shell编程基础3

1 控制结构

if语句的用法如下：

if condition 
then 
    statements
else
    statements
fi

一个简单的例子：

#!/bin/sh

echo “Is it morning? Please answer yes or no”
read tiemofday

if [ $tiemofday = “yes” ]
then 
    echo “Good morning”
else
    echo “Good afternoon”
fi

exit 0

还可以使用 elif 增加分支结构

!bin/sh

echo “Is it morning? Please enter yes or no”
read timeofday

if [ $timeofday = “yes” ] then echo “Good morning” elif [$timeofday = “no” ]
echo “Good afternoon”
else
echo “Sorry,$timeofday not recognized.Enter yes or no”
exit 1
fi

exit 0

for 语句
for 语句的用法示例如下：

for  variable in values
do
    statements
done

while 语句
while语句的语法示例如下：

while condition do 
    statements
done

一个简单的例子
#!/bin/sh

echo “Enter password”
read trythis

while [ “$trythis”  !=  “secret” ] do
    echo “Sorry ,try again. ”
    read trythis
done

exit 0

until 语句

until 语句用法实例

until condition
do
statements
done