System V init is fast becoming the standard in the Linux world to control the startup of software at boot time. This is because it is easier to use and more powerful and flexible than the traditional BSD init.
The init binary is located in /sbin
and not /etc
. This
is important as one might try and upgrade a machine to
System V init without re-installing and reformatting. The
linux kernel looks in /etc
for its init first, so you must
make sure and delete your old init from there if any.
SYSV init also differs from BSD init in that the config
files are in a subdirectory of /etc
instead of residing
directly in /etc
. This directory is called rc.d
. In
there you will find rc.sysinit
and the following
directories:
init.d
contains a bunch of scripts. Basically, you need one
script for each service you may need to start at boot time
or when entering another runlevel. Services include things
like networking, nfs, sendmail, httpd, etc. Services do not
include things like setserial that must only be run once and
then exited. Things like that should go in rc.local
or rc.serial
.
rc.local
should be in /etc/rc.d
if you want one. Most systems
include one even though it doesn't do much. You can also
include an rc.serial in /etc/rc.d
if you need to do serial
port specific things at boot time.
The chain of events is as follows:
init
and
runs the first one it findsinit
runs /etc/rc.d/rc.sysinit
rc.sysinit
does a bunch of necessary things and then
runs rc.serial
(if it exists)init
runs all the scripts for the default runlevel.init
runs rc.local
The default runlevel is decided in /etc/inittab
. You should
have a line close to the top like:
id:3:initdefault:
From this, you'd look in the second column and see that the
default runlevel is 3, as should be the case for most systems.
If you want to change it, you can edit /etc/inittab
by hand
and change the 3. Be very careful when you are messing with the
inittab. If you do mess up, you can get in to fix it by
rebooting and doing:
LILO boot: linux single
This should allow you to boot into single user mode so you can fix it.
Now, how does it run all the right scripts? If you do an
ls -l
on rc3.d
, you might see something like:
lrwxrwxrwx 1 root root 17 13:11 S10network -> ../init.d/network
lrwxrwxrwx 1 root root 16 13:11 S30syslog -> ../init.d/syslog
lrwxrwxrwx 1 root root 14 13:32 S40cron -> ../init.d/cron
lrwxrwxrwx 1 root root 14 13:11 S50inet -> ../init.d/inet
lrwxrwxrwx 1 root root 13 13:11 S60nfs -> ../init.d/nfs
lrwxrwxrwx 1 root root 15 13:11 S70nfsfs -> ../init.d/nfsfs
lrwxrwxrwx 1 root root 18 13:11 S75keytable -> ../init.d/keytable
lrwxrwxrwx 1 root root 23 13:11 S80sendmail -> ../init.d/sendmail.init
lrwxrwxrwx 1 root root 18 13:11 S90lpd -> ../init.d/lpd.init
lrwxrwxrwx 1 root root 11 13:11 S99local -> ../rc.local
What you'll notice is that there are no real ``files'' in the directory. Everything there is a link to one of the scripts in the init.d directory. The links also have an ``S'' and a number at the beginning. The ``S'' means to start this particular script and a ``K'' would mean to stop it. The number is just there for ordering purposes. Init will start all the services based on the order they appear. You can duplicate numbers, but it will only confuse you somewhat. You just need to use a two digit number only, along with an upper case ``S'' or ``K'' to start or stop the services you need to.
How does it start and stop services? Simple. Each of the scripts is written to accept an argument which can be ``start'' and ``stop''. You can execute those scripts by hand in fact with a command like:
/etc/rc.d/init.d/httpd.init stop
to stop the httpd server. Init just reads the name and if it has a ``K'', it calls the script with the ``stop'' argument. If it has an ``S'' it calls the script with a ``start'' argument.
Why all these runlevels?
Some people want an easy way to setup machines to be multi-purpose. I could have a ``server'' runlevel that just runs httpd, sendmail, networking, etc. Then I could have a ``user'' runlevel that runs xdm, networking, etc.
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