shell变量操作

# shell变量操作
## 字符串长度
`${#var}`

## 头部截取
把字符中头部符合`mode`的部分删除  
懒惰模式:`${var#mode}`  
贪婪模式:`${var##mode}`  
```
var="abcabcabc"

echo ${var#*a}
bcabcabc

echo ${var##*a}
bc
```

## 尾部截取
把字符中尾部符合`mode`的部分删除  
懒惰模式:`${var%mode}`  
贪婪模式:`${var%%mode}`  
```
var="abcabcabc"

echo ${var%%c*}
ab

echo ${var%c*}
abcabcab
```

## 替换
`${var/old_string/new_string}`:将字符串中首个`old_string`替换为`new_string`
`${var//old_string/new_string}`:将字符串中所有`old_string`替换为`new_string`

## slice
`${var:start_index:length}`
```bash
var="0123456789"

echo ${var:3}
3456789

echo ${var:3:3}
345

# 后几位不要。 length的公式为：总长度-start_index-不要的位数
echo ${var:3:${#var}-3-2}
34567
```

## 判空 转 默认值
`${var-default}`:当变量未设置时，取default作为表达式的值  
`${var:-default}`:当变量未设置或空串时，取default作为表达式的值  
```
[cqc@emr-header-1 ~]$ unset var
[cqc@emr-header-1 ~]$ echo ${var-default}
default
[cqc@emr-header-1 ~]$ var=""
[cqc@emr-header-1 ~]$ echo ${var-default}

[cqc@emr-header-1 ~]$ echo ${var:-default}
default
```

## 判非空 转 默认值
`${var+default}`:当变量已设置时，取default作为表达式的值
`${var:+default}`:当变量已设置且非空串时，取default作为表达式的值  
```
[cqc@emr-header-1 ~]$ unset var
[cqc@emr-header-1 ~]$ echo ${var+val}

[cqc@emr-header-1 ~]$ var="test"
[cqc@emr-header-1 ~]$ echo ${var+val}
val
[cqc@emr-header-1 ~]$ var=""
[cqc@emr-header-1 ~]$ echo ${var+val}
val
[cqc@emr-header-1 ~]$ echo ${var:+val}

[cqc@emr-header-1 ~]$ 
```

## 赋值
`var=${str=default}`:当str变量未设置时，将default同时赋值给var和str,否则执行var=${str}  
`var=${str:=default}`:当str变量未设置或为空串时，将default同时赋值给var和str,否则执行var=${str}  
```
[cqc@emr-header-1 ~]$ unset str
[cqc@emr-header-1 ~]$ var=${str=default}
[cqc@emr-header-1 ~]$ echo ${var}
default
[cqc@emr-header-1 ~]$ echo ${str}
default

[cqc@emr-header-1 ~]$ str=""
[cqc@emr-header-1 ~]$ var=${str=default}
[cqc@emr-header-1 ~]$ echo ${str}

[cqc@emr-header-1 ~]$ echo ${var}

[cqc@emr-header-1 ~]$

[cqc@emr-header-1 ~]$ str="test"
[cqc@emr-header-1 ~]$ var=${str=default}
[cqc@emr-header-1 ~]$ echo ${var}
test
[cqc@emr-header-1 ~]$ echo ${str}
test
[cqc@emr-header-1 ~]$

[cqc@emr-header-1 ~]$ str=""
[cqc@emr-header-1 ~]$ var=${str:=default}
[cqc@emr-header-1 ~]$ echo ${str}
default
[cqc@emr-header-1 ~]$ echo ${var}
default
```

阿川CH