linux 中if條件判斷語句

 

1、linux中if條件判斷語句，基本結構：

if 條件判斷語句

then

command

fi

 

[root@centos7pc1 test2]# ls
test.sh
[root@centos7pc1 test2]# cat test.sh ## 腳本文件
#!/bin/bash
if [ ! -e xxxx ]        ## if + 條件判斷語句
then                   ## then為基本結構
mkdir xxx &> /dev/null     ## &> 表示標准輸出和標准錯誤輸出
fi                    ## fi為基本結構
[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 [ $? -eq 0 ]            ## 利用$? 判斷上一步的執行結果，上一步成功執行則返回0， 否則返回其他數
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    ## 輸入分數 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!!!