical applications. The use of a leading circumflex in the regular expression is unspecified because many historical implementations have treated it as special, despite their system documentation. For example,

expr foo : ˆfoo expr ˆfoo : ˆfoo 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.

September 1991 posix.fail