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git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@4648 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689
119 lines
5.1 KiB
XML
119 lines
5.1 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../general.ent">
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%general-entities;
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]>
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<sect1 id="ch-scripts-usage">
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<title>How Do These Bootscripts Work?</title>
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<?dbhtml filename="usage.html"?>
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<indexterm zone="ch-scripts-usage">
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<primary sortas="a-Bootscripts">Bootscripts</primary>
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<secondary>usage</secondary></indexterm>
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<para>Linux uses a special booting facility named SysVinit that is
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based on a concept of <emphasis>run-levels</emphasis>. It can be quite
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different from one system to another, so it cannot be assumed that
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because things worked in <insert distro name>, they should work
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the same in LFS too. LFS has its own way of doing things, but it
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respects generally accepted standards.</para>
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<para>SysVinit (which will be referred to as <quote>init</quote> from
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now on) works using a run-levels scheme. There are seven (from 0 to 6)
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run-levels (actually, there are more run-levels, but they are for
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special cases and are generally not used. The init man page describes
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those details), and each one of those corresponds to the actions the
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computer is supposed to perform when it starts up. The default
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run-level is 3. Here are the descriptions of the different run-levels
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as they are 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 <command>xdm</command> or KDE's <command>kdm</command>)
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6: reboot the computer</literallayout>
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<para>The command used to change run-levels is <command>init
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<replaceable>[runlevel]</replaceable></command>, where
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<replaceable>[runlevel]</replaceable> is the target run-level. For
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example, to reboot the computer, a user would issue the <command>init
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6</command> command. The <command>reboot</command> command is an
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alias for it, as is the <command>halt</command> command an alias for
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<command>init 0</command>.</para>
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<para>There are a number of directories under <filename
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class="directory">/etc/rc.d</filename> that look like <filename
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class="directory">rc?.d</filename> (where ? is the number of the
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run-level) and <filename class="directory">rcsysinit.d</filename>, all
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containing a number of symbolic links. Some begin with a
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<emphasis>K</emphasis>, the others begin with an
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<emphasis>S</emphasis>, and all of them have two numbers following the
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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
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scripts are run, from 00 to 99—the lower the number the earlier it
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gets executed. When init switches to another run-level, the
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appropriate services get killed and others get started.</para>
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<para>The real scripts are in <filename
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class="directory">/etc/rc.d/init.d</filename>. They do the actual
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work, and the symlinks all point to them. Killing links and starting
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links point to the same script in <filename
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class="directory">/etc/rc.d/init.d</filename>. This is because the
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scripts can be called with different parameters like
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<parameter>start</parameter>, <parameter>stop</parameter>,
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<parameter>restart</parameter>, <parameter>reload</parameter>, and
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<parameter>status</parameter>. When a K link is encountered, the
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appropriate script is run with the <parameter>stop</parameter>
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argument. When an S link is encountered, the appropriate script is run
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with the <parameter>start</parameter> argument.</para>
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<para>There is one exception to this explanation. Links that start
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with an <emphasis>S</emphasis> in the <filename
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class="directory">rc0.d</filename> and <filename
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class="directory">rc6.d</filename> directories will not cause anything
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to be started. They will be called with the parameter
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<parameter>stop</parameter> to stop something. The logic behind this
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is that when a user is going to reboot or halt the system, nothing
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needs to be started. The system only needs to be stopped.</para>
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<para>These are descriptions of what the arguments make the scripts
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do:</para>
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<variablelist>
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<varlistentry>
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<term><parameter>start</parameter></term>
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<listitem><para>The service is started.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>stop</parameter></term>
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<listitem><para>The service is stopped.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>restart</parameter></term>
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<listitem><para>The service is stopped and then started again.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>reload</parameter></term>
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<listitem><para>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 does not need to be restarted.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>status</parameter></term>
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<listitem><para>Tells if the service is running and with which PIDs.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>Feel free to modify the way the boot process works (after all,
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it is your own LFS system). The files given here are an example of how
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it can be done.</para>
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</sect1>
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