# 4.Shell 字符串
> ## Shell 字符串
字符串是shell编程中最常用最有用的数据类型（除了数字和字符串，也没啥其它类型好用了），字符串可以用单引号，也可以用双引号，也可以不用引号。单双引号的区别跟PHP类似。
> ### 单引号
```
1.str='this is a string'
```
> **单引号字符串的限制：**
>
> * 单引号里的**`任何字符都会原样输出，单引号字符串中的变量是无效`**的；
> * 单引号字串中不能出现单引号（对单引号使用转义符后也不行）。
> ### 双引号
```
1.your_name='qinjx'
2.str="Hello, I know your are \"$your_name\"! \n"
```
> 双引号的优点：
>
> * 双引号里可以有变量
> * 双引号里可以出现转义字符
> ### 拼接字符串
```
1.your_name="qinjx"
2.greeting="hello, "$your_name" !"
3.greeting_1="hello, ${your_name} !"
4.echo $greeting $greeting_1
```
> ### 获取字符串长度
```
1.string="abcd"
2.echo ${#string} #输出 4
```
**实践**
```
1.#!/bin/bash
2.
3.your_name="onetwothree"
4.
5.echo $(#your_name)
```
输出结果
```
1.[root@localhost lcr]# ./helloworld.sh
2../helloworld.sh:行5: 寻找匹配的 `)' 是遇到了未预期的文件结束符
3../helloworld.sh:行8: 语法错误: 未预期的文件结尾
```
**括号不对**
> ### 提取子字符串
以下实例从字符串第 2 个字符开始截取 4 个字符：
```
1.string="runoob is a great site"
2.echo ${string:1:4} # 输出 unoo
```
**实践**
```
1.#!/bin/bash
2.
3.your_name="runoob is a great site"
4.echo ${your_name:5:11}
```
输出结果
```
1.b is a grea
2.[root@localhost lcr]# ./helloworld.sh
3.b is a grea
```
> ### 查找子字符串
查找字符 “i 或 s” 的位置：
```
1.string="runoob is a great company"
2.echo `expr index "$string" is` # 输出 8
```
**注意：** 以上脚本中 “\`” 是**反引号**，而不是单引号 “’”，不要看错了哦。\
**实践**
```
1.#!/bin/bash
2.
3.string="LCR is not loser"
4.
5.echo `expr index "$string" is`
```
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