Linux case语句用法

case 命令
case语句与if-then-else语句的区别。

例1：if-then-else语句，比较繁琐
[19:37:32 root@libin3 libin]# usermod -G root student
[19:22:33 root@libin3 libin]# vim shell55
[19:37:32 root@libin3 libin]# cat shell55 
#!/bin/bash
# look for a value
#
if [ $USER = root ]
then echo "the user is root"
elif [ $USER = student ]
then echo "the user is student"
elif [ $USER = rhce ]
then echo "the user is student"
elif [ $USER =  helloword ]
then echo "the user is helloword"
else 
echo "sorry,no a user exist"
fi

[19:26:24 root@libin3 libin]# chmod u+x shell55
[19:26:31 root@libin3 libin]# ./shell55
the user is root
[student@libin3 libin]$ sudo chmod g+x shell55
[student@libin3 libin]$ ./shell55
the user is student

case命令，就不需要写elif语句检查同一个变量值。case命令会采用下面的列表格式来检查单个变量的多个值。

格式：
case variable in 
pattern1 | pattern2) commands1;; 
pattern3) commands2;; 
*) default commands;; 
esac 
case命令会将指定的变量与不同模式进行比较。如果变量和模式是匹配的，那么shell会执行 
为该模式指定的命令。通过 | 操作符在一行中分隔出多各模式。 * 号会捕获所有与已知模式不匹配的值，就相当于else。

例2：将例1的语句转换成case语句
示例1：
[19:48:36 root@libin3 libin]# vim shell56
#!/bin/bash
#using the case command
#
case $USER in
root | student)
echo "the is $USER";;
rhce)
echo "the is rhce";;
student)
echo "the is student";;
helloword)
echo "the is helloword";;
*)
echo "no is user";;
esac

[19:52:16 root@libin3 libin]# chmod +x shell56
[19:53:11 root@libin3 libin]# ./shell56
the is root

示例2：
[19:56:00 root@libin3 libin]# su - student
[student@libin3 libin]$ sudo chmod g+x /libin/libin/shell56
[student@libin3 libin]$ ./shell56
the is student

例3：这里我们可以得出case语句判断出了libin3.com 为主机名 
[21:11:43 root@libin3 libin]# echo $HOSTNAME
libin3.com
[21:11:45 root@libin3 libin]# vim shell57
#!/bin/bash
# using the case command
#
case $HOSTNAME in
libin2.com | libin4.com )
echo "the is $HOSTNAME";;
libin5.com | libin3.com )
echo "the hostname is libin5.com or $HOSTNAME";;
libin6.com)
echo "the hostname is libin6.com";;
*)
echo "no HOSTNAME";;
esac

[21:04:47 root@libin3 libin]# chmod u+x shell57
[21:05:02 root@libin3 libin]# ./shell57
the hostname is libin5.com or libin3.com