7.7. How Do the System V Bootscripts Work?

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

7.7.1. Configuring Sysvinit

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/rcS.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.

7.7.2. Changing Run Levels

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.