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  • BOOK/bootscripts/common/usage.xml

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    99  <?dbhtml filename="usage.html"?>
    1010
    11   <title>How Do These Bootscripts Work?</title>
     11  <title>How does Systemd work?</title>
    1212
    1313  <indexterm zone="ch-scripts-usage">
    14     <primary sortas="a-Bootscripts">Bootscripts</primary>
     14    <primary sortas="a-systemd-usage">Systemd usage</primary>
    1515  <secondary>usage</secondary></indexterm>
    1616
    17   <para>Linux uses a special booting facility named SysVinit that is
    18   based on a concept of <emphasis>run-levels</emphasis>. It can be quite
    19   different from one system to another, so it cannot be assumed that
    20   because things worked in one particular Linux distribution, they should work
    21   the same in CLFS too. CLFS has its own way of doing things, but it
    22   respects generally accepted standards.</para>
     17  <warning>
     18    <para>Please disregard this page until it is complete and verified.</para>
     19  </warning>
    2320
    24   <para>SysVinit (which will be referred to as <quote>init</quote> from
    25   now on) works using a run-levels scheme. There are seven (numbered 0 to 6)
    26   run-levels (actually, there are more run-levels, but they are for
    27   special cases and are generally not used. See <filename>init(8)</filename>
    28   for more details), and each one of those corresponds to the actions the
    29   computer is supposed to perform when it starts up. The default
    30   run-level is 3. Here are the descriptions of the different run-levels
    31   as they are implemented:</para>
     21  <sect2>
     22    <title>Introduction to Systemd</title>
    3223
    33 <literallayout>0: halt the computer
    34 1: single-user mode
    35 2: multi-user mode without networking
    36 3: multi-user mode with networking
    37 4: reserved for customization, otherwise does the same as 3
    38 5: same as 4, it is usually used for GUI login (like X's <command>xdm</command> or KDE's <command>kdm</command>)
    39 6: reboot the computer</literallayout>
     24 
     25    <para>Systemd is a system management daemon designed exclusively for the
     26    Linux kernel API. In the Linux startup process, it is the first process to
     27    execute in user land; therefore, it is also the parent process of all child
     28    processes in user land.</para>
    4029
    41   <para>The command used to change run-levels is <command>init
    42   <replaceable>[runlevel]</replaceable></command>, where
    43   <replaceable>[runlevel]</replaceable> is the target run-level. For example,
    44   to reboot the computer, a user could issue the <command>init 6</command>
    45   command, which is an alias for the <command>reboot</command> command.
    46   Likewise, <command>init 0</command> is an alias for the
    47   <command>halt</command> command.</para>
     30    <para>Systemd's initialization instructions for each daemon are recorded in
     31    a declarative configuration file rather than a shell script. For
     32    inter-process communication, systemd makes Unix domain sockets and D-Bus
     33    available to the running daemons. Because systemd tracks processes using
     34    Linux cgroups instead of process identifiers (PIDs), daemons cannot "escape"
     35    systemd; not even by double-forking. Systemd is also capable of aggressive
     36    parallelization.</para>
    4837
    49   <para>There are a number of directories under <filename
    50   class="directory">/etc/rc.d</filename> that look like <filename
    51   class="directory">rc?.d</filename> (where ? is the number of the
    52   run-level) and <filename class="directory">rcsysinit.d</filename>, all
    53   containing a number of symbolic links. Some begin with a
    54   <emphasis>K</emphasis>, the others begin with an
    55   <emphasis>S</emphasis>, and all of them have two numbers following the
    56   initial letter. The K means to stop (kill) a service and the S means
    57   to start a service. The numbers determine the order in which the
    58   scripts are run, from 00 to 99&mdash;the lower the number the earlier it
    59   gets executed. When <command>init</command> switches to another run-level,
    60   the appropriate services are either started or stopped, depending on the
    61   runlevel chosen.</para>
     38    <para>Among systemd's auxiliary features are a cron-like job scheduler
     39    called systemd Calendar Timers, and an event logging subsystem called
     40    journal. The system administrator may choose whether to log system events
     41    with systemd or syslog. Systemd's logfile is a binary file. The state of
     42    systemd itself can be preserved in a snapshot for future recall.</para>
    6243
    63   <para>The real scripts are in <filename
    64   class="directory">/etc/rc.d/init.d</filename>. They do the actual work,
    65   and the symlinks all point to them. Killing links and starting links point
    66   to the same script in <filename class="directory">/etc/rc.d/init.d</filename>.
