lfs-book/chapter07/usage.xml

<sect1 id="ch07-usage">
<title>How does the booting process with these scripts work?</title>

<para>
Linux uses a special booting facility named SysVinit. It's based on a
concept of <emphasis>runlevels</emphasis>. It can be widely different 
from one system to another, so  don't assume that because things 
worked in &lt;insert distro name&gt; they should work like that in LFS 
too. LFS has it's own way of doing things, but it respects generally 
accepted standards.
</para>

<para>
SysVinit (which we'll call <emphasis>init</emphasis> from now on) works 
using a runlevels scheme. There are 7 (from 0 to 6) runlevels (actually
there are runlevels but they are for special cases and generally not used. 
Read the init man page for those details), and each one of those 
corresponds to the things you want your computer to do when it starts
up. The default runlevel is 3. Here are the descriptions of the different 
runlevels as they are often implemented:
</para>

<literallayout>
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
</literallayout>

<para>
The command used to change runlevels is <userinput>init
&lt;runlevel&gt;</userinput> where &lt;runlevel>&gt; is 
the target runlevel. For example, to reboot the computer, you'd issue
the init 6 command. The reboot command is just an alias, as is the halt
command an alias to init 0.
</para>

<para>
The /etc/init.d/rcS script is run at every startup of the computer,
before any runlevel is executed and runs the scripts listed in
/etc/rcS.d
</para>

<para>
There are a number of directories under /etc that look like like rc?.d
where ? is the number of the runlevel and rcS.d. Take a look at one of 
them (after you finish this chapter that is, right now there's nothing
there yet). There are a number of symbolic links. Some begin with an K, 
the others begin with an S, and all of them have three 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 000 to 999; the lower the number the sooner it gets
executed. When init switches to another runlevel, the appropriate 
services get killed and others get started.
</para>

<para>
The real scripts are in /etc/init.d. They do all the work, and the
symlinks all point to them. You'll note that killing links and starting 
links point to the same script in /etc/init.d. That's because the scripts 
can be called with different parameters like start, stop, restart, reload, 
status. When a K link is encountered, the appropriate script is run with 
the stop argument. When a S link is encountered, the appropriate script 
is run with the start argument.
</para>

<literallayout>
These are descriptions of what the arguments make the scripts do:
<emphasis>start</emphasis>: The service is started.
<emphasis>stop</emphasis>: The service is stopped.
<emphasis>restart</emphasis>: The service is stopped and then started again.
<emphasis>reload</emphasis>: The configuration of the service is updated. 
Use this after you have modified the configuration file of a service, when 
you don't need/want to restart the service.
<emphasis>status</emphasis>: Tells you if the service is running and with 
which PID's
</literallayout>

<para>
Feel free to modify the way the boot process works (after all it's your
LFS system, not ours). The files here are just an example of how you
can do it in a nice way (well what we consider nice anyway. You may
hate it).
</para>

</sect1>