INFORMATICS

There are basically 8 runlevels in unix.

Run Level: At any given time, the system is in one of the eight possible run levels. A run level is a configuration software under which only a 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 mode user 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, Init m, Init M: Same as init s.

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

The larger query system can check the level of work by:

$ runlevel

$ who -r

The current level of work can be changed by the superuser using the telinit or init command.

The default working level is stored in /etc/inittab at: initdefault :.