source: BOOK/bootscripts/common/usage.xml@ d799833

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  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
  <!ENTITY % general-entities SYSTEM "../../general.ent">
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<sect1 id="ch-scripts-usage">
  <?dbhtml filename="usage.html"?>

  <title>How does systemd work?</title>

  <indexterm zone="ch-scripts-usage">
    <primary sortas="a-systemd-usage">systemd usage</primary>
  <secondary>usage</secondary></indexterm>

  <warning>
    <para>Please disregard this page until it can be written.</para>
  </warning>
<!--
  <para>Linux uses a special booting facility named SysVinit that is
  based on a concept of <emphasis>run-levels</emphasis>. It can be quite
  different from one system to another, so it cannot be assumed that
  because things worked in one particular Linux distribution, they should work
  the same in CLFS too. CLFS has its own way of doing things, but it
  respects generally accepted standards.</para>

  <para>SysVinit (which will be referred to as <quote>init</quote> from
  now on) works using a run-levels scheme. There are seven (numbered 0 to 6)
  run-levels (actually, there are more run-levels, but they are for
  special cases and are generally not used. See <filename>init(8)</filename>
  for more details), and each one of those corresponds to the actions the
  computer is supposed to perform when it starts up. The default
  run-level is 3. Here are the descriptions of the different run-levels
  as they are implemented:</para>

<literallayout>0: halt the computer
1: single-user mode
2: multi-user mode without networking
3: multi-user mode with networking
4: reserved for customization, otherwise does the same as 3
5: same as 4, it is usually used for GUI login (like X's <command>xdm</command> or KDE's <command>kdm</command>)
6: reboot the computer</literallayout>

  <para>The command used to change run-levels is <command>init
  <replaceable>[runlevel]</replaceable></command>, where
  <replaceable>[runlevel]</replaceable> is the target run-level. For example,
  to reboot the computer, a user could issue the <command>init 6</command>
  command, which is an alias for the <command>reboot</command> command.
  Likewise, <command>init 0</command> is an alias for the
  <command>halt</command> command.</para>

  <para>There are a number of directories under <filename
  class="directory">/etc/rc.d</filename> that look like <filename
  class="directory">rc?.d</filename> (where ? is the number of the
  run-level) and <filename class="directory">rcsysinit.d</filename>, all
  containing a number of symbolic links. Some begin with a
  <emphasis>K</emphasis>, the others begin with an
  <emphasis>S</emphasis>, and all of them have two numbers following the
  initial letter. The K means to stop (kill) a service and the S means
  to start a service. The numbers determine the order in which the
  scripts are run, from 00 to 99&mdash;the lower the number the earlier it
  gets executed. When <command>init</command> switches to another run-level,
  the appropriate services are either started or stopped, depending on the
  runlevel chosen.</para>

  <para>The real scripts are in <filename
  class="directory">/etc/rc.d/init.d</filename>. They do the actual work,
  and the symlinks all point to them. Killing links and starting links point
  to the same script in <filename class="directory">/etc/rc.d/init.d</filename>.
  This is because the scripts can be called with different parameters like
  <option>start</option>, <option>stop</option>, <option>restart</option>,
  <option>reload</option>, and <option>status</option>. When a K link is
  encountered, the appropriate script is run with the <option>stop</option>
  argument. When an S link is encountered, the appropriate script is run
  with the <option>start</option> argument.</para>

  <para>There is one exception to this explanation. Links that start
  with an <emphasis>S</emphasis> in the <filename
  class="directory">rc0.d</filename> and <filename
  class="directory">rc6.d</filename> directories will not cause anything
  to be started. They will be called with the parameter
  <option>stop</option> to stop something. The logic behind this
  is that when a user is going to reboot or halt the system, nothing
  needs to be started. The system only needs to be stopped.</para>

  <para>These are descriptions of what the arguments make the scripts
  do:</para>

  <variablelist>
    <varlistentry>
      <term><option>start</option></term>
      <listitem>
        <para>The service is started.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term><option>stop</option></term>
      <listitem>
        <para>The service is stopped.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term><option>restart</option></term>
      <listitem>
        <para>The service is stopped and then started again.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term><option>reload</option></term>
      <listitem>
        <para>The configuration of the service is updated. This is used
        after the configuration file of a service was modified, when the
        service does not need to be restarted.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term><option>status</option></term>
      <listitem>
        <para>Tells if the service is running and with which PIDs.</para>
      </listitem>
    </varlistentry>
  </variablelist>

  <para>Feel free to modify the way the boot process works (after all,
  it is your own CLFS system). The files given here are an example of how
  it can be done.</para>
-->

</sect1>