This section is a brief description of the internals of the boot process. It basically covers in detail how the machine boots using SysV Init and the differences between the original init used in older Linux releases.
Init is the program that gets run by the kernel at boot time. It is in charge of starting all the normal processes that need to run at boot time. These include the gettys that allow you to log in, NFS daemons, FTP daemons, and anything else you want to run when your machine boots.
SysV 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.
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 rc0.d rc1.d rc2.d rc3.d rc4.d rc5.d rc6.d
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.
If you want rc.local, it should be in /etc/rc.d. 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:
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 fix it by rebooting and doing:
LILO boot: linux single
This should allow you to boot into single user mode so you can fix inittab.
Now, how does it run all the right scripts? If you enter 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 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 there just 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 init 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 set up 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.