1 | <sect1 id="ch07-usage">
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2 | <title>How does the booting process with these scripts work?</title>
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3 |
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4 | <para>
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5 | Linux uses a special booting facility named SysVinit. It's based on a
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6 | concept of <emphasis>runlevels</emphasis>. It can be widely different
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7 | from one system to another, so don't assume that because things
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8 | worked in <insert distro name> they should work like that in LFS
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9 | too. LFS has it's own way of doing things, but it respects generally
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10 | accepted standards.
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11 | </para>
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12 |
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13 | <para>
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14 | SysVinit (which we'll call <emphasis>init</emphasis> from now on) works
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15 | using a runlevels scheme. There are 7 (from 0 to 6) runlevels
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16 | (actually, there are more runlevels but they are for special cases and
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17 | generally not used. Read the init man page for those details), and each
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18 | one of those corresponds to the things you want your computer to do when
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19 | it starts up. The default runlevel is 3. Here are the descriptions of the
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20 | different runlevels as they are often implemented:
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21 | </para>
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22 |
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23 | <literallayout>
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24 | 0: halt the computer
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25 | 1: single-user mode
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26 | 2: multi-user mode without networking
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27 | 3: multi-user mode with networking
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28 | 4: reserved for customization, otherwise does the same as 3
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29 | 5: same as 4, it is usually used for GUI login (like X's xdm or KDE's kdm)
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30 | 6: reboot the computer
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31 | </literallayout>
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32 |
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33 | <para>
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34 | The command used to change runlevels is <userinput>init
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35 | <runlevel></userinput> where <runlevel> is
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36 | the target runlevel. For example, to reboot the computer, you'd issue
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37 | the init 6 command. The reboot command is just an alias, as is the halt
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38 | command an alias to init 0.
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39 | </para>
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40 |
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41 | <para>
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42 | The /etc/init.d/rcS script is run at every startup of the computer,
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43 | before any runlevel is executed and runs the scripts listed in
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44 | /etc/rcS.d
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45 | </para>
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46 |
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47 | <para>
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48 | There are a number of directories under /etc that look like like rc?.d
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49 | where ? is the number of the runlevel and rcS.d. Take a look at one of
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50 | them (after you finish this chapter that is, right now there's nothing
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51 | there yet). There are a number of symbolic links. Some begin with an K,
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52 | the others begin with an S, and all of them have three numbers following
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53 | the initial letter. The K means to stop (kill) a service, and the S means
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54 | to start a service. The numbers determine the order in which the scripts
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55 | are run, from 000 to 999; the lower the number the sooner it gets
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56 | executed. When init switches to another runlevel, the appropriate
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57 | services get killed and others get started.
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58 | </para>
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59 |
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60 | <para>
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61 | The real scripts are in /etc/init.d. They do all the work, and the
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62 | symlinks all point to them. You'll note that killing links and starting
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63 | links point to the same script in /etc/init.d. That's because the scripts
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64 | can be called with different parameters like start, stop, restart, reload,
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65 | status. When a K link is encountered, the appropriate script is run with
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66 | the stop argument. When a S link is encountered, the appropriate script
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67 | is run with the start argument.
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68 | </para>
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69 |
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70 | <para>
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71 | These are descriptions of what the arguments make the scripts do:
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72 | </para>
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73 |
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74 | <itemizedlist>
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75 |
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76 | <listitem><para>
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77 | <emphasis>start</emphasis>: The service is started.
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78 | </para></listitem>
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79 |
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80 | <listitem><para>
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81 | <emphasis>stop</emphasis>: The service is stopped.
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82 | </para></listitem>
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83 |
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84 | <listitem><para>
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85 | <emphasis>restart</emphasis>: The service is stopped and then started again.
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86 | </para></listitem>
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87 |
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88 | <listitem><para>
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89 | <emphasis>reload</emphasis>: The configuration of the service is updated.
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90 | Use this after you have modified the configuration file of a service, when
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91 | you don't need/want to restart the service.
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92 | </para></listitem>
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93 |
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94 | <listitem><para>
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95 | <emphasis>status</emphasis>: Tells you if the service is running and with
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96 | which PID's.
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97 | </para></listitem>
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98 |
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99 | </itemizedlist>
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100 |
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101 | <para>
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102 | Feel free to modify the way the boot process works (after all it's your
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103 | LFS system, not ours). The files here are just an example of how you
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104 | can do it in a nice way (well what we consider nice anyway. You may
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105 | hate it).
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106 | </para>
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107 |
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108 | </sect1>
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109 |
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