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.

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

About the Author

Vinu Thomas is a consultant on Web design and Internet Technologies.

His website is http://www.vinuthomas.com. You can read more articles

on Linux @

http://www.vinuthomas.com/sections-listarticles-6.html

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