Run levels in Linux

Different Run levels in Linux and how to switch between them

What are different Run levels in Linux and how to switch between them?

Run levels:-Run levels define what processes or services to run automatically while the system boots up. This is defined in /etc/inittab file.

N. B: – 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 and starts 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 or Changing between different runlevels:-

Method-1: Changing run level temporarily without reboot.

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

Linux Runlevels Explained

A runlevel is one of the modes that a Unix -based operating system will run in. Each runlevel has a certain number of services stopped or started, giving the user control over the behavior of the machine. Conventionally, seven runlevels exist, numbered from zero to six.
After the Linux kernel has booted, the init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.

Run Level	Mode	Action
0	Halt	Shuts down system
1	Single-User Mode	Does not configure network interfaces, start daemons, or allow non-root logins
2	Multi-User Mode	Does not configure network interfaces or start daemons.
3	Multi-User Mode with Networking	Starts the system normally.
4	Undefined	Not used/User-definable
5	X11	As runlevel 3 + display manager(X)
6	Reboot	Reboots the system
Standard run levels for Red Hat based distributions

Most Linux servers lack a graphical user interface and therefore start in runlevel 3. Servers with a GUI & desktop Unix systems start runlevel 5.When a server is issued a reboot command, it enters runlevel 6.

Init scripts

Init (short for initialization) is the program on Unix and Unix-like systems that spawns all other processes. It runs as a daemon and typically has PID 1.

The /etc/inittab file is used to set the default run level for the system. This is the runlevel that a system will start up on upon reboot. The applications that are started by init are located in the /etc/rc.d folder. Within this directory there is a separate folder for each run level, eg rc0.d, rc1.d, and so on.

 

 

chkconfig

The chkconfig tool is used in Red Hat based systems (like CentOS) to control what services are started at which runlevels. Running the command chkconfig –list will display a list of services whether they are enabled or disabled for each runlevel.

root@host:~ # chkconfig --list
filelimits 0:off 1:off 2:on 3:on 4:on 5:on 6:off
syslog 0:off 1:off 2:on 3:on 4:on 5:on 6:off
gpm 0:off 1:off 2:on 3:on 4:on 5:on 6:off
cpanel 0:off 1:off 2:off 3:on 4:on 5:on 6:off
kudzu 0:off 1:off 2:off 3:on 4:on 5:on 6:off
ntpd 0:off 1:off 2:off 3:off 4:off 5:off 6:off
netfs 0:off 1:off 2:off 3:on 4:on 5:on 6:off
network 0:off 1:off 2:on 3:on 4:on 5:on 6:off
netplugd 0:off 1:off 2:off 3:off 4:off 5:off 6:off
rawdevices 0:off 1:off 2:off 3:on 4:on 5:on 6:off
ipchains 0:off 1:off 2:on 3:on 4:on 5:on 6:off
iptables 0:off 1:off 2:on 3:on 4:on 5:on 6:off
crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off
anacron 0:off 1:off 2:on 3:on 4:on 5:on 6:off
cups 0:off 1:off 2:on 3:on 4:on 5:on 6:off
xfs 0:off 1:off 2:on 3:on 4:on 5:on 6:off
xinetd 0:off 1:off 2:off 3:on 4:on 5:on 6:off
httpd 0:off 1:off 2:off 3:on 4:off 5:on 6:off

Single User mode is a mode that a multi-user system (like a Linux server) can be booted into the operating system as a superuser. Booting a system into this mode does not start networking, but can be used to make changes to any configuration files on the server. One of the most common usages for single-user mode is to change the root password for a server on which the current password is unknown.

Runlevels are an important part of the core of the Linux operating system. While not something the average administrator will work with on a daily basis, understanding runlevels gives the administrator another layer of control and flexibility over the servers they manage.