Bash控制结构

1. if-else语句

#!/bin/bash

#if ... fi 语句;  
if [ $a != $b ]
then
    echo "a != b"  
fi  

#if ... else ... fi 语句;  
if [ $a == $b ]  
then  
    echo "a is equal to b"  
else  
    echo "a is not equal to b"  
fi  

#if ... elif ... else ... fi 语句;  
if [ $a == $b ]  
then  
    echo "a is equal to b"  
elif [ $a -gt $b ]  
then  
    echo "a is greater than b"  
elif [ $a -lt $b ]  
then  
    echo "a is less than b"  
else  
    echo "None of the condition met"  
fi  

2. case-esac语句

#!/bin/bash

read -p "Please input a number" aNum
case $aNum in
    1)  echo 'You select 1'
        ;;    #不要忘记加上这一行
    2)  echo 'You select 2'
        ;;
    3)  echo 'You select 3'
        ;;
    4)  echo 'You select 4'
        ;;
    *)  echo 'You do not select a number
        between 1 to 4'
        ;;
esac

3. for循环

#!/bin/bash

#for循环一般格式为:  
#for 变量 in 列表  
#do  
#   command1  
#   command2  
#    ...  
#    commandN  
#done  
#列表是一组值(数字、字符串等)组成的序列,每个值通过空格分隔。每循环一次,就将列表中的下一个值赋给变量。  
#in 列表是可选的,如果不用它,for 循环使用命令行的位置参数。  

#for loop in 1 2 3 4 5   
for loop in 1 2 tt 4 5 kk  
do  
    echo "The value is: $loop"  
done   
#The value is: 1  
#The value is: 2  
#The value is: tt  
#The value is: 4  
#The value is: 5  
#The value is: kk  

num=0  
for str in 'This is a string'  
do  
    num=`expr $num + 1`  
    echo $str $num   #-->This is a string 1   ;只执行一次  
done   

4. while循环

#!/bin/bash   
  
COUNTER=0  
while [ $COUNTER -lt 3 ]  
do  
    COUNTER=`expr $COUNTER + 1`   
done  

5. until循环

#!/bin/bash   

#until 循环执行一系列命令直至条件为 true 时停止。until 循环与 while 循环在处理方式上刚好相反。一般while循环优于until循环,但在某些时候,也只是极少数情况下,until 循环更加有用。  
  
a=0  
until [ ! $a -lt 5 ]  
do  
   echo $a   #--> 0 1 2 3 4 5  
   a=`expr $a + 1`  
done  

6. break,continue跳出循环

 

posted @ 2017-02-28 19:53  弋痕夕的残影  阅读(437)  评论(0编辑  收藏  举报