RUNLEVEL

1. How to check current runlevel on which you are logged in:

who -r

or

runlevel

2. Check services which are enabled in which runlevel:

chkconfig

3. What are the runlevels existing in a Linux system configuration and its meanings:

ID	Name	Description
0	Halt	Shuts down the system.
1	Single-user Mode	Mode for administrative tasks.[2][2]
2	Multi-user Mode	Does not configure network interfaces and does not export networks services.[3]
3	Multi-user Mode with Networking	Starts the system normally.[4]
4	Not used/User-definable	For special purposes.
5	Start the system normally with appropriate display manager. ( with GUI )	Same as runlevel 3 + display manager.
6	Reboot	Reboots the system.

Runlevels design changes and it depends on the Operating system that we are using like UBUNTU, REDHAT.

4. How to change the runlevel via command line without rebooting:

telenit <runlevel>

or

init <runlevel>