在我们的shell 编程中，一个必不可少的操作就是针对于字符串的操作，

重要有字符串替换，计算字符串长度 等等。。。

原文地址：

https://blog.csdn.net/github_33736971/article/details/53980123

下面对这些操作进行一个总结。

${#string}
返回$string的长度

${string:position}
在$string中，从$position位置之后开始提取子串

${string:position:length}
在$string中，从$position位置之后开始提取$length长度的子串

[root@localhost SHELL]# NAME="ni hao,ming tian"
[root@localhost SHELL]# echo $NAME
ni hao,ming tian
[root@localhost SHELL]# echo ${NAME}
ni hao,ming tian
[root@localhost SHELL]# echo ${#NAME}
17
[root@localhost SHELL]# echo ${NAME:3}
hao,ming tian
[root@localhost SHELL]# echo ${NAME:3:6}
hao,m
[root@localhost SHELL]# 

${string#substring}
从变量$string开头开始删除最短匹配$substring子串

${string##substring}
从变量$string开头开始删除最长匹配$sunstring子串

[root@localhost SHELL]# NAME=abcABC123ABCabcde
[root@localhost SHELL]# echo $NAME
abcABC123ABCabcde
[root@localhost SHELL]# echo ${NAME#a*C}
123ABCabcde
[root@localhost SHELL]# echo ${NAME##a*C}
abcde
[root@localhost SHELL]# 

• ${string%substring}
  从变量$string结尾开始删除最短匹配$substring子串

  ${string%%substring}
  从变量$string结尾开始删除最长匹配$substring子串

  [root@localhost SHELL]# NAME=abcABC123ABCabc
  [root@localhost SHELL]# echo $NAME
  abcABC123ABCabc
  [root@localhost SHELL]# echo ${NAME%%a*c}
  
  [root@localhost SHELL]# echo ${NAME%a*c}
  abcABC123ABC
  [root@localhost SHELL]# echo ${NAME#b*c}
  abcABC123ABCabc
  [root@localhost SHELL]# echo ${NAME#bc}
  abcABC123ABCabc
  [root@localhost SHELL]# echo ${NAME%bc}
  abcABC123ABCa
  [root@localhost SHELL]# echo ${NAME%%bc}
  abcABC123ABCa
  [root@localhost SHELL]# echo ${NAME%C*c}
  abcABC123AB
  [root@localhost SHELL]# echo ${NAME%%C*c}
  abcAB
  [root@localhost SHELL]# echo ${NAME%%C*b}
  abcABC123ABCabc
  [root@localhost SHELL]# echo ${NAME}
  abcABC123ABCabc
  [root@localhost SHELL]# 

  • 注意：在进行#或##匹配时，$string的首字符必须是被删除子串$substring的第一个字符，不然无法匹配删除；
    在进行%或者%%匹配时，$string的最后一个字符必须是被删除子串$substring的最后一个字符，不然无法进行删除操作；

    ${parameter/parttern/string}
    用string来替换第一个匹配的pattern

    ${parameter/#parttern/string}
    从开头匹配parameter变量中的pattern，匹配上后就用string来替换匹配的pattern

    ${parameter/%pattern/string}
    从结尾开始匹配parameter变量中的pattern，匹配上后就用string替换匹配的pattern

    ${parameter//pattern/string}
    用string来替换parameter变量中所有匹配的pattern

    [root@localhost SHELL]# NAME="I am a student student"
    [root@localhost SHELL]# echo ${NAME//student/teacher}
    I am a teacher teacher
    [root@localhost SHELL]# echo ${NAME/#student/teacher}
    I am a student student
    [root@localhost SHELL]# echo ${NAME/%student/teacher}
    I am a student teacher
    [root@localhost SHELL]# echo ${NAME/%I am /teacher}
    I am a student student
    [root@localhost SHELL]# echo ${NAME/#I am /teacher}
    teachera student student
    [root@localhost SHELL]# echo ${NAME/#I am a/teacher}
    teacher student student
    [root@localhost SHELL]# echo $NAME
    I am a student student
    [root@localhost SHELL]#