Linux Login and Switch User

login as: root
root@192.168.1.103's password:
Last login: Mon Aug 19 19:54:25 2013 from 192.168.1.105
[root@localhost ~]# su
[root@localhost ~]# sakib
bash: sakib: command not found
[root@localhost ~]# su sakib
su: user sakib does not exist
[root@localhost ~]# su test
[test@localhost root]$ ls
ls: cannot open directory .: Permission denied
[test@localhost root]$ su root
Password:
[root@localhost ~]#