linux 中if条件判断语句

 

1、linux中if条件判断语句，基本结构：

if 条件判断语句

then

command

fi

 

[root@centos7pc1 test2]# ls
test.sh
[root@centos7pc1 test2]# cat test.sh   ## 脚本文件
#!/bin/bash
if [ ! -e xxxx ]        ## if + 条件判断语句
then                   ## then为基本结构
mkdir xxx &> /dev/null     ## &> 表示标准输出和标准错误输出
fi                    ## fi为基本结构
[root@centos7pc1 test2]# bash test.sh
[root@centos7pc1 test2]# ls
test.sh  xxx
[root@centos7pc1 test2]# bash test.sh
[root@centos7pc1 test2]# ls
test.sh  xxx

 

2、if .... else

[root@centos7pc1 test2]# ls
test.sh
[root@centos7pc1 test2]# cat test.sh   
#!/bin/bash
ping -c 3 $1 &> /dev/null     
if [ $? -eq 0 ]            ## 利用$? 判断上一步的执行结果，上一步成功执行则返回0， 否则返回其他数
then
echo "$1 is online!"
else
echo "$1 is not online!"
fi
[root@centos7pc1 test2]# bash test.sh www.baidu.com
www.baidu.com is online!

 

3、if ... elif ... else

[root@centos7pc1 test2]# ls
test.sh
[root@centos7pc1 test2]# cat test.sh
#!/bin/bash
read -p "please input your score: " SCORE    ## 输入分数
if [ $SCORE -ge 80 ] && [ $SCORE -le 100 ]
then
echo "execllent!"
elif [ $SCORE -ge 60 ] && [ $SCORE -lt 80 ]   
then
echo "pass!"
elif [ $SCORE -ge 0 ] && [ $SCORE -lt 60 ]
then
echo "failture!"
else
echo "out of the score range!!!"
fi
[root@centos7pc1 test2]# bash test.sh
please input your score: 89
execllent!
[root@centos7pc1 test2]# bash test.sh
please input your score: 78
pass!
[root@centos7pc1 test2]# bash test.sh
please input your score: 33
failture!
[root@centos7pc1 test2]# bash test.sh
please input your score: 234
out of the score range!!!