| 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>Linux uses a special booting facility named SysVinit. It's based on a
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| 5 | concept of <emphasis>runlevels</emphasis>. It can be widely different
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| 6 | from one system to another, so it can not be assumed that because things
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| 7 | worked in <insert distro name> they should work like that in LFS
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| 8 | too. LFS has it's own way of doing things, but it respects generally
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| 9 | accepted standards.</para>
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| 10 |
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| 11 | <para>SysVinit (which we'll call <emphasis>init</emphasis> from now on) works
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| 12 | using a runlevels scheme. There are 7 (from 0 to 6) runlevels
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| 13 | (actually, there are more runlevels but they are for special cases and
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| 14 | generally not used. The init man page describes those details), and each
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| 15 | one of those corresponds to the things the computer is supposed to do when
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| 16 | it starts up. The default runlevel is 3. Here are the descriptions of the
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| 17 | different runlevels as they are often implemented:</para>
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| 18 |
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| 19 | <literallayout>0: halt the computer
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| 20 | 1: single-user mode
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| 21 | 2: multi-user mode without networking
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| 22 | 3: multi-user mode with networking
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| 23 | 4: reserved for customization, otherwise does the same as 3
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| 24 | 5: same as 4, it is usually used for GUI login (like X's xdm or KDE's kdm)
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| 25 | 6: reboot the computer</literallayout>
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| 26 |
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| 27 | <para>The command used to change runlevels is <userinput>init
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| 28 | <runlevel></userinput> where <runlevel> is
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| 29 | the target runlevel. For example, to reboot the computer, a user would issue
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| 30 | the init 6 command. The reboot command is just an alias, as is the halt
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| 31 | command an alias to init 0.</para>
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| 32 |
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| 33 | <para>The /etc/init.d/rcS script is run at every startup of the computer,
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| 34 | before any runlevel is executed and runs the scripts listed in
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| 35 | /etc/rcS.d</para>
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| 36 |
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| 37 | <para>There are a number of directories under /etc that look like like rc?.d
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| 38 | where ? is the number of the runlevel and rcS.d which contain a number of s
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| 39 | ymbolic links. Some begin with an K, the others begin with an S, and all
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| 40 | of them have three numbers following the initial letter. The K means to
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| 41 | stop (kill) a service, and the S means to start a service. The numbers
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| 42 | determine the order in which the scripts are run, from 000 to 999; the
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| 43 | lower the number the sooner it gets executed. When init switches to
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| 44 | another runlevel, the appropriate services get killed and others get
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| 45 | started.</para>
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| 46 |
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| 47 | <para>The real scripts are in /etc/init.d. They do all the work, and the
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| 48 | symlinks all point to them. Killing links and starting links point to
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| 49 | the same script in /etc/init.d. That's because the scripts can be
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| 50 | called with different parameters like start, stop, restart, reload,
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| 51 | status. When a K link is encountered, the appropriate script is run with
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| 52 | the stop argument. When a S link is encountered, the appropriate script
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| 53 | is run with the start argument.</para>
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| 54 |
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| 55 | <para>There is one exception. Links that start with an S in the
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| 56 | rc0.d and rc6.d directories will not cause anything to be started. They
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| 57 | will be called with the paramater <emphasis>stop</emphasis> to stop
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| 58 | something. The logic behind it is that when you are going to reboot or
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| 59 | halt the system, you don't want to start anything, only stop the
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| 60 | system.</para>
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| 61 |
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| 62 | <para>These are descriptions of what the arguments make the
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| 63 | scripts do:</para>
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| 64 |
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| 65 | <itemizedlist>
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| 66 |
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| 67 | <listitem><para><emphasis>start</emphasis>: The service is
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| 68 | started.</para></listitem>
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| 69 |
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| 70 | <listitem><para><emphasis>stop</emphasis>: The service is
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| 71 | stopped.</para></listitem>
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| 72 |
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| 73 | <listitem><para><emphasis>restart</emphasis>: The service is
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| 74 | stopped and then started again.</para></listitem>
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| 75 |
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| 76 | <listitem><para><emphasis>reload</emphasis>: The configuration
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| 77 | of the service is updated.
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| 78 | This is used after the configuration file of a service was modified, when
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| 79 | the service doesn't need to be restarted.</para></listitem>
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| 80 |
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| 81 | <listitem><para><emphasis>status</emphasis>: Tells if the service
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| 82 | is running and with which PID's.</para></listitem>
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| 83 |
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| 84 | </itemizedlist>
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| 85 |
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| 86 | <para>Feel free to modify the way the boot process works (after all it's your
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| 87 | LFS system, not ours). The files here are just an example of how it can be
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| 88 | done in a nice way (well what we consider nice anyway. You may hate it).</para>
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| 89 |
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| 90 | </sect1>
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| 91 |
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