Q. How to define the Linux system runlevel and how do I determine which runlevel my system is currently in?

A. You need to use runlevel command to find the current and previous system runlevel. Usaully 0-6 runlevel are used by all Linux distributions:

=> 0 : Halt system

=> 1 : Take system to single-user mode (good for Linux system maintenance)

=> 2 : User defined or distribution like Debian use it

=> 3 : Full multi-user mode (text mode login)

=> 4 : Not used/user-defined

=> 5 : Full multi-user GUI mode login

=> 6 : Reboot system

Please note that runlevels 0, 1, and 6 are reserved. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to get the system down into single user mode.

Task: Determine which runlevel my system is currently in

Type runlevel command:
$ runlevel

Output:

N 3

runlevel command reads the system utmp file (typically /var/run/utmp) to locate the runlevel record, and then prints the previous and current system runlevel on its standard output, separated by a single space. If there is no previous system runlevel, the letter N will be printed instead.

Task: Change runlevel

Use init command to change the runlevel. For example runlevel 1 is used to get the system down into single user mode:
# init 1