What is Runlevel and how to change it

Linux runlevel controls what are the processes & services to be started automatically by the system or Init when system start. The runlevel are from 0 to 6. The standard Linux kernel supports seven different runlevels, as shown below.

Runlevel  0 : Shutdown the system or halt

Runlevel  1 : Single-user or troubleshooting  mode.

Runlevel  2 : Multi-user mode without NFS.

Runlevel  3 : Multi-user mode with NFS.

Runlevel  4 : Unused, Undefined 

Runlevel  5 : Graphical  or X11 user interface.

Runlevel  6 : Reboot the system.

Find Out Current Run Level Command

Type the following command:
# who -r

#runlevel

How to Change Run Level 

Use the init command to change rune levels:
# init 1

On most Linux server system default run level is 3 and on most Linux Desktop system default run level is 5. The default run level is defined by the initdefault line at  /etc/inittab file.  To change the default run level, edit /etc/inittab file,

# vi /etc/inittab

Set initdefault to 5 for X Windows

id:5:initdefault: