Changeset 774f0a37

Jan 24, 2014, 8:52:12 AM (9 years ago)
William Harrington <kb0iic@…>
clfs-3.0.0-systemd, clfs-3.0.0-sysvinit, master, systemd, sysvinit

Add some detailed information regarding systemd usage. It could use some polishing.

1 edited


  • BOOK/bootscripts/common/usage.xml

    r3a85bbd r774f0a37  
    1717  <warning>
    18     <para>Please disregard this page until it can be written.</para>
     18    <para>Please disregard this page until it is complete and verified.</para>
    1919  </warning>
    20 <!--
    21   <para>Linux uses a special booting facility named SysVinit that is
    22   based on a concept of <emphasis>run-levels</emphasis>. It can be quite
    23   different from one system to another, so it cannot be assumed that
    24   because things worked in one particular Linux distribution, they should work
    25   the same in CLFS too. CLFS has its own way of doing things, but it
    26   respects generally accepted standards.</para>
    28   <para>SysVinit (which will be referred to as <quote>init</quote> from
    29   now on) works using a run-levels scheme. There are seven (numbered 0 to 6)
    30   run-levels (actually, there are more run-levels, but they are for
    31   special cases and are generally not used. See <filename>init(8)</filename>
    32   for more details), and each one of those corresponds to the actions the
    33   computer is supposed to perform when it starts up. The default
    34   run-level is 3. Here are the descriptions of the different run-levels
    35   as they are implemented:</para>
     21  <sect2>
     22    <title>Introduction to Systemd</title>
    37 <literallayout>0: halt the computer
    38 1: single-user mode
    39 2: multi-user mode without networking
    40 3: multi-user mode with networking
    41 4: reserved for customization, otherwise does the same as 3
    42 5: same as 4, it is usually used for GUI login (like X's <command>xdm</command> or KDE's <command>kdm</command>)
    43 6: reboot the computer</literallayout>
     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>
    45   <para>The command used to change run-levels is <command>init
    46   <replaceable>[runlevel]</replaceable></command>, where
    47   <replaceable>[runlevel]</replaceable> is the target run-level. For example,
    48   to reboot the computer, a user could issue the <command>init 6</command>
    49   command, which is an alias for the <command>reboot</command> command.
    50   Likewise, <command>init 0</command> is an alias for the
    51   <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>
    53   <para>There are a number of directories under <filename
    54   class="directory">/etc/rc.d</filename> that look like <filename
    55   class="directory">rc?.d</filename> (where ? is the number of the
    56   run-level) and <filename class="directory">rcsysinit.d</filename>, all
    57   containing a number of symbolic links. Some begin with a
    58   <emphasis>K</emphasis>, the others begin with an
    59   <emphasis>S</emphasis>, and all of them have two numbers following the
    60   initial letter. The K means to stop (kill) a service and the S means
    61   to start a service. The numbers determine the order in which the
    62   scripts are run, from 00 to 99&mdash;the lower the number the earlier it
    63   gets executed. When <command>init</command> switches to another run-level,
    64   the appropriate services are either started or stopped, depending on the
    65   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>
    67   <para>The real scripts are in <filename
    68   class="directory">/etc/rc.d/init.d</filename>. They do the actual work,
    69   and the symlinks all point to them. Killing links and starting links point
    70   to the same script in <filename class="directory">/etc/rc.d/init.d</filename>.
    71   This is because the scripts can be called with different parameters like
    72   <option>start</option>, <option>stop</option>, <option>restart</option>,
    73   <option>reload</option>, and <option>status</option>. When a K link is
    74   encountered, the appropriate script is run with the <option>stop</option>
    75   argument. When an S link is encountered, the appropriate script is run
    76   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>
    78   <para>There is one exception to this explanation. Links that start
    79   with an <emphasis>S</emphasis> in the <filename
    80   class="directory">rc0.d</filename> and <filename
    81   class="directory">rc6.d</filename> directories will not cause anything
    82   to be started. They will be called with the parameter
    83   <option>stop</option> to stop something. The logic behind this
    84   is that when a user is going to reboot or halt the system, nothing
    85   needs to be started. The system only needs to be stopped.</para>
     47  </sect2>
    87   <para>These are descriptions of what the arguments make the scripts
    88   do:</para>
     49  <sect2>
     50    <title>Systemctl</title>
    90   <variablelist>
    91     <varlistentry>
    92       <term><option>start</option></term>
    93       <listitem>
    94         <para>The service is started.</para>
    95       </listitem>
    96     </varlistentry>
     52    <para><command>systemctl</command> is the main command used to introspect
     53     and control Systemd.</para>
    98     <varlistentry>
    99       <term><option>stop</option></term>
    100       <listitem>
    101         <para>The service is stopped.</para>
    102       </listitem>
    103     </varlistentry>
     55    <variablelist>
    105     <varlistentry>
    106       <term><option>restart</option></term>
    107       <listitem>
    108         <para>The service is stopped and then started again.</para>
    109       </listitem>
    110     </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>
    112     <varlistentry>
    113       <term><option>reload</option></term>
    114       <listitem>
    115         <para>The configuration of the service is updated. This is used
    116         after the configuration file of a service was modified, when the
    117         service does not need to be restarted.</para>
    118       </listitem>
    119     </varlistentry>
     64      <varlistentry>
     65        <term>List failed units:</term>
     66        <listitem>
     67          <para><command>systemctl --failed</command></para>
     68        </listitem>
     69      </varlistentry>
    121     <varlistentry>
    122       <term><option>status</option></term>
    123       <listitem>
    124         <para>Tells if the service is running and with which PIDs.</para>
    125       </listitem>
    126     </varlistentry>
    127   </variablelist>
     71      <varlistentry>
     72        <term>List avilable unit files:</term>
     73        <listitem>
     74          <para><command>systemctl list-unit-files</command></para>
     75        </listitem>
     76      </varlistentry>
    129   <para>Feel free to modify the way the boot process works (after all,
    130   it is your own CLFS system). The files given here are an example of how
    131   it can be done.</para>
    132 -->
     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>
     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>
     92      <varlistentry>
     93        <term>Restart a unit:</term>
     94        <listitem>
     95          <para><command>systemctl restart </command><replaceable>unit</replaceable></para>
     96        </listitem>
     97      </varlistentry>
     99      <varlistentry>
     100        <term>Reload unit configuration:</term>
     101        <listitem>
     102          <para><command>systemctl reload </command><replaceable>unit</replaceable></para>
     103        </listitem>
     104      </varlistentry>
     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>
     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>
     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>
     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>
     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>
     141    </variablelist>
     143    <para>For more information regarding systemd, please refer to the systemd
     144    and related man-pages and
     145    <ulink url="">Systemd at
     146    FedoraProject</ulink> for documentation, examples, features, and other
     147    information.</para>
     149  </sect2>
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