typora/note/Shell/for 循环语法.md

### 概览
- 列表循环：用于将一组命令执行已知的次数
- 不带列表循环，执行时由用户指定参数和参数的个数
- 类C风格循环
- break continue exit
- 
### 列表循环
- 方式一
```shell
# 无步长
for variable in {list} 
do
	command1
	command2
	...
done

# 有步长
for variable in {list..步长}
do
	command1
	command2
	...
done

```

- 方式二
```
for variable in a b c
do
	command1
	command2
	...
done
```
- 示例
```
#! /bin/env bash

# 步长为2
for i in {1..5..2}
do
	echo $i
done

#! /bin/env bash
for i in 1 2 3 4 5
do
	echo $i
done
```

### 不带列表循环
- 方式一
```
for variable
do
	command
	command
	...
done 
```

- 示例
```
#! /bin/env bash

for i
do
	echo hello
done
```
- 自动用$@ 用户输入的所有参数进行循环
```
me@me-EQ59:~/shell_demo/scripts/shell01$ bash -x for_not_list.sh a b c
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
me@me-EQ59:~/shell_demo/scripts/shell01$
```

```
me@me-EQ59:~/shell_demo/scripts/shell01$ bash -x for_not_list.sh {1..5}
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
me@me-EQ59:~/shell_demo/scripts/shell01$
```

```
me@me-EQ59:~/shell_demo/scripts/shell01$ bash -x for_not_list.sh `seq 5`
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
+ for i in "$@"
+ echo hello
hello
me@me-EQ59:~/shell_demo/scripts/shell01$
```

### 类C风格循环
- 方式一
```
# expr1 定义变量并赋初值
# expr 决定是否进行循环
# expr3 决定循环变量如何改变，决定循环什么时候退出 
for (( expr1;expr2;expr4 ))
do
	command
	command
	...
done
```

- 示例
```
for (( i=1;i<=5;i++ ))
do
	echo $i
done
```

```
me@me-EQ59:~/shell_demo/scripts/shell01$ for ((i=0;i<=5;i++));do echo $i;done
0
1
2
3
4
5
me@me-EQ59:~/shell_demo/scripts/shell01$ for ((i=0;i<=5;i++));do [ $((i%2)) -eq 0 ] && echo $i;done
0
2
4
me@me-EQ59:~/shell_demo/scripts/shell01$
```