    67   This is because the scripts can be called with different parameters like
    68   <option>start</option>, <option>stop</option>, <option>restart</option>,
    69   <option>reload</option>, and <option>status</option>. When a K link is
    70   encountered, the appropriate script is run with the <option>stop</option>
    71   argument. When an S link is encountered, the appropriate script is run
    72   with the <option>start</option> argument.</para>
     44    <para>Systemd provides a replacement for sysvinit, pm-utils, inetd, acpid,
     45    syslog, watchdog, cron and atd, and obsoletes ConsoleKit.</para>
    7346
    74   <para>There is one exception to this explanation. Links that start
    75   with an <emphasis>S</emphasis> in the <filename
    76   class="directory">rc0.d</filename> and <filename
    77   class="directory">rc6.d</filename> directories will not cause anything
    78   to be started. They will be called with the parameter
    79   <option>stop</option> to stop something. The logic behind this
    80   is that when a user is going to reboot or halt the system, nothing
    81   needs to be started. The system only needs to be stopped.</para>
     47  </sect2>
    8248
    83   <para>These are descriptions of what the arguments make the scripts
    84   do:</para>
     49  <sect2>
     50    <title>Systemctl</title>
    8551
    86   <variablelist>
    87     <varlistentry>
    88       <term><option>start</option></term>
    89       <listitem>
    90         <para>The service is started.</para>
    91       </listitem>
    92     </varlistentry>
     52    <para><command>systemctl</command> is the main command used to introspect
     53     and control Systemd.</para>
    9354
    94     <varlistentry>
    95       <term><option>stop</option></term>
    96       <listitem>
    97         <para>The service is stopped.</para>
    98       </listitem>
    99     </varlistentry>
     55    <variablelist>
    10056
    101     <varlistentry>
    102       <term><option>restart</option></term>
    103       <listitem>
    104         <para>The service is stopped and then started again.</para>
    105       </listitem>
    106     </varlistentry>
     57      <varlistentry>
     58        <term>List running units:</term>
     59        <listitem>
     60          <para><command>systemctl</command> or <command>systemctl list-units</command></para>
     61        </listitem>
     62      </varlistentry>
    10763
    108     <varlistentry>
    109       <term><option>reload</option></term>
    110       <listitem>
    111         <para>The configuration of the service is updated. This is used
    112         after the configuration file of a service was modified, when the
    113         service does not need to be restarted.</para>
    114       </listitem>
    115     </varlistentry>
     64      <varlistentry>
     65        <term>List failed units:</term>
     66        <listitem>
     67          <para><command>systemctl --failed</command></para>
     68        </listitem>
     69      </varlistentry>
    11670
    117     <varlistentry>
    118       <term><option>status</option></term>
    119       <listitem>
    120         <para>Tells if the service is running and with which PIDs.</para>
    121       </listitem>
    122     </varlistentry>
    123   </variablelist>
     71      <varlistentry>
     72        <term>List available unit files:</term>
     73        <listitem>
     74          <para><command>systemctl list-unit-files</command></para>
     75        </listitem>
     76      </varlistentry>
    12477
    125   <para>Feel free to modify the way the boot process works (after all,
    126   it is your own CLFS system). The files given here are an example of how
    127   it can be done.</para>
     78      <varlistentry>
     79        <term>Activate a unit immediately:</term>
     80        <listitem>
     81          <para><command>systemctl start</command> <replaceable>unit</replaceable></para>
     82        </listitem>
     83      </varlistentry>
     84
     85      <varlistentry>
     86        <term>Stop a unit immediately:</term>
     87        <listitem>
     88          <para><command>systemctl stop </command><replaceable>unit</replaceable></para>
     89        </listitem>
     90      </varlistentry>
     91
     92      <varlistentry>
     93        <term>Restart a unit:</term>
     94        <listitem>
     95          <para><command>systemctl restart </command><replaceable>unit</replaceable></para>
     96        </listitem>
     97      </varlistentry>
     98
     99      <varlistentry>
     100        <term>Reload unit configuration:</term>
     101        <listitem>
     102          <para><command>systemctl reload </command><replaceable>unit</replaceable></para>
     103        </listitem>
     104      </varlistentry>
     105
     106      <varlistentry>
     107        <term>Show status of a unit:</term>
     108        <listitem>
     109          <para><command>systemctl status </command><replaceable>unit</replaceable></para>
     110        </listitem>
     111      </varlistentry>
     112
     113      <varlistentry>
     114        <term>Check if a unit is enabled or disabled:</term>
     115        <listitem>
     116          <para><command>systemctl is-enabled </command><replaceable>unit</replaceable></para>
     117        </listitem>
     118      </varlistentry>
     119
     120      <varlistentry>
     121        <term>Enable a unit to start during boot:</term>
     122        <listitem>
     123          <para><command>systemctl enable </command><replaceable>unit</replaceable></para>
     124        </listitem>
     125      </varlistentry>
     126
     127      <varlistentry>
     128        <term>Disable a unit to not start during boot:</term>
     129        <listitem>
     130          <para><command>systemctl disable </command><replaceable>unit</replaceable></para>
     131        </listitem>
     132      </varlistentry>
     133
     134      <varlistentry>
     135        <term>Reload systemd and scan for new or changed units:</term>
     136        <listitem>
     137          <para><command>systemctl daemon-reload</command></para>
     138        </listitem>
     139      </varlistentry>
     140
     141    </variablelist>
     142
     143    <para>For more information regarding systemd, please refer to the systemd
     144    and related man-pages and
     145    <ulink url="https://fedoraproject.org/wiki/Systemd">Systemd at
     146    FedoraProject</ulink> for documentation, examples, features, and other
     147    information.</para>
     148
     149  </sect2>
    128150
    129151</sect1>
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