Runlevels

Runlevels

The term runlevel refers to a mode of OS initialization in Unix or similar operating systems. Runlevels are sort of like profiles that your computer uses to determine which services to launch in the background when you boot. The most commonly used runlevels are Runlevel 5 and Runlevel 3.

Runlevels may vary from one OS to another. The runlevels in Red Hat Linux are –

0	Halt	Shuts down the machine.
1	Single-User Mode	Typically used for recovery. 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 and provides the BASH shell.
4	Not used/User-definable	For special purposes
5	X11	Full graphical mode managed by X-Server.
6	Reboot	Reboots the computer.

Setting the Runlevel

The runlevel that the system would log into by default is defined in /etc/inittab

#### /etc/inittab ####

init:5:initdefault

This line indicates that the system would log into runlevel 5 by default. Changing the value to something else would also change the default runlevel.