shell 字符串操作 + 变量替换
本篇内容包括：
- 字符串操作
- 变量替换
1.字符串操作
| 表达式 | 含义 |
| ${#string} | 获取字符串 string 的长度 |
| ${string:position} | 从字符串 string position 处截取子串 |
| ${string:position:length} | 从字符串 string position 处截取length 长度的子串 |
| ${string#substring} | 从字符串 string 的开头，删除最短匹配 substring 的子串 |
| ${string##substring} | 从字符串 string 的开头，删除最长匹配 substring 的子串 |
| ${string%substring} | 从字符串 string 的结尾，删除最短匹配 substring 的子串 |
| ${string%%substring} | 从字符串 string 的结尾，删除最长匹配 substring 的子串 |
| ${string/substring/replacement} | 在字符串 string 中，使用 replacement 替换 substring |
| ${string//substring/replacement} | 在字符串 string 中，使用 replacement，代替所有匹配的 substring |
| ${string/#substring/replacement} | 条件替换 ： 如果字符串 string 的前缀匹配 substring ， 那么就用 replacement 来代替匹配到的 substring |
| ${string/%substring/replacement} | 条件替换 ： 如果字符串 string 的后缀匹配 substring ，那么就用 replacement 来代替匹配到的 substring |
1.获取长度：
zhang@zhang-virtual-machine:~/Desktop$ string='my blog nickName is xiao mu xi zi'
zhang@zhang-virtual-machine:~/Desktop$ echo $string
my blog nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~/Desktop$ echo ${#string}
33
zhang@zhang-virtual-machine:~/Desktop$
2.字符串截取：
zhang@zhang-virtual-machine:~/Desktop$ echo $string
my blog nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~/Desktop$ echo ${string:8}
nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~/Desktop$ echo ${string:8:8}
nickName
zhang@zhang-virtual-machine:~/Desktop$
3.字符串替换 ：
zhang@zhang-virtual-machine:~/Desktop$ echo $string
my blog nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~/Desktop$ echo ${string#my}
blog nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~/Desktop$ echo ${string#my blog}
nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~/Desktop$ echo ${string##my blog}
nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~/Desktop$ echo ${string##zi}
my blog nickName is xiao mu xi zi
zhang@zhang-virtual-machine:~$ echo ${string%zi}
my blog nickName is xiao mu xi
zhang@zhang-virtual-machine:~$ echo ${string%%xi zi}
my blog nickName is xiao mu
zhang@zhang-virtual-machine:~$
zhang@zhang-virtual-machine:~/Desktop$ greet='hello,world,hello,linux'
zhang@zhang-virtual-machine:~/Desktop$ echo $greet
hello,world,hello,linux
zhang@zhang-virtual-machine:~/Desktop$ echo ${greet/hello/welcome to}
welcome to,world,hello,linux
zhang@zhang-virtual-machine:~/Desktop$ echo ${greet//hello/welcome to}
welcome to,world,welcome to,linux
zhang@zhang-virtual-machine:~/Desktop$ echo $greet
hello,world,hello,linux
zhang@zhang-virtual-machine:~/Desktop$ echo $greet
hello,world,hello,linux
zhang@zhang-virtual-machine:~/Desktop$ echo ${greet/#world/php}
hello,world,hello,linux
zhang@zhang-virtual-machine:~/Desktop$ echo ${greet/#hello/welcome to}
welcome to,world,hello,linux
zhang@zhang-virtual-machine:~/Desktop$ echo ${greet/%linux/php}
hello,world,hello,php
zhang@zhang-virtual-machine:~/Desktop$
2.变量替换
| 表达式 | 含义 |
| ${var-DEFAULT} | 如果 var 没有被声明，那么就以 DEFAULT 作为其默认值 |
| ${var:-DEFAULT} | 如果var没有被声明或者其值为空， 那么就以 DEFAULT 作为其默认值 |
| ${var=DEFAULT} | 如果var没有被声明，那么就以 DEFAULT 作为其默认值 |
| ${var:=DEFAULT} | 如果var没有被声明或者其值为空，那么就以 DEFAULT 作为其默认值 |
| ${var+OTHERVALUE} | 如果var声明了， 那么其值就是 OTHERVALUE ，否则就为空字符串 |
| ${var:+OTHERVALUE} | 如果var被设置了， 那么其值就是 OTHERVALUE ，否则就为空字符串 |
| ${!varprefix*} | 匹配之前所有以varprefix开头进行声明的变量 |
| ${!varprefix@} | 匹配之前所有以varprefix开头进行声明的变量 |
zhang@zhang-virtual-machine:~/Desktop$ echo $name
zhang@zhang-virtual-machine:~/Desktop$ echo ${name-zhang}
zhang
zhang@zhang-virtual-machine:~/Desktop$ name=
zhang@zhang-virtual-machine:~/Desktop$ echo $name
zhang@zhang-virtual-machine:~/Desktop$ echo ${name:-zhang}
zhang
zhang@zhang-virtual-machine:~/Desktop$ echo ${name=zhang}
zhang@zhang-virtual-machine:~/Desktop$ echo ${name:=zhang}
zhang
zhang@zhang-virtual-machine:~/Desktop$
zhang@zhang-virtual-machine:~/Desktop$ echo $lastName
zhang@zhang-virtual-machine:~/Desktop$ echo ${lastName+guoping}
zhang@zhang-virtual-machine:~/Desktop$ lastName='xiao mu xi zi'
zhang@zhang-virtual-machine:~/Desktop$ echo ${lastName+guoping}
guoping
zhang@zhang-virtual-machine:~/Desktop$ echo ${lastName:+guoping}
guoping
zhang@zhang-virtual-machine:~/Desktop$
zhang@zhang-virtual-machine:~/Desktop$ varprefixName='zhang'
zhang@zhang-virtual-machine:~/Desktop$ varprefixLastName='guoping'
zhang@zhang-virtual-machine:~/Desktop$ fullName='guoping zhang'
zhang@zhang-virtual-machine:~/Desktop$ echo ${!varprefix*}
varprefixLastName varprefixName
zhang@zhang-virtual-machine:~/Desktop$
zhang@zhang-virtual-machine:~$ skillDynamicLanguage='php'
zhang@zhang-virtual-machine:~$ skillStaticLanguage='C++'
zhang@zhang-virtual-machine:~$ echo ${!Lanugage@}
zhang@zhang-virtual-machine:~$ echo ${!skill@}
skillDynamicLanguage skillStaticLanguage
zhang@zhang-virtual-machine:~$
以上所列，有点冷门(即用的比较少，有些系统脚本偶尔会用到)，不过掌握后还是能省去不少麻烦。