Run level in Linux

A runlevel is a preset operating state on a Unix-like operating system. A system can be booted into (i.e., started up into) any of several runlevels, each of which is represented by a single digit integer. Each runlevel designates a different system configuration and allows access to a different combination of processes (i.e., instances of executing programs).

There are differences in the runlevels according to the operating system. Seven runlevels are supported in the standard Linux kernel (i.e., core of the operating system).

If you are moving to higher run levels, you may make additional services available to users, while moving to a lower run level will causes to services (daemons) to become unavailable. On the production server run level 3 is the normally used and rarely changed. However, some administrative tasks require the administrator to move system to run level 1 i.e single user mode.

Find current run level

[root@linux1 ~]# who -r

         run-level 5  2016-05-31 17:41                   last=S

[root@linux1 ~]#

You can also runlevel command to find out your current run level.

[root@linux1 ~]# runlevel

N 5

[root@linux1 ~]#

Runlevel and Its Usage

The Init is the parent of all processes with PID # 1. Its primary purpose is to create processes from a script stored in the file /etc/inittab file. This file usually has entries which cause init to spawn gettys on each line that users can log in. A runlevel is nothing but a software configuration of the Linux system which allows only a selected group of processes to exist. The processes spawned by init for each of these runlevels are defined in the /etc/inittab file. Init can be in one of eight runlevels as follows:

·         Runlevel 0 is halt

·         Runlevel 1 is single-user

·         Runlevels 2-5 are multi-user (some distro uses RUN level 5 to start X [KDE/Gnome])

·         Runlevel 6 is for rebooting system

Note: telinit or init can be used to change the run level but you will lost the connection, so think twice before changing the runlevel command.

By default Linux boots either to runlevel 3 or to runlevel 5. The former permits the system to run all services except for a GUI. The latter allows all services including a GUI.

For example, typing the init 3 command will move system to run level 3:

# init 3

Default runlevel. The runlevels used by RHS 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)

It is always wise to make a backup copy of /etc/inittab or any other configuration file before attempting to modify it manually.

# vi /etc/inittab

Set initdefault to 5, so that you can boot to X next time when Linux comes up:

id:5:initdefault:

Save and close the file. Reboot the system to see changes:

# reboot