Bash的循环与分支

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一、循环

1.for循环

for week in Monday Tuesday Wednesday Thursday Friday Saturday Sunday    #<span style="color:#CC66CC;">每个占一行</span>
do
echo $week
done

for week in<strong><span style="color:#CC66CC;"> "</span></strong>Monday Tuesday Wednesday Thursday Friday Saturday Sunday<span style="color:#CC66CC;"><strong>"</strong></span>   #<span style="color:#CC66CC;">打印在一行</span>
do
echo $week
done

结果为：

Monday

Tuesday

Wednesday

Thursday

Friday

Saturday

Sunday

Monday Tuesday Wednesday Thursday Friday Saturday Sunday

2.while循环

#!/bin/bash

var=0
lim=10

while [ "$var" -lt "$lim" ]
do
echo -n "$var"
var=`expr $var + 1`
# 或者   var=$((var +1))
# 或者   let "var += 1" 等等
done

 从0递增到10

3.until循环

#!/bin/bash

e=end

until [ "$var" = "$e" ]
do
echo "($e to exit)"
read var
echo "#1 = $var"
done

输入的字符串为 end 时，退出。

二、嵌套循环

#!/bin/bash

out=1

for a in 1 2 3 4 5
do
echo "$out in outer"
in=1
for b in 1 2
do
echo "$in in inner"
let in+=1
done
let out+=1
done

三、循环控制

break  跳出整个循环

continue 跳出本次循环

四、测试与分支

1.case (in) / esac

在shell中的case结构与C/C++中的switch结构是相同的。它允许通过判断来选择代码块中多条路径中的一条。

它的作用和多个if/then/else语句的作用相同，是它们的简化结构，特别适用于创建菜单。

case块以esac（case的反向拼写）结尾。

#!/bin/bash

read key
<span style="color:#CC66CC;"><strong>case "$key" in</strong></span>
[[:lower:]] ) echo "Lower letter";;
[[:upper:]] ) echo "Upper letter";;
[0-9]) echo "Digit";;
* ) echo "Whitepace";;
<strong><span style="color:#CC66CC;">esac</span></strong>

只有当 [] 中出现的被输入，那么输出相应的。 比如输入e，则返回 Lower letter。

2.select

select结构是建立菜单的另一种工具，这种结构是从ksh中引入的。

echo
select fruit in "apple" "pear" "strawberry" "watermelon"
do
echo " your favorite is $fruit"
break
done

结果为：
1) apple

2) pear

3) strawberry

4) watermelon

#? 4

 your favorite is watermelon