if..then是最常见的条件判断语句，简而言之，就是当符合某个条件判断的时候，就予以进行某项工作。

1.if-then格式

if-then格式1：

if [ 条件判断表达式 ];then

   当条件判断表达式成立时，需执行的命令

fi

if-then格式2：

if [ 条件判断表达式 ];then

   当条件判断表达式成立时，需执行的命令

else

  当条件判断表达式不成立时，需执行的命令

fi

if-then格式3：

if [ 条件判断表达式一 ];then

  当条件判断表达式一成立时，需执行的命令

elif [ 条件判断表达式二 ];then

  当条件判断表达式二成立时，需执行的命令

else

  当条件判断表达式一和二不成立时，需执行的命令

fi

2.样例

[root@kibana ~]# cat if.sh 
#!/bin/bashread -p "Please input(Y/N): " yn
if [ "$yn" == "Y" ] || [ "$yn" == "y" ];thenecho "Yes,continue!"exit 0
fiif [ "$yn" == "N" ] || [ "$yn" == "n" ];thenecho "No,interrupt!"exit 0
fiecho "I don't know what your choice is!" && exit 0 
[root@kibana ~]# 
[root@kibana ~]# sh if.sh 
Please input(Y/N): y
Yes,continue!
[root@kibana ~]# sh if.sh 
Please input(Y/N): n
No,interrupt!
[root@kibana ~]# sh if.sh 
Please input(Y/N): dfa
I don't know what your choice is!
[root@kibana ~]# 

[root@kibana ~]# cat if-0.sh 
#!/bin/bashread -p "Please input: " yn
if [ "$yn" == "hello" ];thenecho "Hello,ztj!"
elseecho "The input that you input is \"$yn\"!" 
fi
[root@kibana ~]# sh if-0.sh 
Please input: hello
Hello,ztj!
[root@kibana ~]# sh if-0.sh 
Please input: ddd
The input that you input is "ddd"!
[root@kibana ~]# 

[root@kibana ~]# cat if-1.sh 
#!/bin/bashread -p "Please input(Y/N): " yn
if [ "$yn" == "Y" ] || [ "$yn" == "y" ];thenecho "Yes,continue!"
elif [ "$yn" == "N" ] || [ "$yn" == "n" ];thenecho "No,interrupt!"
elseecho "I don't know what your choice is!" 
fi
[root@kibana ~]# sh if-1.sh 
Please input(Y/N): y
Yes,continue!
[root@kibana ~]# sh if-1.sh 
Please input(Y/N): n
No,interrupt!
[root@kibana ~]# sh if-1.sh 
Please input(Y/N): dd
I don't know what your choice is!
[root@kibana ~]#