1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!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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3 | <!ENTITY % general-entities SYSTEM "../general.ent">
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4 | %general-entities;
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5 | ]>
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6 | <sect1 id="ch-scripts-usage">
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7 | <title>How Do These Bootscripts Work?</title>
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8 | <?dbhtml filename="usage.html"?>
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9 |
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10 | <indexterm zone="ch-scripts-usage">
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11 | <primary sortas="a-Bootscripts">Bootscripts</primary>
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12 | <secondary>usage</secondary></indexterm>
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13 |
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14 | <para>Linux uses a special booting facility named SysVinit that is based on a
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15 | concept of <emphasis>run-levels</emphasis>. It can be quite different from one
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16 | system to another, so it cannot be assumed that because things worked in one
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17 | particular Linux distribution, they should work the same in LFS too. LFS has its
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18 | own way of doing things, but it respects generally accepted standards.</para>
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19 |
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20 | <para>SysVinit (which will be referred to as <quote>init</quote> from now on)
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21 | works using a run-levels scheme. There are seven (numbered 0 to 6) run-levels
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22 | (actually, there are more run-levels, but they are for special cases and are
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23 | generally not used. See <filename>init(8)</filename> for more details), and each one
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24 | of those corresponds to the actions the computer is supposed to perform when it
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25 | starts up. The default run-level is 3. Here are the descriptions of the
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26 | different run-levels as they are implemented:</para>
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27 |
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28 | <literallayout>0: halt the computer
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29 | 1: single-user mode
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30 | 2: multi-user mode without networking
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31 | 3: multi-user mode with networking
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32 | 4: reserved for customization, otherwise does the same as 3
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33 | 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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34 | 6: reboot the computer</literallayout>
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35 |
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36 | <para>The command used to change run-levels is <command>init
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37 | <replaceable>[runlevel]</replaceable></command>, where
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38 | <replaceable>[runlevel]</replaceable> is the target run-level. For example, to
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39 | reboot the computer, a user could issue the <command>init 6</command> command,
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40 | which is an alias for the <command>reboot</command> command. Likewise,
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41 | <command>init 0</command> is an alias for the <command>halt</command>
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42 | command.</para>
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43 |
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44 | <para>There are a number of directories under <filename
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45 | class="directory">/etc/rc.d</filename> that look like <filename
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46 | class="directory">rc?.d</filename> (where ? is the number of the run-level) and
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47 | <filename class="directory">rcsysinit.d</filename>, all containing a number of
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48 | symbolic links. Some begin with a <emphasis>K</emphasis>, the others begin with
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49 | an <emphasis>S</emphasis>, and all of them have two numbers following the
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50 | initial letter. The K means to stop (kill) a service and the S means to start a
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51 | service. The numbers determine the order in which the scripts are run, from 00
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52 | to 99—the lower the number the earlier it gets executed. When
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53 | <command>init</command> switches to another run-level, the appropriate services
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54 | are either started or stopped, depending on the runlevel chosen.</para>
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55 |
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56 | <para>The real scripts are in <filename
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57 | class="directory">/etc/rc.d/init.d</filename>. They do the actual
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58 | work, and the symlinks all point to them. Killing links and starting
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59 | links point to the same script in <filename
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60 | class="directory">/etc/rc.d/init.d</filename>. This is because the
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61 | scripts can be called with different parameters like
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62 | <parameter>start</parameter>, <parameter>stop</parameter>,
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63 | <parameter>restart</parameter>, <parameter>reload</parameter>, and
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64 | <parameter>status</parameter>. When a K link is encountered, the
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65 | appropriate script is run with the <parameter>stop</parameter>
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66 | argument. When an S link is encountered, the appropriate script is run
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67 | with the <parameter>start</parameter> argument.</para>
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68 |
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69 | <para>There is one exception to this explanation. Links that start
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70 | with an <emphasis>S</emphasis> in the <filename
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71 | class="directory">rc0.d</filename> and <filename
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72 | class="directory">rc6.d</filename> directories will not cause anything
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73 | to be started. They will be called with the parameter
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74 | <parameter>stop</parameter> to stop something. The logic behind this
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75 | is that when a user is going to reboot or halt the system, nothing
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76 | needs to be started. The system only needs to be stopped.</para>
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77 |
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78 | <para>These are descriptions of what the arguments make the scripts
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79 | do:</para>
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80 |
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81 | <variablelist>
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82 | <varlistentry>
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83 | <term><parameter>start</parameter></term>
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84 | <listitem><para>The service is started.</para></listitem>
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85 | </varlistentry>
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86 |
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87 | <varlistentry>
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88 | <term><parameter>stop</parameter></term>
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89 | <listitem><para>The service is stopped.</para></listitem>
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90 | </varlistentry>
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91 |
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92 | <varlistentry>
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93 | <term><parameter>restart</parameter></term>
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94 | <listitem><para>The service is stopped and then started again.</para></listitem>
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95 | </varlistentry>
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96 |
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97 | <varlistentry>
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98 | <term><parameter>reload</parameter></term>
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99 | <listitem><para>The configuration of the service is updated.
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100 | This is used after the configuration file of a service was modified, when
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101 | the service does not need to be restarted.</para></listitem>
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102 | </varlistentry>
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103 |
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104 | <varlistentry>
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105 | <term><parameter>status</parameter></term>
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106 | <listitem><para>Tells if the service is running and with which PIDs.</para></listitem>
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107 | </varlistentry>
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108 | </variablelist>
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109 |
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110 | <para>Feel free to modify the way the boot process works (after all,
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111 | it is your own LFS system). The files given here are an example of how
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112 | it can be done.</para>
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113 |
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114 | </sect1>
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115 |
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