Changing runlevels with init and telinit
The init or telinit command sends signals to the executing init process, instructing it to change to a specified runlevel. You must be logged in as the superuser to use the init command.
Generally, you will use a runlevel change for the following reasons:
To shut down the system using runlevel 0
To go to single-user mode using runlevel 1
To reboot the system using runlevel 6