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What I understand from runlevels is that in each level some programs have permission to be executed and some don't. runlevel 3 boots system into CLI and runlevel 5 starts the default GUI.

But I don't understand what do 0(shutdown) and 6(reboot) mean?(Start in shutdown/reboot mode?!). What happens If set initdefault to these values?

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If you start in these, the system will shut down/reboot as soon as it enters the runlevel. A runlevel is essentially just a way of specifying actions you want to take when you enter/leave a certain state, in that respect, once those runlevels are entered they execute programs that prepare the computer to shut down or reboot, respectively.

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You can check this out first hand in


If a script name starts with an "S", it runs at startup; if it starts with a "K", it runs at shutdown (or, when moving down a level). Check out this answer and especially the comments (corrections) made by James O'Gorman.

Edit - be sure to check out Dave Sherohman's comment below.

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I think it would be clearer to say that "S" scripts run when the system enters that runlevel and "K" scripts run when it leaves that runlevel (by entering a different one). "Startup" and "shutdown" sound like you're talking about system startup/shutdown, which is not the case here. –  Dave Sherohman Nov 15 '12 at 11:01
Hej! Yes, I agree. Made an edit. –  Emanuel Berg Nov 15 '12 at 16:42
I still think it is unclear. Moving from runlevel 1 to 2 (in Debian, that's traditionally the transition from single-user to multi-user-without-X) runs the runlevel 1 K scripts (followed by the runlevel 2 S scripts), but certainly doesn't move "down a level". Same for runlevel 3 or 5 to 6, or 3 to 0, or any of a myriad other combinations. It is much better to simply reason about what scripts are run when entering (S, if any exist) or exiting (K, again if any exist) a specific runlevel. –  Michael Kjörling Nov 15 '12 at 19:37
@MichaelKjörling: Yes: "S" for enter (you could think of "start" although it could originally be something else - well, doesn't matter as long as you remember it), and "K" for exit ("kill"). This seems to be straightforward enough, so I'll make an effort to be less "relaxed" when I describe things in the future. –  Emanuel Berg Nov 15 '12 at 19:49
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