expr (将参数作为表达式求值)

expr EXPRESSION

expr OPTION

Options

Expressions

expr prints the value of EXPRESSION to standard output. A blank line below separates increasing precedence groups.

EXPRESSION may be:

Notes About Usage

• Be aware that many operators need to be escaped or quoted to be interpreted correctly by shells.
• Comparisons are arithmetic if both ARGs are numbers, otherwise the comparisons are lexicographical.
• Pattern matches return the string matched between \( and \) or null; if \( and \) are not used, they return the number of characters matched or 0.
• Exit status is 0 if EXPRESSION is neither null nor 0, 1 if EXPRESSION is null or 0, 2 if EXPRESSION is syntactically invalid, and 3 if an error occurred.

expr text : '.*'

Performs a regular expression match. The regular expression after the colon is matched to the text before the colon. The returned output is the number of characters that matched. Here, the regular expression '.*' represents "any number of any character", therefore the result is:

4

expr text : tex

Returns the number of characters from the regular expression after the colon which appear in the text before the colon. Here, the regular expression 'tex' represents "exactly the consecutive characters t, e, and x", so the output would be:

3

expr text : '\(.*\)'

Here, the regular expression '\(.*\)' represents "The actual text (whatever appears in between the parentheses, which are escaped with backslashes) which matches the pattern .*, which itself represents any number of any character." Matched against the text text, this returns the string exactly:

text

expr 5 = 5

Returns 1 (true) if the expressions are equivalent, or 0 (false) if they are not. Here, the values 5 and 5 are equal, and therefore equivalent, so the output will be:

1

expr '5' = '5'

Here, two strings are being compared for equivalence. If the strings match exactly, character-for-character, the result will be 1 (true). Otherwise, the result will be 0. Here, the result is:

1

expr 5 \> 10

Here, the result is 1 (true) if 5 is less than 10, otherwise the result is 0. The "less than" symbol ("<") is preceded by a backslash ("\") to protect it from the shell, which would otherwise interpret it as a redirectionoperator. In this example, 5 is not greater than 10, so the output is:

0

expr 5 \!= 5

Just as the = operator tests for equivalence, the != operator tests for non-equivalence. If the two values being tested are not equivalent, the result is true (1), otherwise the result is false (0). 5 is equivalent to 5, so the result is false:

0

expr 5 \!= "5"

"Equivalence" is not the same as "equality". 5 is a number, and "5" is a string, so technically they are not "equal," but expr considers them equivalent because it reads the string for its contents, sees that it is a number, and uses the value of that number in the comparison. So the value 5 is equivalent to a string containing the number "5". Therefore the answer here is false; they are not non-equivalent:

0

This next example will show, in a series of commands, how expr can be used to increment the value of a variable.

If we define a variable named count, setting it to zero:

count=0

...we can output the value of that variable with the echo command:

echo $count

0

...now we can increment it by setting it to the value of an expr evaluation, which returns the value of the variable, plus one:

count=`expr $count + 1`

...and we can check the updated value with another echo:

echo $count

1