用法概要

The expr utility evaluates expressions as specified by its arguments. After evaluation, the result is written on the standard output. Each token of the expression is a separate argument, so terms of the expression must be separated by blanks. Characters special to the shell must be escaped. Note: 0 is returned to indicate a zero value, rather than the null string. Strings containing blanks or other special characters should be quoted. Integer-valued arguments may be preceded by a unary minus sign. Internally, integers are treated as 32-bit, two's-complement numbers.

The operators and keywords are listed below. Characters that need to be escaped are preceded by `\'. The list is in order of increasing precedence, with equal precedence operators grouped within { } symbols.

expr \| expr: Returns the evaluation of the first expr if it is neither NULL nor 0; otherwise, returns the evaluation of the second expr if it is not NULL; otherwise, 0.
expr \& expr: Returns the first expr if neither expr is NULL or 0, otherwise returns 0.
expr { =, \, \ , \<, \<=, != } expr: Returns the result of an integer comparison if both arguments are integers, otherwise returns the result of a lexical comparison.
expr { +, - } expr: Addition or subtraction of integer-valued arguments.
expr { \, /, % } expr: Multiplication, division, or remainder of the integer-valued arguments.
string : regular-expression
match string regular-expression: The two forms of the matching operator above are synonymous. The matching operators : and match compare the first argument with the second argument which must be a regular expression. Regular expression syntax is the same as that of regexp(5), except that all patterns are “anchored” (treated as if they begin with ^) and therefore ^ is not a special character, in that context. Normally, the matching operator returns the number of characters matched (0 on failure). Alternatively, the \ . . . \ pattern symbols can be used to return a portion of the first argument.
substr string integer-1 integer-2: Extracts the substring of string starting at position integer-1 and of length integer-2 characters. If integer-1 has a value greater than the length of string, expr returns a null string. If you try to extract more characters than there are in string, expr returns all the remaining characters from string. Beware of using negative values for either integer-1 or integer-2 as expr tends to run forever in these cases.
index string character-list: Reports the first position in string at which any one of the characters in character-list matches a character in string.
length string: Returns the length (that is, the number of characters) of string.
( expr ): Parentheses may be used for grouping.

示例

示例 1 Adding an integer to a shell variable

Add 1 to the shell variable a.

a='expr $a + 1'

示例 2 Returning a path name segment

Return the last segment of a path name (that is, the filename part). Watch out for / alone as an argument: expr will take it as the division operator (see BUGS below).

# 'For $a equal to either "/usr/abc/file" or just "file"'
expr  $a  :  '.*/\  \  $a

示例 3 Using // characters to simplify the expression

The addition of the // characters eliminates any ambiguity about the division operator and simplifies the whole expression.

# A better representation of example 2.
expr  //$a  :  '.*/\

示例 4 Returning the value of a variable

Returns the number of characters in $VAR.

expr  $VAR  :  '.*'

退出状态

expr returns the following exit codes:

0: If the expression is neither NULL nor 0.
1: If the expression is NULL or 0.
2: For invalid expressions.

属性

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Availability	compatibility/ucb

另请参见

sh(1), test(1), attributes(5), regexp(5)

诊断

syntax error: for operator/operand errors
non-numeric argument: if arithmetic is attempted on such a string
division by zero: if an attempt to divide by zero is made

已知问题

After argument processing by the shell, expr cannot tell the difference between an operator and an operand except by the value. If $a is an =, the command:

expr  $a  =  '='

looks like:

expr  =  =  =

as the arguments are passed to expr (and they will all be taken as the = operator). The following works:

expr  X$a  =  X=

Note: the match, substr, length, and index operators cannot themselves be used as ordinary strings. That is, the expression:

example% expr index expurgatorious length
syntax error
example%

generates the `syntax error' message as shown instead of the value 1 as you might expect.

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