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I never thought about this before, but I was wondering why they chose runlevel 2. Every other distro and OS I've used default to runlevel 3 (with the exception of AIX which also defaults to runlevel 2).

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I'm confused, I think you want to edit your question. "Every other distro and OS I've used default to runlevel 2" – uther May 26 '12 at 20:11
You're right. That was a typo. – Robert Silvie II May 26 '12 at 20:11
Not an answer to the why but Debian does not differentiate between runlevels 2-5. And if i would have to guess 2 is just the lowest available number so they choose to go with it. – Ulrich Dangel May 26 '12 at 20:16
up vote 9 down vote accepted

The Debian distribution (and hence Ubuntu, which is derived from it) does not define any differences between runlevels 2-5 as a matter of policy. It is up to the local system administrator to make use of runlevels as they see fit.

Since there is no difference between runlevels 2-5, a default runlevel 2 was chosen.

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Most distributions imho default to runlevel 5 (RedHat, Fedora, SLES, OpenSuSE, Ubuntu) - meaning login with runnung X11 and GUI. You have to specify otherwise, if you do not want a GUI.

But the definition of runlevels does not seem to be fixed.

I rember Solaris used runlevel 5 for poweroff, 0 for halt (without power off).

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