Run Levels in Linux (inittab):

So
here goes another article for Linux. “ Run Levels in Linux”

“Runlevels” as the name suggests is a type of Linux Operating systemyou want to boot when you start a computer like text mode environment
, graphical mode, etc. In linux we can switch to another run level as
per your need from the CLI. Run levels define what processes or services to run automatically while
the system boots up. This is defined in /etc/inittab file.

The init process is the last step in the boot procedure and has pid of
‘1’.’ “init” is responsible for starting system processes
as per defined in the /etc/inittab file.

“init” process checks which default run level is defined in
/etc/inittab andstarts the system in that run level which means all the services
defined for that run level gets executed.

So, in the above screenshot for my CentOS, it shows that the default
init level is 5 which means that whenever I boot my system it will
boot in graphical mode. We can change the default mode as per our
requirement. The file is stored in /etc/inittab and can be changed
using vi editor.

“init” process checks which default run level is defined in
/etc/inittab andstarts the system in that run level which means all the services
defined for that run level gets executed.

There are 7 different run levels present (run level 0-6) in Linux system
for different purpose. The descriptions are given below.

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

Most desktop Linux distributions boot into run level 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 run level 3,
which starts the text based login prompt as it is advisable not to
install graphical windows in a server as lots of space goes waste and
also it takes lot of resource to run.

Each run level is defined inside its own directory structure. These
directories are located in the /etc/rc.d/ directory, under which you
have rc1.d, rc2.d… rc6.d directories where the number from 0 to 6
corresponds to the specific run level. Inside each directory symbolic
links are defined to a  to master initscripts found in
/etc/init.d or /etc/rc.d/init.d.

Switching between different runlevels:-

Method-1:
Changing run level without reboot (temporarily).

We
can use init command to change rune levels without rebooting the
system.

Ex:-if
we are currently in run level 3 and want to go to run level 1, just
we need to execute

#
init 1

Or
if you want to shutdown a machine you can take help of run level ‘0’
.Just you need to execute

#init
0

Remember
this change is not permanent and on next reboot you will get your
default runlevel.

Method-2:
Changing run level permanently

If
you want to change your default run level then

Open
the file /etc/inittab and edit entry initdefault:

#
vi /etc/inittab

Let’s
set initdefault to 5, so that you can boot to X next time when Linux
comes up:

id:5:initdefault:

Method-3:-Change
run level at boot time

You
can also change the run level at boot time. If your system uses LILO
as the boot manager, you can append the run level 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 run level(in
our case 5) to the end of the boot command as shown:

kernel
/vmlinuz-2.6.18-164.el5 ro root=LABEL=/ rhgb quiet  5