在Shell中提供了三种常用的循环语句：for循环、while循环、until循环，以及一种特殊循环select（之前我误认为它应该属于判断）。
和c语言一样，可以使用break，continue。

for循环

用法一：直接遍历列表

$ cat fordemo.sh 
#!/bin/bash

for var in $*
do
    echo $var
done

用法二：略写的计数

$ cat fordemo.sh 
#!/bin/bash

for var in {1..5}
do
    echo $var
done
$ ./fordemo.sh
1
2
3
4
5

注意此处in后面为{ }，并且中间为2个点：..
用法三：跳跃式的略写计数

$ cat fordemo.sh 
#!/bin/bash

for var in {1..100..10}
do
    echo $var
done
$ ./fordemo.sh
1
11
21
31
41
51
61
71
81
91

用法四：配合seq命令的跳跃式略写计数

$ cat fordemo.sh 
#!/bin/bash

for var in $(seq 1 10 100)
do
    echo $var
done
$ ./fordemo.sh
1
11
21
31
41
51
61
71
81
91

用法五：无{list}，把传参作为默认list

$ cat fordemo.sh 
#!/bin/bash

for var
do
    echo $var
done

与用法一demo功能相同

用法六：类c风格的计数循环

$ cat fordemo.sh 
#!/bin/bash

for ((i=1; i<10; i++))
do
    echo $i
done
$ ./fordemo.sh
1
2
3
4
5
6
7
8
9

注意用法六里面for后面为(( ))

while循环

基本格式：

while [[表达式]]
do
    command
    command
done

demo:

$ cat fordemo.sh 
#!/bin/bash

i=0
while [[ $i -lt 8 ]]
#或者while (( $i -lt 8 ))，while (( $i < 8 ))
do
   
   let "i++"
   echo $i
done
$ ./fordemo.sh
1
2
3
4
5
6
7
8

while后面使用[[ ]]时，既可以使用测试比较运算符，又可以使用普通的运算符；但是while后面使用(( ))时，只可以使用普通的运算符。

unitl循环

基本格式：

until 表达式
do 
    command
    command
done

用法与while大同小异

select结构用于判断

select功能与case类似，于用户交互性更强

• 基本结构：

select var in {list}
do
    command
    ...
break
done

或者

select var
do
    command
    ...
break
done

这种方式通过命令行来传递select参数列表
demo：

$ cat selectdomo.sh 
#!/bin/bash

echo "What is your favorite clolor?"
select color in "red" "blue" "green" "white" "blank"
do
    break
done
echo "You have selected $color"
$ ./selectdomo.sh 
What is your favorite clolor?
1) red
2) blue
3) green
4) white
5) blank
#? 5 #5为我手动输入
You have selected blank
$ cat selectdomo.sh 
#!/bin/bash

echo "What is your favorite clolor?"
select color 
do
    break
done
echo "You have selected $color"
 ./selectdomo.sh  red blank
What is your favorite clolor?
1) red
2) blank
#? 2
You have selected blank
$ cat selectdemo.sh 
#!/bin/bash

export PS3="Please,input:"
list=(first two three four five)
select choice in ${list[*]}
do
    echo -n  "Your choice is $choice."
    echo " Copy That!"
    break
done