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 | <?dbhtml filename="usage.html" dir="chapter07"?>
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4 |
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5 | <para>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 it can not be assumed 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.</para>
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11 |
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12 | <para>SysVinit (which we'll call <emphasis>init</emphasis> from now on) works
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13 | using a runlevels scheme. There are 7 (from 0 to 6) runlevels
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14 | (actually, there are more runlevels but they are for special cases and
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15 | generally not used. The init man page describes those details), and each
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16 | one of those corresponds to the things the computer is supposed to do when
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17 | it starts up. The default runlevel is 3. Here are the descriptions of the
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18 | different runlevels as they are often implemented:</para>
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19 |
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20 | <literallayout>0: halt the computer
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21 | 1: single-user mode
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22 | 2: multi-user mode without networking
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23 | 3: multi-user mode with networking
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24 | 4: reserved for customization, otherwise does the same as 3
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25 | 5: same as 4, it is usually used for GUI login (like X's xdm or KDE's kdm)
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26 | 6: reboot the computer</literallayout>
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27 |
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28 | <para>The command used to change runlevels is <userinput>init
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29 | <runlevel></userinput> where <runlevel> is
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30 | the target runlevel. For example, to reboot the computer, a user would issue
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31 | the init 6 command. The reboot command is just an alias, as is the halt
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32 | command an alias to init 0.</para>
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33 |
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34 | <para>The /etc/init.d/rcS script is run at every startup of the computer,
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35 | before any runlevel is executed and runs the scripts listed in
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36 | /etc/rcS.d</para>
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37 |
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38 | <para>There are a number of directories under /etc that look like like rc?.d
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39 | where ? is the number of the runlevel and rcS.d which contain a number of
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40 | symbolic links. Some begin with an K, the others begin with an S, and all
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41 | of them have three numbers following the initial letter. The K means to
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42 | stop (kill) a service, and the S means to start a service. The numbers
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43 | determine the order in which the scripts are run, from 000 to 999; the
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44 | lower the number the sooner it gets executed. When init switches to
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45 | another runlevel, the appropriate services get killed and others get
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46 | started.</para>
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47 |
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48 | <para>The real scripts are in /etc/init.d. They do all the work, and the
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49 | symlinks all point to them. Killing links and starting links point to
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50 | the same script in /etc/init.d. That's because the scripts can be
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51 | called with different parameters like start, stop, restart, reload,
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52 | status. When a K link is encountered, the appropriate script is run with
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53 | the stop argument. When a S link is encountered, the appropriate script
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54 | is run with the start argument.</para>
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55 |
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56 | <para>There is one exception. Links that start with an S in the
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57 | rc0.d and rc6.d directories will not cause anything to be started. They
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58 | will be called with the parameter <emphasis>stop</emphasis> to stop
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59 | something. The logic behind it is that when you are going to reboot or
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60 | halt the system, you don't want to start anything, only stop the
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61 | system.</para>
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62 |
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63 | <para>These are descriptions of what the arguments make the
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64 | scripts do:</para>
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65 |
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66 | <itemizedlist>
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67 |
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68 | <listitem><para><emphasis>start</emphasis>: The service is
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69 | started.</para></listitem>
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70 |
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71 | <listitem><para><emphasis>stop</emphasis>: The service is
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72 | stopped.</para></listitem>
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73 |
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74 | <listitem><para><emphasis>restart</emphasis>: The service is
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75 | stopped and then started again.</para></listitem>
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76 |
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77 | <listitem><para><emphasis>reload</emphasis>: The configuration
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78 | of the service is updated.
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79 | This is used after the configuration file of a service was modified, when
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80 | the service doesn't need to be restarted.</para></listitem>
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81 |
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82 | <listitem><para><emphasis>status</emphasis>: Tells if the service
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83 | is running and with which PID's.</para></listitem>
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84 |
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85 | </itemizedlist>
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86 |
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87 | <para>Feel free to modify the way the boot process works (after all it's your
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88 | LFS system, not ours). The files here are just an example of how it can be
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89 | done in a nice way (well what we consider nice anyway. You may hate it).</para>
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90 |
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91 | </sect1>
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92 |
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