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Created Jul 30, 2020
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Runlevels in UNIX
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.
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