what are init 0 init 1 init 2 init 3 init 4 init 5 init 6 init s init S init m

The best solution to know about these init levels is to understand the " man init " command output on Unix.

There are basically 8 runlevels in unix. I will briefly tell some thing about the different init levels and their use.

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 runlevel

init 2 : No network but multitasking support is present .

init 3 : Network is present multitasking is present but with out GUI .

init 4 : It is similar to runlevel 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 runlevel 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.

We can take it from above that 4 options(S,s,M,m) are synonymous.