[H-GEN] Re: init scripts
Andrae Muys
A.Muys at mailbox.uq.edu.au
Mon Jul 28 22:53:15 EDT 1997
On Tue, 29 Jul 1997, Peter McCarthy wrote:
> Hi how do the damn init scripts in Red Hat work. My goal is to get
> named running. It apears in the rc.initd dir but what is the mechanism
> to get it off the ground at boot ?
>
(aside: Please, it makes it easier to read if you format your mail for 80
columns, easier to reply if you use 75.)
Please note that RedHat provides a section in it's Linux Tips on the boot
process. You can read it at
<http://www.redhat.com/support/docs/rhl/Tips/Boot-Process-Tips/Boot-Process-Tips.html>
This post is quite long, so I've reproduced the quick summary at the end
here so you don't _have_ to read the whole thing unless you want to :).
-------------------------------------------------------------------------
Remember...
/etc/inittab - Is the root of the intialisation sequence.
/etc/rc.d/rc - Is the common base of all runlevel configuration scripts.
/etc/rc.d/init.d/ - Is where the physical scripts lie.
/etc/rc.d/rc?.d/ - Contain the appropriate symbolic links to start/stop
services.
/etc/sysconfig/ - Contains the configuration information used by the rc.d
scripts to initialise the system, and the services/daemons run on it.
-------------------------------------------------------------------------
OK, my system is a standard RedHat 4.0 install, patched. So I'll draw on
my own init files as a generic example.
So a normal reboot entails...
'In the beginning was the POST, then the bootloader, then the kernel
bootstrap. Much deep magic is performed to summon the kernel from the
depths of the root device.' :) Then as soon as the kernel get's it's act
together it spawns 'init' and slips into the background (dealing with
interrupts, and system calls and such). init then looks through inittab
for the default runlevel, and the system initialisation scripts and finds
id:3:initdefault:
Which specifies that the default run-level should be 3.
si::sysinit:/etc/rc.d/rc.sysinit
Which means that init should run /etc/rc.d/rc.sysinit when initialising
the system (ie now).
Now would be a good idea to have a look at rc.sysinit, notice the line
if [ -f /etc/sysconfig/network ]; then
. /etc/sysconfig/network
Now /etc/sysconfig/network is where you configure your basic networking.
Mine contains
NETWORKING=yes
HOSTNAME=reynier.meeting.humbug.org.au
GATEWAYDEV=eth0
GATEWAY=192.168.105.1
Note that the effect of the 'if' statement in rc.sysinit is to define
these 4 values as shell variables in the script. This is common trick
used throughout RedHats initialisation scripts. In fact you'll find that
the files under /etc/sysconfig/ are regularly included in this manner in
the rc.d scripts.
In the case of rc.sysinit, the only one they are interested in is
HOSTNAME. Read through the script and you should find you recognise much
of it from the bootup messages which flash past everytime you reboot.
You'll also spot some interesting bit's and pieces, especially how to
diable the fsck's without actually modifiying a single script. (As this
isn't really a good idea, I'll leave it as an exercise for the reader :).
OK, back to inittab now. init has finished initialising the system
(/etc/rc.d/rc.sysinit), and it is now ready to change to the correct
runlevel. Now it is my belief that there are 8 run levels.
0 - Which halts the system.
1 - Which is single user mode, should drop you to a single root shell (ie
it is one good reason for physical security if you want a secure machine).
2 - Multi-user without NFS mounts (IIRC limited networking?? Real
sysadmins can discuss the use of 2, I don't really know enough about it).
3 - Is full multi-user, with all the trimmings. (Your default should
probably be this or 5)
4 - Typically unused (ie often defaults to 3).
5 - X, often runs xdm etc, set the default run level to 5 if you want X to
run automatically.
6 - reboot.
The eighth is actually S, which is startup. ie this is what we've been in
so far. So because the default run-level in this case is 3
(id:3:initdefault:) we are actually changing from runlevel S to runlevel
3.
OK, now look further down /etc/inittab till you see a group of lines
similar to the following...
l0:0:wait:/etc/rc.d/rc 0
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6
Note that this section is VERY different to the BSD init model used by
Slackware and many older distributions.
There are a few things which should be recognised here.
a) All runlevel changes are listed as 'wait'. This means that init will
wait until the config script for that runlevel has finished before
continuing.
b) Under RedHat (ie under the SysVinit model) one script handles ALL
run level changes.
c) That script is called /etc/rc.d/rc and it takes the new runlevel as the
first and only arguement.
So lets go have a look at this script.
Unfortunately my scripting ability isn't exactly crash hot, so I get a
little confused by the mess at the top, however the end result is to set
the variable 'runlevel' to the new runlevel we are changing to.
So we continue to the important part (If someone would be willing to
explain to me exactly how those five lines work I'd appreciate it).
if [ -d /etc/rc.d/rc$runlevel.d ]; then
This checks to see if there is a directory called '/etc/rc.d/rc[0123456].d'
In this case as we are going to runlevel 3, it is looking for a directory
called '/etc/rc.d/rc3.d'.
