source: chapter07/usage.xml@ 9966f76

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<sect1 id="ch-scripts-usage">
<title>How do these bootscripts work?</title>
<?dbhtml filename="usage.html"?>

<indexterm zone="ch-scripts-usage">
<primary sortas="a-Bootscripts">Bootscripts</primary>
<secondary>usage</secondary></indexterm>

<para>Linux uses a special booting facility named SysVinit. It's based on a
concept of <emphasis>run-levels</emphasis>. It can be widely different 
from one system to another, so it can't be assumed that because things 
worked in &lt;insert distro name&gt; they should work like that in LFS 
too. LFS has its 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 run-levels scheme. There are 7 (from 0 to 6) run-levels
(actually, there are more run-levels but they are for special cases and 
generally not used. The <command>init</command> man page describes those details), and each
one of those corresponds to the things the computer is supposed to do when 
it starts up. The default run-level is 3. Here are the descriptions of the 
different run-levels 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 run-levels is <command>init
&lt;runlevel&gt;</command> where &lt;runlevel&gt; is the target run-level. For
example, to reboot the computer, a user would issue the <userinput>init
6</userinput> command. The <command>reboot</command> command is just an alias for
it, as is the <command>halt</command> command an alias for <command>init
0</command>.</para>

<para>There are a number of directories under <filename class="directory">/etc/rc.d</filename>
that look like like <filename class="directory">rc?.d</filename> (where ? is the 
number of the run-level) and <filename class="directory">rcsysinit.d</filename>
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 sooner it gets executed. When init switches to another run-level, the
appropriate services get killed and others get started.</para>

<para>The real scripts are in <filename class="directory">/etc/rc.d/init.d</filename>.
They do all the work, and the symlinks all point to them. Killing links and starting links 
point to the same script in <filename class="directory">/etc/rc.d/init.d</filename>.
That's because the scripts can be called with different parameters like 
<parameter>start</parameter>, <parameter>stop</parameter>, 
<parameter>restart</parameter>, <parameter>reload</parameter>,
<parameter>status</parameter>. When a K link is encountered, the appropriate 
script is run with the <parameter>stop</parameter> argument. When an S link is 
encountered, the appropriate script is run with the <parameter>start</parameter> 
argument.</para>

<para>There is one exception. 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 <parameter>stop</parameter> to stop
something. The logic behind it is that when you are going to reboot or
halt the system, you don't want to start anything, only stop the
system.</para>

<para>These are descriptions of what the arguments make the 
scripts do:</para>

<variablelist>
<varlistentry>
<term><parameter>start</parameter></term>
<listitem><para>The service is started.</para></listitem>
</varlistentry>

<varlistentry>
<term><parameter>stop</parameter></term>
<listitem><para>The service is stopped.</para></listitem>
</varlistentry>

<varlistentry>
<term><parameter>restart</parameter></term>
<listitem><para>The service is stopped and then started again.</para></listitem>
</varlistentry>

<varlistentry>
<term><parameter>reload</parameter></term>
<listitem><para>The configuration of the service is updated.
This is used after the configuration file of a service was modified, when 
the service doesn't need to be restarted.</para></listitem>
</varlistentry>

<varlistentry>
<term><parameter>status</parameter></term>
<listitem><para>Tells if the service is running and with which PIDs.</para></listitem>
</varlistentry>
</variablelist>

<para>Feel free to modify the way the boot process works (after all, it's your 
own LFS system). The files given here are just an example of how it can be 
done in a nice way (well, what we consider nice -- you may hate it).</para>

</sect1>