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 LFS too. LFS 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
During the kernel initialization, the first program that is run is
either specified on the command line or, by default init. This program reads the
initialization file /etc/inittab.
Create this file with:
cat > /etc/inittab << "EOF"
# Begin /etc/inittab
id:3:initdefault:
si::sysinit:/etc/rc.d/init.d/rc S
l0:0:wait:/etc/rc.d/init.d/rc 0
l1:S1:wait:/etc/rc.d/init.d/rc 1
l2:2:wait:/etc/rc.d/init.d/rc 2
l3:3:wait:/etc/rc.d/init.d/rc 3
l4:4:wait:/etc/rc.d/init.d/rc 4
l5:5:wait:/etc/rc.d/init.d/rc 5
l6:6:wait:/etc/rc.d/init.d/rc 6
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
su:S016:once:/sbin/sulogin
1:2345:respawn:/sbin/agetty --noclear tty1 9600
2:2345:respawn:/sbin/agetty tty2 9600
3:2345:respawn:/sbin/agetty tty3 9600
4:2345:respawn:/sbin/agetty tty4 9600
5:2345:respawn:/sbin/agetty tty5 9600
6:2345:respawn:/sbin/agetty tty6 9600
# End /etc/inittab
EOF
An explanation of this initialization file is in the man page for
inittab. For LFS, the key
command that is run is rc. The intialization file above
will instruct rc to
run all the scripts starting with an S in the /etc/rc.d/rcsysinit.d directory followed by all
the scripts starting with an S in the /etc/rc.d/rc?.d directory where the question mark
is specified by the initdefault value.
As a convenience, the rc script reads a library of
functions in /lib/lsb/init-functions.
This library also reads an optional configuration file,
/etc/sysconfig/rc.site. Any of the
system configuration file parameters described in subsequent
sections can be alternatively placed in this file allowing
consolidation of all system parameters in this one file.
As a debugging convenience, the functions script also logs all
output to /run/var/bootlog. Since the
/run directory is a tmpfs, this file
is not persistent across boots, however it is appended to the more
permanent file /var/log/boot.log at
the end of the boot process.
Changing run-levels is done with 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. K links and S 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 LFS system). The files given here are an example of how it can be done.