Linux Logging in, Logging Out, and Shutting down
Once you have completed your system install and booted your system, you should see a login prompt on your monitor. When you did your Linux install you should have set a root password. You may have also created a user with a password. Therefore to log in, you will want to type the name of a user or "root" for the login name and enter the appropriate password. If you logged in as a normal user and know the root password and want to use administration commands, you may use the command "su" to become a "super user". Some systems also support the "sudo" command, which allows administrative privileges on a command by command basis.
Linux Shell levels and the su command
The command, "su" will allow a normal user to enter a new shell level as the root user or as another user if they know the root user's or that user's password respectively. To become the root user, type "su" then you will be prompted for the root password. To become another user, type "su username". You must enter either that user's password to become that user. Every time you use the su command you enter a new shell level which means you have invoked a new running copy of the shell program, such as bash. You can see this change by typing the command "env" and looking at the value of the environment variable "SHLVL". This value increments when you use the su command and decrements when you use the "exit" command to exit that shell environment. You can also see the shell level value by typing "printenv SHLVL".
Use the command "logout" to exit a given session. If you have logged in, then typed "su" to become a superuser or another user, you may need to type "exit" until your SHLVL environment value is 1. Then you can type "logout" to exit your session. The "exit" command will take you back to previous shell levels.
Shutting Linux Down
The system is intended to be shutdown by the system administrator using the shutdown command in one of the forms shown below. Many systems are set up to capture the <CTRL><ALT><DEL> keystroke combination to issue the shutdown command through the init program. This will work on most systems if the root user is logged in. Examples of using the shutdown command are shown below.
shutdown -h now
shutdown -r +10 "Rebooting in 10 minutes"
shutdown -r 13:00