基本命令_创建组/用户
*only root user could use this commad
1 groupadd -g test -- create a group name test
2 usermod -d /home/cici -G test cici
-- change the user cici's login direcotry as? /home/cici and add the user? cici to test group
3 su cici
4 groups -- check the goup for user cici
