Init : someone could comment this ?

[Les Mikesell]

Lennart Poettering wrote:

>> What's wrong with numerical runlevels other than Linux flavors starting the 
>> network in level 2 instead of 3?
> 
> They don't make any sense, that's wrong with them.

1 as single user, 2 as multiuser makes reasonable sense, with networking 
as the logical next step.

> Why only 6 of them?

Until someone decided to make X something special you only needed 3 
steps that you could jump among: single user, multiuser, 
multiuser+network.  But with a scheme based on directory names and the 
inherent sort order of a wildcard expansion, there initially wasn't a 
real limit.

> I never understood why a process like "shutdown" or "reboot" should be
> considered a "level". Do you?

runlevel 0 = quit sounds extremely logical.  6=reboot was someone just 
running out of ideas.

> Nobody knows what the actually mean.

Unless they read a SysV manual back when they fit in your pocket?

> They are using numeric ids for no
> reason.

Their sort order is well known.

> I mean, people invented stuff like DNS for not having to
> deal with random numbers that often. Why should they deal with random
> numbers when dealing with init systems, then?
> 
> Almost all distributions use only 2 or 3 of them. 
> 
> Their configuration is plain awful.
> 
> They're totally awkward, because there are numeric levels and the
> magic runlevel S.

Well, yeah - but then there is q also - and necessarily so...

> Also, init doesn't really have any information what service is running
> in which one is not for the current runlevel.

That's the point.  Init should not know anything specifically except for 
the stuff it manages directly per inittab.

 > To work around that the
> configuration is highly redundant with lots of symlinks.

Which lets it do anything it needs to do, finding out only when it needs 
to know.  All very good things.

> Then, on the philosophical level, having a single set of "runlevels"
> just doesn't cut it, because services these days should only be
> started maybe when a certain HW is around, or when some specific user
> software runs that wants to make use of it. How do runlevels fit into
> that? They are too static to reflect this dynamic way of starting and
> stopping services properly.
> 
> In short, they don't make any sense. They should die.

They make sense for what they do - otherwise how can I tell the system I 
want to go down to single user mode with a clear definition of what will 
still be running?  And I should be able to tell it I want multiuser mode 
but with no networking, but Linux distros screw that up.   But init 
isn't limited to those, it also does things specified in inittab.  If 
you want magic stuff to happen on certain events, respawn the thing that 
  detects the event starting at the desired runlevel and let that 
program do whatever is supposed to happen next.  Getty watching for a 
carrier detect hardware event is the original example - and it doesn't 
need to know anything except what to exec next and doesn't need to wait 
around for results.  Processes are supposed to be cheap enough that if 
you need one you can use it - don't make init need to know anything else.

-- 
   Les Mikesell
    lesmikesell at gmail.com