1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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4 | <!ENTITY % general-entities SYSTEM "../general.ent">
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5 | %general-entities;
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6 | ]>
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7 |
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8 | <sect1 id="ch-scripts-usage">
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9 | <?dbhtml filename="usage.html"?>
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10 |
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11 | <title>How Do These Bootscripts Work?</title>
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12 |
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13 | <indexterm zone="ch-scripts-usage">
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14 | <primary sortas="a-Bootscripts">Bootscripts</primary>
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15 | <secondary>usage</secondary>
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16 | </indexterm>
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17 |
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18 | <para>Linux uses a special booting facility named SysVinit that is based on a
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19 | concept of <emphasis>run-levels</emphasis>. It can be quite different from one
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20 | system to another, so it cannot be assumed that because things worked in one
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21 | particular Linux distribution, they should work the same in LFS too. LFS has its
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22 | own way of doing things, but it respects generally accepted standards.</para>
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23 |
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24 | <para>SysVinit (which will be referred to as <quote>init</quote> from now on)
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25 | works using a run-levels scheme. There are seven (numbered 0 to 6) run-levels
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26 | (actually, there are more run-levels, but they are for special cases and are
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27 | generally not used. See <filename>init(8)</filename> for more details), and
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28 | each one of those corresponds to the actions the computer is supposed to
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29 | perform when it starts up. The default run-level is 3. Here are the
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30 | descriptions of the different run-levels as they are implemented:</para>
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31 |
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32 | <literallayout>0: halt the computer
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33 | 1: single-user mode
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34 | 2: multi-user mode without networking
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35 | 3: multi-user mode with networking
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36 | 4: reserved for customization, otherwise does the same as 3
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37 | 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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38 | 6: reboot the computer</literallayout>
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39 |
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40 | <para>The command used to change run-levels is <command>init
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41 | <replaceable><runlevel></replaceable></command>, where
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42 | <replaceable><runlevel></replaceable> is the target run-level. For example, to
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43 | reboot the computer, a user could issue the <command>init 6</command> command,
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44 | which is an alias for the <command>reboot</command> command. Likewise,
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45 | <command>init 0</command> is an alias for the <command>halt</command>
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46 | command.</para>
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47 |
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48 | <para>There are a number of directories under <filename
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49 | class="directory">/etc/rc.d</filename> that look like <filename
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50 | class="directory">rc?.d</filename> (where ? is the number of the run-level) and
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51 | <filename class="directory">rcsysinit.d</filename>, all containing a number of
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52 | symbolic links. Some begin with a <emphasis>K</emphasis>, the others begin with
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53 | an <emphasis>S</emphasis>, and all of them have two numbers following the
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54 | initial letter. The K means to stop (kill) a service and the S means to start a
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55 | service. The numbers determine the order in which the scripts are run, from 00
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56 | to 99—the lower the number the earlier it gets executed. When
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57 | <command>init</command> switches to another run-level, the appropriate services
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58 | are either started or stopped, depending on the runlevel chosen.</para>
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59 |
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60 | <para>The real scripts are in <filename
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61 | class="directory">/etc/rc.d/init.d</filename>. They do the actual work, and
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62 | the symlinks all point to them. Killing links and starting links point to
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63 | the same script in <filename class="directory">/etc/rc.d/init.d</filename>.
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64 | This is because the scripts can be called with different parameters like
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65 | <parameter>start</parameter>, <parameter>stop</parameter>,
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66 | <parameter>restart</parameter>, <parameter>reload</parameter>, and
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67 | <parameter>status</parameter>. When a K link is encountered, the appropriate
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68 | script is run with the <parameter>stop</parameter> argument. When an S link
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69 | is encountered, the appropriate script is run with the
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70 | <parameter>start</parameter> argument.</para>
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71 |
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72 | <para>There is one exception to this explanation. Links that start
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73 | with an <emphasis>S</emphasis> in the <filename
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74 | class="directory">rc0.d</filename> and <filename
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75 | class="directory">rc6.d</filename> directories will not cause anything
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76 | to be started. They will be called with the parameter
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77 | <parameter>stop</parameter> to stop something. The logic behind this
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78 | is that when a user is going to reboot or halt the system, nothing
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79 | needs to be started. The system only needs to be stopped.</para>
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80 |
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81 | <para>These are descriptions of what the arguments make the scripts
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82 | do:</para>
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83 |
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84 | <variablelist>
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85 |
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86 | <varlistentry>
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87 | <term><parameter>start</parameter></term>
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88 | <listitem>
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89 | <para>The service is started.</para>
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90 | </listitem>
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91 | </varlistentry>
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92 |
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93 | <varlistentry>
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94 | <term><parameter>stop</parameter></term>
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95 | <listitem>
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96 | <para>The service is stopped.</para>
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97 | </listitem>
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98 | </varlistentry>
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99 |
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100 | <varlistentry>
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101 | <term><parameter>restart</parameter></term>
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102 | <listitem>
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103 | <para>The service is stopped and then started again.</para>
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104 | </listitem>
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105 | </varlistentry>
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106 |
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107 | <varlistentry>
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108 | <term><parameter>reload</parameter></term>
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109 | <listitem>
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110 | <para>The configuration of the service is updated.
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111 | This is used after the configuration file of a service was modified, when
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112 | the service does not need to be restarted.</para>
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113 | </listitem>
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114 | </varlistentry>
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115 |
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116 | <varlistentry>
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117 | <term><parameter>status</parameter></term>
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118 | <listitem>
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119 | <para>Tells if the service is running and with which PIDs.</para>
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120 | </listitem>
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121 | </varlistentry>
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122 |
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123 | </variablelist>
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124 |
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125 | <para>Feel free to modify the way the boot process works (after all,
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126 | it is your own LFS system). The files given here are an example of how
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127 | it can be done.</para>
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128 |
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129 | </sect1>
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