mirror of https://github.com/tLDP/LDP
93 lines
4.1 KiB
XML
93 lines
4.1 KiB
XML
<sect1 id="ch07-usage">
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<title>How does the booting process with these scripts work?</title>
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<?dbhtml filename="usage.html" dir="chapter07"?>
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<para>Linux uses a special booting facility named SysVinit. It's based on a
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concept of <emphasis>runlevels</emphasis>. It can be widely different
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from one system to another, so it can't be assumed that because things
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worked in <insert distro name> they should work like that in LFS
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too. LFS has its own way of doing things, but it respects generally
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accepted standards.</para>
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<para>SysVinit (which we'll call <emphasis>init</emphasis> from now on) works
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using a runlevels scheme. There are 7 (from 0 to 6) runlevels
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(actually, there are more runlevels but they are for special cases and
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generally not used. The init man page describes those details), and each
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one of those corresponds to the things the computer is supposed to do when
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it starts up. The default runlevel is 3. Here are the descriptions of the
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different runlevels as they are often implemented:</para>
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<literallayout>0: halt the computer
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1: single-user mode
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2: multi-user mode without networking
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3: multi-user mode with networking
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4: reserved for customization, otherwise does the same as 3
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5: same as 4, it is usually used for GUI login (like X's xdm or KDE's kdm)
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6: reboot the computer</literallayout>
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<para>The command used to change runlevels is <userinput>init
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<runlevel></userinput> where <runlevel> is
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the target runlevel. For example, to reboot the computer, a user would issue
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the init 6 command. The reboot command is just an alias, as is the halt
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command an alias to init 0.</para>
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<para>The /etc/init.d/rcS script is run at every startup of the computer,
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before any runlevel is executed and runs the scripts listed in
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/etc/rcS.d</para>
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<para>There are a number of directories under /etc that look like rc?.d
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where ? is the number of the runlevel and rcS.d which contain a number of
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symbolic links. Some begin with an K, the others begin with an S, and all
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of them have three numbers following the initial letter. The K means to
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stop (kill) a service, and the S means to start a service. The numbers
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determine the order in which the scripts are run, from 000 to 999; the
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lower the number the sooner it gets executed. When init switches to
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another runlevel, the appropriate services get killed and others get
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started.</para>
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<para>The real scripts are in /etc/init.d. They do all the work, and the
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symlinks all point to them. Killing links and starting links point to
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the same script in /etc/init.d. That's because the scripts can be
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called with different parameters like start, stop, restart, reload,
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status. When a K link is encountered, the appropriate script is run with
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the stop argument. When a S link is encountered, the appropriate script
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is run with the start argument.</para>
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<para>There is one exception. Links that start with an S in the
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rc0.d and rc6.d directories will not cause anything to be started. They
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will be called with the parameter <emphasis>stop</emphasis> to stop
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something. The logic behind it is that when you are going to reboot or
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halt the system, you don't want to start anything, only stop the
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system.</para>
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<para>These are descriptions of what the arguments make the
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scripts do:</para>
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<itemizedlist>
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<listitem><para><emphasis>start</emphasis>: The service is
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started.</para></listitem>
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<listitem><para><emphasis>stop</emphasis>: The service is
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stopped.</para></listitem>
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<listitem><para><emphasis>restart</emphasis>: The service is
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stopped and then started again.</para></listitem>
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<listitem><para><emphasis>reload</emphasis>: The configuration
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of the service is updated.
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This is used after the configuration file of a service was modified, when
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the service doesn't need to be restarted.</para></listitem>
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<listitem><para><emphasis>status</emphasis>: Tells if the service
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is running and with which PID's.</para></listitem>
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</itemizedlist>
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<para>Feel free to modify the way the boot process works (after all it's your
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LFS system, not ours). The files here are just an example of how it can be
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done in a nice way (well what we consider nice anyway. You may hate it).</para>
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</sect1>
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