Using su
The su command allows users to open a terminal window, and from that terminal start a sub shell in which the user has another identity. To perform administrative tasks, for instance, you can log in with a normal user account and type su to open a root shell. This brings the benefit that only in the root shell root privileges are used.
If just the command su is typed, the username root is implied. But su can be used to run tasks as another user as well. Type su linda to open a subshell as the user linda, for example. When using su as an ordinary user, you are prompted for a password and after entering that you have acquired the credentials of the target user:
[linda@localhost ~]$ su Password:
[root@localhost linda]#
When using su , a sub shell is started. This is an environment where you are able to work as the target user account, but environment settings for that user account have not been set. If you need complete access to the entire environment of the target user account, you can use su - to start a login shell. If you start a login shell, all scripts that make up the user environment are processed, which makes you work in an environment that is exactly the same as when logging in as that user. (使用su - 切换到其它用户，不要使用su直接切换)
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Using su - is better than using su . When the - is used, a login shell is started, without the - , some variables may not be set correctly. So, you are better off using su - immediately.