#### 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
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#### 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
```
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#### 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
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#### 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 $n
done
```
while 语句
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#### while 循环语法格式
```shell
int=5
while(($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
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#### 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
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#### 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
```
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#### 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。
```