Linux uses a special booting facility named SysVinit that is based on a concept of run-levels. It can be quite different from one system to another, so it cannot be assumed that because things worked in one particular Linux distribution, they should work the same in CLFS too. CLFS has its own way of doing things, but it respects generally accepted standards.
SysVinit (which will be referred to as “init” from now on) works using a run-levels
scheme. There are seven (numbered 0 to 6) run-levels (actually, there
are more run-levels, but they are for special cases and are generally
not used. See init(8) for more
details), and each one of those corresponds to the actions the
computer is supposed to perform when it starts up. The default
run-level is 3. Here are the descriptions of the different run-levels
as they are implemented:
0: halt the computer
1: single-user mode
2: multi-user mode without networking
3: multi-user mode with networking
4: reserved for customization, otherwise does the same as 3
5: same as 4, it is usually used for GUI login (like X's xdm or KDE's kdm)
6: reboot the computer
The command used to change run-levels is init [runlevel], where
[runlevel] is the target
run-level. For example, to reboot the computer, a user could issue
the init 6 command,
which is an alias for the reboot command. Likewise,
init 0 is an alias for
the halt command.
There are a number of directories under /etc/rc.d that look like rc?.d (where ? is the number of the run-level) and
rcsysinit.d, all containing a number of
symbolic links. Some begin with a K, the others begin with an S, and all of them have two numbers
following the initial letter. The K means to stop (kill) a service
and the S means to start a service. The numbers determine the order
in which the scripts are run, from 00 to 99—the lower the
number the earlier it gets executed. When init switches to another run-level,
the appropriate services are either started or stopped, depending on
the runlevel chosen.
The real scripts are in /etc/rc.d/init.d. They do the actual work, and the
symlinks all point to them. Killing links and starting links point to
the same script in /etc/rc.d/init.d.
This is because the scripts can be called with different parameters
like start, stop, restart,
reload, and status. When a K link is encountered, the appropriate
script is run with the stop argument.
When an S link is encountered, the appropriate script is run with the
start argument.
There is one exception to this explanation. Links that start with an
S in the rc0.d and rc6.d
directories will not cause anything to be started. They will be
called with the parameter stop to stop
something. The logic behind this is that when a user is going to
reboot or halt the system, nothing needs to be started. The system
only needs to be stopped.
These are descriptions of what the arguments make the scripts do:
start
The service is started.
stop
The service is stopped.
restart
The service is stopped and then started again.
reload
The configuration of the service is updated. This is used after the configuration file of a service was modified, when the service does not need to be restarted.
status
Tells if the service is running and with which PIDs.
Feel free to modify the way the boot process works (after all, it is your own CLFS system). The files given here are an example of how it can be done.