Linux/Security/Login

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There are several ways to login as a user or change to another user.

Login

The simplest way to login is to actually login. Just use the monitor and keyboard, or connect with a terminal (ssh) over the network.

Most installations have a root user for maintenance, and at least one normal user for daily use.

Now if you are logged in as a normal user and decide to do some maintenance, you can logout with <ctrl>d and then login as root. Another option is to stay loggedin as the normal user and login on another session as root. On hardware another session is created with <alt>Fn where n is function key 1-12. With a terminal application a second session has to be started.

Sudo

The sudo command is for executing one command as root. From within the session of the normal user execute:

sudo <command>

It will ask for the password of the currently loggedin normal user.

Su

Another option is to become root for a while, while still being loggedin as a normal user

su

The password of root will asked, and after that you are in a session of user root. To leave the root session type <ctrl>d, and you are back in the normal user's session.

The other way around, if loggedin as root, su can be used to start a normal user's session. This can be useful for testing if that user is allowed to do the things you expect.

su <username> 

No password is asked, and to leave the user's session, use <ctrl>d.