LeRoy Cressy on 2 Nov 2005 01:34:47 -0000

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Re: [PLUG] Editing System V init

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Stephen Gran wrote:
> On Tue, Nov 01, 2005 at 06:11:09PM -0500, Art Alexion said:
>>By default, my distro tries to run an ntp update script before it
>>initializes ppp.  The documentation on the distro web site shows you how
>>to disable the ntp init script, but I don't want to do that.  I want to
>>edit the startup so that it runs _after_ ppp is initialized.
>>I installed the webmin init module, but need to read about the runlevel
>>and 'start at' and 'stop at' settings before I make any changes.  Google
>>hasn't helped me find documentation.  Can anyone recommend a place?
> Without knowing more about what your distro is, I invite you,
> semi-facetiously, to investigate the lovely command 'mv'.  The beauty of
> a simple
> mv /etc/rc2.d/S23ntp /etc/rc2.d/S53ntp
> beats all of webmin, for me.
> On a more serious note, if you know what your distros default run level
> is, just push the ntp start link to something slightly later than the
> ppp start link.  That is the simplest solution.
> The way it works, roughly (and totally solution dependant, but most
> linux distros do it this way these days) is that the kernel boots, and
> at some point hands thing off to init.  init runs all of the scripts
> that start with S in the rcS.d directory, and then switches to the
> 'default' run level - what that default is is completely distribution
> dependant, although I think Redhat-alikes all use 5.
> Then init runs all scripts in rc5.d directory that begin with an S with
> the start argument (and really, it should also run all scripts that begin
> with a K with the stop argument, but many don't).  The only real things
> to know about the hacked SysV init that most linux distros use is that
> a) scripts whose names start with S should get run with the start
>    argument
> b) scripts whose names start with K should get run with the stop
>    argument  
> c) All scripts in a directory are run in numerical order
> d) At boot, S is first, then default run level.
> That's the basics for managing it.
I have seen sever questions about linux startup in the past and I think
that it is a good time to answer some general questions.

1.	Kernel loads init which reads /etc/inittab
2.	/etc/inittab references to the startup script with a lines
	l0:0:wait:/etc/init.d/rc 0
	l1:1:wait:/etc/init.d/rc 1
	l2:2:wait:/etc/init.d/rc 2
	l3:3:wait:/etc/init.d/rc 3
	l4:4:wait:/etc/init.d/rc 4
	l5:5:wait:/etc/init.d/rc 5
	l6:6:wait:/etc/init.d/rc 6
	# Normally not reached, but fallthrough in case of emergency.

	/etc/init.d/rc is a shell script which starts the startup		
	programs in order.  Reading both inittab and the rc script
	will teach you how the system starts.  It is also good to
	read the man page for init which gives the details of the
	boot process.

Steve gave the quick info you wanted, but if you switch distros sometime
in the future, you will need to see where that distro puts the init
scripts.  It seems that each distro is slitely different.
- --
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