</variablelist>
</refsect1>
+ <refsect1>
+ <title>Concepts</title>
+
+ <para>systemd provides a dependency system between
+ various entities called "units". Units encapsulate
+ various objects that are relevant for system boot-up
+ and maintainance. The majority of units are configured
+ in unit configuration files, whose syntax and basic
+ set of options is described in
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ however some are created automatically from other
+ configuration or dynamically from system state. Units
+ may be active (meaning started, bound, plugged in, ...
+ depending on the unit type), or inactive (meaning
+ stopped, unbound, unplugged, ...), as well is in the
+ process of being activated or deactivated,
+ i.e. between the two states. The following unit types
+ are available:</para>
+
+ <orderedlist>
+ <listitem><para>Service units, which control
+ daemons and the processes they consist of. For
+ details see
+ <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Socket units, which
+ encapsulate local IPC or network sockets in
+ the system, useful for socket-based
+ activation. For details about socket units see
+ <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ for details on socket-based activation and
+ other forms of activation, see
+ <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Target units are useful to
+ group units, or provide well-known
+ synchronization points during boot-up, see
+ <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Device units expose kernel
+ devices in systemd and may be used to
+ implement device-based activation. For details
+ see
+ <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Mount units control mount
+ points in the file system, for details see
+ <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Automount units provide
+ automount capabilities, for on-demand mounting
+ of file systems as well as parallelized
+ boot-up. See
+ <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Snapshot units can be used to
+ temporarily save the state of the set of
+ systemd units, which later may be restored by
+ activating the saved snapshot unit. For more
+ information see
+ <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Timer units are useful for
+ triggering activation of other units based on
+ timers. You may find details in
+ <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Swap units are very similar to
+ mount units and encapsulated memory swap
+ partitions or files of the operating
+ systemd. They are described in <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Path units may be used
+ activate other services when file system
+ objects change or are modified. See
+ <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ </orderedlist>
+
+ <para>Units are named as their configuration
+ files. Some units have special semantics. A detailed
+ list you may find in
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+
+ <para>On boot systemd activates the target unit
+ <filename>default.target</filename> whose job it is to
+ activate on-boot services and other on-boot units by
+ pulling them in via dependencies. Usually the unit
+ name is just an alias (symlink) for either
+ <filename>graphical.target</filename> (for
+ fully-featured boots into the UI) or
+ <filename>multi-user.target</filename> (for limited
+ console-only boots for use in embedded or server
+ environments, or similar; a subset of
+ graphical.target). However it is at the discretion of
+ the administrator to configure it as an alias to any
+ other target unit. See
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ for details about these target units.</para>
+
+ <para>For more information about the concepts and
+ ideas behind systemd please refer to the <ulink
+ url="http://0pointer.de/blog/projects/systemd.html">Original
+ Announcement Document</ulink>.</para>
+ </refsect1>
+
<refsect1>
<title>Directories</title>
~ianmdlvl / elogind.git / blobdiff