Debian/Ubutnu run levels(Using inittab)

In ubuntu, you can't find any file name inittab under /etc.
ubuntu call it as UpStart(http://upstart.ubuntu.com/)

First, we look at the run levels in ubuntu


* 0 - Halt
* 1 - Single
* 2 - Full multi-user with display manager (GUI)
* 3 - Full multi-user with display manager (GUI)
* 4 - Full multi-user with display manager (GUI)
* 5 - Full multi-user with display manager (GUI)
* 6 - Reboot

refer the following link and look at the section 10.5

http://www.debian.org/doc/FAQ/ch-customizing.en.html

If you want to know the current runlevel, just give

zoobave@zoobave-desktop:~$ runlevel
N 5

There are two ways are available to change the current runlevel
I)use the telinit command

zoobave@zoobave-desktop:~$ sudo telinit 3

II) Create/Edit the /etc/initab file and change the the initdefault value in the
id:3:initdefault entry

# /etc/inittab: init( configuration.
# $Id: inittab,v 1.91 2002/01/25 13:35:21 miquels Exp $

# The default runlevel.
id:3:initdefault:

# Boot-time system configuration/initialization script.
# This is run first except when booting in emergency (-b) mode.
si::sysinit:/etc/init.d/rcS

# What to do in single-user mode.
~~:S:wait:/sbin/sulogin

# /etc/init.d executes the S and K scripts upon change
# of runlevel.
#
# Runlevel 0 is halt.
# Runlevel 1 is single-user.
# Runlevels 2-5 are multi-user.
# Runlevel 6 is reboot.

l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
l4:4:wait:/etc/init.d/rc 4
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6
# Normally not reached, but fallthrough in case of emergency.
z6:6:respawn:/sbin/sulogin

# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

# Action on special keypress (ALT-UpArrow).
#kb::kbrequest:/bin/echo "Keyboard Request--edit /etc/inittab to let this work."

# What to do when the power fails/returns.
pf:owerwait:/etc/init.d/powerfail start
pn:owerfailnow:/etc/init.d/powerfail now
po:owerokwait:/etc/init.d/powerfail stop

# /sbin/getty invocations for the runlevels.
#
# The "id" field MUST be the same as the last
# characters of the device (after "tty").
#
# Format:
# :::
#
# Note that on most Debian systems tty7 is used by the X Window System,
# so if you want to add more getty's go ahead but skip tty7 if you run X.
#
1:2345:respawn:/sbin/mingetty --noclear --autologin matt tty1
2:23:respawn:/sbin/mingetty tty2
3:23:respawn:/sbin/mingetty tty3
4:23:respawn:/sbin/mingetty tty4
5:23:respawn:/sbin/mingetty tty5
6:23:respawn:/sbin/mingetty tty6

But, there is no difference between runlevels 2,3,4 and 5. If you want to customize, add/remove the entries in /etc/rcN.d/ directory
For, example, if you don't want to start your X server during runlevel 3, just remove the S13gdm file under /etc/rc3.d/ directory

zoobave@zoobave-desktop:~$ sudo rm /etc/rc5.d/S13gdm