LInux：循环语句

if-else语句

#### if 语句语法格式

```shell
if [ $a -gt $b ];
then 
    echo "a>b"
fi

if [ $a -gt $b ];
then 
    echo "a>b"
    echo "a!=b"
    echo "true"
fi
```

if-else语句

#### if-else 语句语法格式

```shell
if [ $a -gt $b ];
then 
    echo "a>b"
else
    echo "a<=b"
fi

if [ $a -gt $b ];
then 
    echo "a>b"
    echo "a!=b"
    echo "true"
else
    echo "a<=b"
fi

if ((a>b));
then 
    echo "true"
    echo "a>b"
else
    echo "a<=b"
fi
```

#### if else语法格式

```shell
if [ $a -gt $b ];
then 
    echo "a>b"
elif [$a -eq $b]
then
    echo "a=b"
else
    echo "a<b"
fi
```

for 循环

#### for循环一般格式为

```shell
for m in 1 2 3 4;
do 
   echo "values is $m"
done

for n in This is a dong;
do 
   echo $n 
done
```

while 语句

#### while 循环语法格式

```shell
int=5
while(($int>=2));
do 
   echo "$int"
   let "int--"
done

echo "按下CTRL+D退出"
echo "输入你喜欢的电影"
while read film;
do
   echo "${film} is ok"
done
```

无限循环

shell 复制代码

while True;
do
  echo "ok"
done

for ((::))

until循环

#### until 循环执行一系列命令直至条件为 true 时停止

```shell
a=0
until [ ! $a -lt 10 ];
do
    echo $a
    a=$((a+1))
done
```

case ... esac

shell 复制代码

echo "输入1到4之间的数字"
read -p "数字：" num
case $num in 
    1) 
        echo '1'
    ;;
    2) 
        echo '2'
    ;;
    3) 
        echo '3'
    ;;
    4) 
        echo '4'
    ;;
esac   

s1='iambot'
case $s1 in
    'iambot') echo 'bot'
    ;;
    'iampm') echo 'pm'
    ;;
    'iampl') echo 'pl'
    ;;
esac

跳出循环

#### break 命令允许跳出所有循环

```shell
echo "welcome to Amusement park"
echo "please inter a number between 1-4"
while :
do
    read -p "please inter your number: " num
    case $num in
        1|2|3|4)
            echo "you choose 1-4 you are great"
            ;;
        *)
            echo "you choose others you are bad"
            break
            ;;
    esac
done
```

#### continue 命令不会跳出所有循环，仅仅跳出当前循环。

```shell
echo "welcome to Amusement park"
echo "please inter a number between 1-4"
while :
do
    read -p "please inter your number: " num
    case $num in
        1|2|3|4)
            echo "you choose 1-4 you are great"
            ;;
        *)
            echo "you choose others you are bad"
            continue
            echo 'game over'
            ;;
    esac
done
#运行代码发现，当输入大于5的数字时，该例中的循环不会结束，语句 echo "游戏结束" 永远不会被执行。

```