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111 lines
3.9 KiB
XML
111 lines
3.9 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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<para>
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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 not 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 it's own way of doing things, but it respects generally
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accepted standards.
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</para>
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<para>
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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:
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</para>
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<literallayout>
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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
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</literallayout>
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<para>
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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.
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</para>
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<para>
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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
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</para>
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<para>
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There are a number of directories under /etc that look like like rc?.d
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where ? is the number of the runlevel and rcS.d. A user might take a look
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at one of
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them (after this chapter is finished, right now there's nothing
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there yet). There are a number of symbolic links. Some begin with an K,
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the others begin with an S, and all of them have three numbers following
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the initial letter. The K means to stop (kill) a service, and the S means
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to start a service. The numbers determine the order in which the scripts
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are run, from 000 to 999; the lower the number the sooner it gets
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executed. When init switches to another runlevel, the appropriate
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services get killed and others get started.
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</para>
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<para>
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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
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links point to the same script in /etc/init.d. That's because the scripts
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can be 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.
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</para>
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<para>
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These are descriptions of what the arguments make the scripts do:
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</para>
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<itemizedlist>
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<listitem><para>
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<emphasis>start</emphasis>: The service is started.
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</para></listitem>
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<listitem><para>
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<emphasis>stop</emphasis>: The service is stopped.
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</para></listitem>
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<listitem><para>
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<emphasis>restart</emphasis>: The service is stopped and then started again.
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</para></listitem>
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<listitem><para>
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<emphasis>reload</emphasis>: The configuration 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.
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</para></listitem>
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<listitem><para>
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<emphasis>status</emphasis>: Tells if the service is running and with
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which PID's.
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</para></listitem>
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</itemizedlist>
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<para>
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Feel free to modify the way the boot process works (after all it's the users
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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
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in a nice way (well what we consider nice anyway. A user may hate it).
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</para>
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</sect1>
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