@TOC

一、expr命令介绍

expr是一个用于数学表达式计算工具命令行工具，它可以对整数、小数和字符串进行算术操作、逻辑运算、比较操作和字符串操作等。

二、expr的命令帮助

1. expr的help帮助信息

expr的help帮助信息

[root@docker ~]# expr --help
Usage: expr EXPRESSION
  or:  expr OPTION

      --help     display this help and exit
      --version  output version information and exit

Print the value of EXPRESSION to standard output.  A blank line below
separates increasing precedence groups.  EXPRESSION may be:

  ARG1 | ARG2       ARG1 if it is neither null nor 0, otherwise ARG2

  ARG1 & ARG2       ARG1 if neither argument is null or 0, otherwise 0

  ARG1 < ARG2       ARG1 is less than ARG2
  ARG1 <= ARG2      ARG1 is less than or equal to ARG2
  ARG1 = ARG2       ARG1 is equal to ARG2
  ARG1 != ARG2      ARG1 is unequal to ARG2
  ARG1 >= ARG2      ARG1 is greater than or equal to ARG2
  ARG1 > ARG2       ARG1 is greater than ARG2

  ARG1 + ARG2       arithmetic sum of ARG1 and ARG2
  ARG1 - ARG2       arithmetic difference of ARG1 and ARG2

  ARG1 * ARG2       arithmetic product of ARG1 and ARG2
  ARG1 / ARG2       arithmetic quotient of ARG1 divided by ARG2
  ARG1 % ARG2       arithmetic remainder of ARG1 divided by ARG2

  STRING : REGEXP   anchored pattern match of REGEXP in STRING

  match STRING REGEXP        same as STRING : REGEXP
  substr STRING POS LENGTH   substring of STRING, POS counted from 1
  index STRING CHARS         index in STRING where any CHARS is found, or 0
  length STRING              length of STRING
  + TOKEN                    interpret TOKEN as a string, even if it is a
                               keyword like 'match' or an operator like '/'

  ( EXPRESSION )             value of EXPRESSION

Beware that many operators need to be escaped or quoted for shells.
Comparisons are arithmetic if both ARGs are numbers, else 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.

GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
For complete documentation, run: info coreutils 'expr invocation'

2. expr的常用运算符

expr的常用运算符

expr的常用运算符：

加法运算：+
减法运算：-
乘法运算：\*
除法运算：/
取模（取余）运算：%

三、expr命令的基本使用

1. 查询expr工具版本

查询expr工具版本

[root@docker ~]# expr --version
expr (GNU coreutils) 8.22
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Parker, James Youngman, and Paul Eggert.

2. 算术运算

可以使用加、减、乘、除、模运算符进行基本的算术运算

• 加法运算

[root@docker ~]# expr 8 + 9
17

• 减法运算

[root@docker ~]# expr 8 -  9
-1

• 乘法运算

[root@docker ~]# expr 8  \*  9
72

• 除法运算

[root@docker ~]# expr 12  /  6
2

• 取余运算

[root@docker ~]# expr 13  %  6
1

3. 比较操作

使用expr进行比较操作，判断正确输出0，判断错误输出为1

[root@docker ~]# expr 13 \<  20
1
[root@docker ~]# expr 25 \<  20
0

4. 字符串操作

可以使用字符串操作符对字符串进行操作，其中，：“：”表示正则匹配，正则表达式中，“.”表示任意字符，“*”表示匹配任意次数。
需要注意的是，在expr中，各种操作符与数字或字符串之间必须用空格隔开。而且expr对于特殊字符，需要进行转义处理（如上述例子中的“\”符号）。

[root@docker ~]# expr "hello" : 'he'
2
[root@docker ~]# expr "hello" : '.*'
5