source: BOOK/bootscripts/common/usage.xml @ 3f8be484

clfs-1.2clfs-2.1clfs-3.0.0-systemdclfs-3.0.0-sysvinitsystemdsysvinit
Last change on this file since 3f8be484 was 3f8be484, checked in by Jim Gifford <clfs@…>, 13 years ago

r627@server (orig r625): jim | 2005-10-31 12:59:34 -0800
Import of Cross-LFS Book

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1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
3  "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
4  <!ENTITY % general-entities SYSTEM "../../general.ent">
5  %general-entities;
6]>
7
8<sect1 id="ch-scripts-usage">
9  <?dbhtml filename="usage.html"?>
10
11  <title>How Do These Bootscripts Work?</title>
12
13  <indexterm zone="ch-scripts-usage">
14    <primary sortas="a-Bootscripts">Bootscripts</primary>
15  <secondary>usage</secondary></indexterm>
16
17  <para>Linux uses a special booting facility named SysVinit that is
18  based on a concept of <emphasis>run-levels</emphasis>. It can be quite
19  different from one system to another, so it cannot be assumed that
20  because things worked in one particular Linux distribution, they should work
21  the same in LFS too. LFS has its own way of doing things, but it
22  respects generally accepted standards.</para>
23
24  <para>SysVinit (which will be referred to as <quote>init</quote> from
25  now on) works using a run-levels scheme. There are seven (numbered 0 to 6)
26  run-levels (actually, there are more run-levels, but they are for
27  special cases and are generally not used. See <filename>init(8)</filename>
28  for more details), and each one of those corresponds to the actions the
29  computer is supposed to perform when it starts up. The default
30  run-level is 3. Here are the descriptions of the different run-levels
31  as they are implemented:</para>
32
33<literallayout>0: halt the computer
341: single-user mode
352: multi-user mode without networking
363: multi-user mode with networking
374: reserved for customization, otherwise does the same as 3
385: same as 4, it is usually used for GUI login (like X's <command>xdm</command> or KDE's <command>kdm</command>)
396: reboot the computer</literallayout>
40
41  <para>The command used to change run-levels is <command>init
42  <replaceable>[runlevel]</replaceable></command>, where
43  <replaceable>[runlevel]</replaceable> is the target run-level. For example,
44  to reboot the computer, a user could issue the <command>init 6</command>
45  command, which is an alias for the <command>reboot</command> command.
46  Likewise, <command>init 0</command> is an alias for the
47  <command>halt</command> command.</para>
48
49  <para>There are a number of directories under <filename
50  class="directory">/etc/rc.d</filename> that look like <filename
51  class="directory">rc?.d</filename> (where ? is the number of the
52  run-level) and <filename class="directory">rcsysinit.d</filename>, all
53  containing a number of symbolic links. Some begin with a
54  <emphasis>K</emphasis>, the others begin with an
55  <emphasis>S</emphasis>, and all of them have two numbers following the
56  initial letter. The K means to stop (kill) a service and the S means
57  to start a service. The numbers determine the order in which the
58  scripts are run, from 00 to 99&mdash;the lower the number the earlier it
59  gets executed. When <command>init</command> switches to another run-level,
60  the appropriate services are either started or stopped, depending on the
61  runlevel chosen.</para>
62
63  <para>The real scripts are in <filename
64  class="directory">/etc/rc.d/init.d</filename>. They do the actual work,
65  and the symlinks all point to them. Killing links and starting links point
66  to the same script in <filename class="directory">/etc/rc.d/init.d</filename>.
67  This is because the scripts can be called with different parameters like
68  <option>start</option>, <option>stop</option>, <option>restart</option>,
69  <option>reload</option>, and <option>status</option>. When a K link is
70  encountered, the appropriate script is run with the <option>stop</option>
71  argument. When an S link is encountered, the appropriate script is run
72  with the <option>start</option> argument.</para>
73
74  <para>There is one exception to this explanation. Links that start
75  with an <emphasis>S</emphasis> in the <filename
76  class="directory">rc0.d</filename> and <filename
77  class="directory">rc6.d</filename> directories will not cause anything
78  to be started. They will be called with the parameter
79  <option>stop</option> to stop something. The logic behind this
80  is that when a user is going to reboot or halt the system, nothing
81  needs to be started. The system only needs to be stopped.</para>
82
83  <para>These are descriptions of what the arguments make the scripts
84  do:</para>
85
86  <variablelist>
87    <varlistentry>
88      <term><option>start</option></term>
89      <listitem>
90        <para>The service is started.</para>
91      </listitem>
92    </varlistentry>
93
94    <varlistentry>
95      <term><option>stop</option></term>
96      <listitem>
97        <para>The service is stopped.</para>
98      </listitem>
99    </varlistentry>
100
101    <varlistentry>
102      <term><option>restart</option></term>
103      <listitem>
104        <para>The service is stopped and then started again.</para>
105      </listitem>
106    </varlistentry>
107
108    <varlistentry>
109      <term><option>reload</option></term>
110      <listitem>
111        <para>The configuration of the service is updated. This is used
112        after the configuration file of a service was modified, when the
113        service does not need to be restarted.</para>
114      </listitem>
115    </varlistentry>
116
117    <varlistentry>
118      <term><option>status</option></term>
119      <listitem>
120        <para>Tells if the service is running and with which PIDs.</para>
121      </listitem>
122    </varlistentry>
123  </variablelist>
124
125  <para>Feel free to modify the way the boot process works (after all,
126  it is your own LFS system). The files given here are an example of how
127  it can be done.</para>
128
129</sect1>
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