Runlevels in Linux

Run-level defines the state in which O.S. and its services are running.

There are total 7 run-levels in linux and system can be booted in any of these run-levels.

Run-level	Description
0	Halt / Shutdown
1	Single User Mode
2	Multi User without NFS
3	Full Multiuser
4	Unused
5	X11 (Graphical Mode)
6	Reboot

By default linux boots in either run-level 3 or 5

                       

The init program is used to handle all the run-levels and its process id is 1.

You can also check the process id of init using below command

[root@server]$ ps -aux | grep init

root         1  0.0  0.1  31032  1880 ?        Ss   Sep30   0:19 /sbin/init  

The /etc/inittab file contains the information regarding the run-levels.

Issue the command cat /etc/inittab

                       

            # Default runlevel. The runlevels used are:

#   0 - halt (Do NOT set initdefault to this)

#   1 - Single user mode

#   2 - Multiuser, without NFS (The same as 3, if you do not have networking)

#   3 - Full multiuser mode

#   4 - unused

#   5 - X11

#   6 - reboot (Do NOT set initdefault to this)

#

id:5:initdefault:                      

Here, see the last line in file “id:5:initdefault:” which states that the run-level of the system is 5. You can also change the run-level to 3 or 1 as per your convenience but keep in mind that these changes are permanent.

To change the run-level temporarily simply you can type the command “init run-level” i.e. init 3.

How to  check the current run-level ?

runlevel command is used to check the current run-level.

[root@server]$ runlevel

N 3

Here, N means previous run-level and 3 is the current run-level.

You can also use the command who –r

[root@server]$ who -r

         run-level 3  2014-09-30 04:23