# How Do These 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 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.