Added run level explanation

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@227 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

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+<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>
+