System Run Levels


  • At bootup, init reads /etc/inittab file and executes all scripts for default run level.
    On Unix, there are 7 conventional run levels (0,1,2,3,4,5,6). However, 10 levels are possible (0 -- 9).

  • On Debian, 
    
Runlevel  0  - is halt.
Runlevel  1  - is single-user.
Runlevels 2-5  are multi-user.
Runlevel  6  - is reboot.

  • On the other Linux systems, such as RedHat, 
    
Runlevel  0 - halt 
Runlevel  1 - Single user mode
Runlevel  2 - Multiuser, without NFS (The same as 3, if you do not have networking)
Runlevel  3 - Full multiuser mode
Runlevel  4 - unused
Runlevel  5 - X11
Runlevel  6 - reboot

    File /etc/inittab specifues what init should do at startup. 
    
# The default runlevel (2 in this  case).
id:2:initdefault:
    Entries in /etc/inittab take the following format: 
    
id:run-level:action:process
    id: a unique identifier
    run-level: 0 -- 6
    action: sysinit - run process at bootup
    once - run process once upon entering specific runlevel
    respawn - restart process whenever it is terminated
    wait - like "once", but awaits completion of process before continuing
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