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init - Switching System Runlevels
The init command, followed by a number "#", is used to switch the system's runlevel. In most distributions, only the superuser is allowed to execute this command. The definition of runlevels might vary across different distributions. You can refer to the init --helpdocumentation for specific details. However, there is a general consensus as follows:
Runlevel 0: Shutdown
Runlevel 1: Single-user maintenance mode (only root can log in)
Runlevel 3: Text mode
Runlevel 5: Graphical interface mode
Runlevel 6: Reboot
Example::
$ init 1←Entering maintenance mode (runlevel=1)
For additional init command examples, you can check out this link.