If there isn't such a directory then you skip all the way to the 'fi' on
the last line (ie the script finishes there). If there is (and if you do
a `ls /etc/rc.d`, you'll note that these directories do exist), then it
proceeds to shutdown those process'/services which are not required in the
new runlevel.
The key lines in this section are
for i in /etc/rc.d/rc$runlevel.d/K*; do
this loops through once for every file in /etc/rc.d/rc3.d which begins
with a 'K'.
subsys=${i#/etc/rc.d/rc$runlevel.d/K??}
[ ! -f /var/lock/subsys/$subsys ] && continue
If the specific subsystem is actually up at the moment then ...
$i stop
it executes the script (`/etc/rc.d/rc?.d/K*`) passing 'stop' as the first
parameter.
So, in other words, there is a subdirectory for every runlevel under rc.d.
And when you change runlevels it automatically executes every file in that
directory beginning with a K asking to to 'stop'.
The second half should be obvious now,
for i in /etc/rc.d/rc$runlevel.d/S*; do
# Check if the script is there.
[ ! -f $i ] && continue
# Check if the subsystem is already up.
subsys=${i#/etc/rc.d/rc$runlevel.d/S??}
[ -f /var/lock/subsys/$subsys ] && continue
Exactly the same as the first half, only now we are looking for files
beginning with a 'S'.
$i start
And instead of telling it to 'stop', we tell it to 'start'.
So hey presto we now have a simple way to start and stop different
process', daemons, jobs', etc each time we change runlevels. The only
thing that matters is whether the first letter is a K or an S.
Now this would be powerful, but the SysVinit model goes further, so
naturally RedHat does to.
do an `ls -l /etc/rc.d/rc2.d`
lrwxrwxrwx 1 root root 15 Mar 9 04:21 K10nfsfs -> ../init.d/nfsfs
lrwxrwxrwx 1 root root 18 Mar 9 04:21 S01keytable -> ../init.d/keytable
lrwxrwxrwx 1 root root 17 Mar 9 04:21 S10network -> ../init.d/network
lrwxrwxrwx 1 root root 16 Mar 9 04:24 S30syslog -> ../init.d/syslog
.
.
V
Note that they are all symbolic links! That's right, every single file in
/etc/rc.d/rc?.d/ is a symbolic link. And they all point to scripts in
/etc/rc.d/init.d. So in other words if you want to change how the network
is brought up (for instance), you only have to change ONE file
(/etc/rc.d/init.d/network) and you have changed the config for EVERY
runlevel which uses networking..
Now compare `ls -l /etc/rc.d/rc1.d` with `ls -l /etc/rc.d/rc3.d` (ie
single user mode, with normal multi-user mode).
`ls -l /etc/rc.d/rc1.d`
lrwxrwxrwx 1 root root 15 Mar 9 04:21 K10nfsfs -> ../init.d/nfsfs
lrwxrwxrwx 1 root root 13 Mar 9 11:45 K15gpm -> ../init.d/gpm
lrwxrwxrwx 1 root root 23 Mar 9 11:30 K30sendmail -> ../init.d/sendmail.init
lrwxrwxrwx 1 root root 14 Mar 9 11:51 K50inet -> ../init.d/inet
`ls -l /etc/rc.d/rc3.d` (I've changed the order for clarity).
lrwxrwxrwx 1 root root 15 Mar 9 04:21 S70nfsfs -> ../init.d/nfsfs
lrwxrwxrwx 1 root root 13 Mar 9 11:45 S85gpm -> ../init.d/gpm
lrwxrwxrwx 1 root root 23 Mar 9 11:30 S80sendmail -> ../init.d/sendmail.init
lrwxrwxrwx 1 root root 14 Mar 9 11:51 S50inet -> ../init.d/inet
Notice that both the START and the STOP scripts are simply LINKS TO THE
*SAME SCRIPT*.
Remember we pass 'start' or 'stop' to the script when we run it. So we
can use the SAME SCRIPT for both starting and stopping the service.
Have a look in a few of the scripts under /etc/rc.d/init.d, you'll see
that they follow the same basic format.
[Basic initialisation]
[Check for basic error conditions]
[if ($1 == start) then start service]
[if ($1 == stop then stop service]
[Any necessary clean up]
[exit]
If you look in '/etc/rc.d/init.d/inet', you'll see this basic format.
Take note of the first two lines (excluding comments), especially
. /etc/sysconfig/network
As I said before, this is the standard way RedHat init scripts handle
common config info.
Well I hope I haven't been too long winded, and if you've read this far
thank-you for your time, I hope it has been helpful. You should be able
to find your way around RedHat initialisation reasonably well now.
Remember...
/etc/inittab - Is the root of the intialisation sequence.
/etc/rc.d/rc - Is the common base of all runlevel configuration scripts.
/etc/rc.d/init.d/ - Is where the physical scripts lie.
/etc/rc.d/rc?.d/ - Contain the appropriate symbolic links to start/stop
services.
/etc/sysconfig/ - Contains the configuration information used by the rc.d
scripts to initialise the system, and the services/daemons run on it.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Andrae Muys
andrae at humbug.org.au
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