Linux Runlevels

Linux systems today generally use eight runlevels. Runlevels
define what services or processes should be running on the
system. The init process can run the system in one of eight
runlevels. The system runs only one of the eight runlevels
at a time. The main runlevels are from 0 – 6. Here's what
each runlevel is for

Runlevel 0: Halt System - To shutdown the system
Runlevel 1: Single user mode
Runlevel 2: Basic multi user mode without NFS
Runlevel 3: Full multi user mode (text based)
Runlevel 4: unused
Runlevel 5: Multi user mode with Graphical User Interface
Runlevel 6: Reboot System

Runlevels 1 and 2 are generally used for debugging purposed
only, and are not used during normal operations. Most desktop
linux distributions boot into runlevel 5, which starts up the
Graphical Login Prompt. This allows the user to use the system
with X-Windows server enabled. Most servers boot into runlevel
3, which starts the text based login prompt.

Linux runlevels can be changed on the fly using the init tool.
If you want to switch from text based operations to the
Graphical Interface, you just have to type in 'telinit 5' in the
root prompt. This will bring up the Graphical Interface in your
system.

Each runlevel can be configured by the system administrator. The
"/etc/inittab" file has information on which runlevel to start
the system at and lists the processes to be run at each runlevel.

Each runlevel has its own directory structure where you can
define the order in which the services start. These directories
are located in the /etc/rc.d/ directory, under which you have
rc1.d, rc2.d, rc3.d…. rc6.d directories where the number from
0 through 6 that corresponds to the runlevel. Inside each directory
are symbolic links that point to master initscripts found in
/etc/init.d or /etc/rc.d/init.d.

You can also change the runlevel at boot time. If your system uses
LILO as the boot manager, you can append the runlevel to the boot
command :
LILO: linux 3 or
LILO: linux 5

If your system uses GRUB, you can change the boot runlevel by
pressing the `e' key to edit the boot configuration. append the
runlevel to the end of the boot command as shown:
kernel /vmlinuz ro root=/dev/hda1 5