Shell 流程控制

if...else 语句

总共有三类语句

• if then......fi
• if then......else......fi
• if then......elif then......else......fi

#!/bin/bash

a=1
b=2

if [ $a == $b ]
then
    echo "$a is equal $b"
elif [ $a -gt $b ]
then
    echo "$a is greater than $b"
else
    echo "$a is lower than $b"
fi

$ sh test.sh
1 is lower than 2

for 语句

• in 后面可以接列表， 可以是数字和字符串， 但是每个值必须用空格分开

for var in item1 item2 ... itemN
do
    command
done

#!/bin/bash


for value in 1 2 3 4
do
    echo "value is "$value
done

# sh test.sh
value is 1
value is 2
value is 3
value is 4

while 语句

条件为真，则执行command步骤，再循环回到顶部，再次判断真假。

while condition
do
    command
done

#!/bin/bash

a=3
while [ $a -lt 5 ]
do
    a=`expr $a + 1`
    echo $a
done

#$ sh test.sh
4
5

until 语句

until 循环执行一系列命令直至条件为 true 时停止

until condition
do
    command
done

#!/bin/bash

value=0
until [ ! $value -lt 4 ]
do
    echo "value is "$value
    value=`expr $value + 1`
done

# sh test.sh
value is 0
value is 1
value is 2
value is 3

case选择语句

• 一旦模式匹配，则执行完匹配模式相应命令后(从模式到;;)不再继续其他模式
• 无一匹配模式，使用星号 * 捕获该值，再执行后面的命令

case 值 in
模式1)
    command
    ;;
模式2）
    command
    ;;
*）
    command
    ;;
esac

#!/bin/bash

num=1
case $num in
1)
    echo "You get the num is 1"
;;
2)
    echo "You get the num is 2"
;;
*)
    echo "You get the num is others"
;;
esac
#sh test.sh
You get the num is others

跳出循环

• break： 跳出所有循环（终止执行后面的所有循环）
• continue：仅仅跳出当前循环