Runlevels in UNIX

runlevels

There are basically 8 runlevels in Unix. 

Run Level: At any given time, the system is in one of eight possible run levels. A run level is a software configuration under which only a selected group of processes exists. Processes spawned by init for each of these run levels are defined in /etc/inittab. init can be in one of eight run levels, 0-6 and S or s (S and s are identical). The run level changes when a privileged user runs /sbin/init.


init 0 : Shutdown (goes thru the /etc/rc0.d/* scripts then halts)

init 1: Single user mode or emergency mode means no network no multitasking is present in this mode only root has access in this run level

init 2: No network but multitasking support is present.

init 3: Network is present multitasking is present but without GUI.

init 4: It is similar to run level 3; It is reserved for other purposes in research.

init 5: Network is present multitasking and GUI is present with sound etc.

init 6: This run level is defined to system restart.

init s: Tells the init command to enter the maintenance mode. When the

system enters maintenance mode from another run level, only the system console

is used as the terminal.

init S: Same as init s.

init m: Same as init s and init S.

init M: Same as init s or init S or init m.