The Runlevels in Linux

Many Windows users are familiar with different modes of execution of their operating system:
a. Safe mode
b. Command prompt only
c. Safe mode with network support
d. Safe mode with vga

And other such options that are presented during the boot-up process. Similarly, Linux has different modes of executing as well, which are known as runlevels. But unlike Windows, we can change runlevels on a fly.

Definition for runlevel : The term runlevel refers to a mode of operation in one of the computer operating system that implement Unix System V-style initialization. This system replaces the traditional /etc/rc and /etc/rc.local scripts used in BSD UNIX. Conventionally, seven runlevels exist, numbered from zero to six, plus an alias runlevel named ‘S’ and/or ‘s’.

Runlevels control services started by the initialization process. The number of runlevels and services started on those runlevels varies with Linux distributions. Information about the runlevels of a particular distribution is listed in file /etc/inittab.

The contents of the file include the following:

D	Name	Description
0	Halt	Shuts down the system.
1	Single-user Mode	Mode for administrative tasks.
2	Multi-user Mode	Does not configure network interfaces and does not export networks services.
3	Multi-user Mode with Networking	Starts the system normally.
4	Not used/User-definable	For special purposes.
5	Start the system normally with appropriate display manager. ( with GUI )	As runlevel 3 + display manager.
6	Reboot	Reboots the system.