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I'm reading about SysV init and I see there are levels 0-6.

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I understand that level 4 could be defined by the OS operator for special purposes, but I'm having trouble thinking of any examples.

Has anyone seen runlevel 4 used in the wild? What was it used for? I assume a trigger for changing the runlevel to 4 also had to be created, since the system would never automatically go into level 4?

Philip Kirkbride
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    Slackware uses runlevel 4 for [multiuser with display manager](http://www.slackware.com/config/init.php), but if you’re only interested in systems strictly adhering to the LSB definition, that’s not relevant. – Stephen Kitt Jan 02 '20 at 16:44
  • @StephenKitt none the less, I find that interesting. It's as if they thought level 4 was pointless and shifted 5 to 4. It may be pointless for me to specify sysV, as `systemd` still makes use of these same run levels. – Philip Kirkbride Jan 02 '20 at 16:46
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    Slackware is older than the LSB, I don’t know off-hand if there was a “reference runlevel 5” when runlevel 4 was chosen for this in Slackware ;-). – Stephen Kitt Jan 02 '20 at 16:48
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    [JdeBP’s page on the topic](http://jdebp.eu/FGA/system-5-rc-problems.html) suggests there wasn’t. – Stephen Kitt Jan 02 '20 at 17:01
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    Strictly, this is the van Smoorenburg clone that you are asking about, as that's what that table applies to. Actual AT&T Unix System 5 `init`/`rc` was a completely different kettle of fish. Note that Wikipedia is iffy on this subject, as is all too common. – JdeBP Jan 02 '20 at 17:49
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    This topic may be better over on the [Retrocomputing SE](https://retrocomputing.stackexchange.com/). There aren't any major Unices still using SysVInit. – Warren Young Jan 02 '20 at 20:45

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