What are the various runlevels in linux OS?

Following are the run-levels which you could refer:

0 — Halt

1 — Single-user mode

2 — not used (user-definable)

3 — full multiuser mode

4 — not used (user-definable)

5 — full multiuser mode (with an X-based login screen)

6 — Reboot

 

If you use a command line login screen, then you are in run level 3.

 

If you use a graphical login screen, you are operating in run level 5.

The default run level can be changed by editing the /etc/inittab file.

 

It contains a line near the top of the file similar to the following:

Id:5:initdefault:

Change the number in this line to the desired run level.

 

You will have to reboot the system for the changes to take effect.