<para>systemd is a system and session manager for
Linux operating systems. When run as first process on
- boot (as PID 1) it may act as init system that brings
- up and maintains userspace.</para>
+ boot (as PID 1), it acts as init system that brings
+ up and maintains userspace services.</para>
- <para>For compatibility with SysV if systemd is called
+ <para>For compatibility with SysV, if systemd is called
as <command>init</command> and a PID that is not
- 1 it will execute <command>telinit</command> and pass
+ 1, it will execute <command>telinit</command> and pass
all command line arguments unmodified. That means
<command>init</command> and <command>telinit</command>
are mostly equivalent when invoked from normal login sessions. See
D-Bus interfaces
repository. Optionally the interface
name for the introspection data may be
- specified. If omitted the
+ specified. If omitted, the
introspection data for all interfaces
is dumped.</para></listitem>
</varlistentry>
</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>Processes systemd spawns ared placed in
+ individual Linux control groups named after the unit
+ which they belong to in the private systemd
+ hierarchy. (see <ulink
+ url="http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>
+ for more information about control groups, or short
+ "cgroups"). systemd uses this to effectively keep
+ track of processes. Control group information is
+ maintained in the kernel, and is accessible via the
+ file system hierarchy (beneath
+ <filename>/cgroup/systemd/</filename>), or in tools
+ such as
+ <citerefentry><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ (<command>ps xawf -eo pid,user,cgroup,args</command>
+ is particularly useful to list all processes and the
+ systemd units they belong to.).</para>
+
+ <para>systemd is compatible with the SysV init system
+ to a large degree: SysV init scripts are supported and
+ simply read as an alternative (though limited)
+ configuration file format. The SysV
+ <filename>/dev/initctl</filename> interface is
+ provided, and comaptibility implementations of the
+ various SysV client tools available. In addition to
+ that various established Unix functionality such as
+ <filename>/etc/fstab</filename> or the
+ <filename>utmp</filename> database are
+ supported.</para>
+
+ <para>systemd has a minimal transaction system: if a
+ unit is requested to start up or shut down it will add
+ it and all its dependencies to a temporary
+ transaction. Then, it will verify if the transaction
+ is consistent (i.e. whether the ordering of all units
+ is cycle-free). If it is not, systemd will try to fix
+ it up, and removes non-essential jobs from the
+ transaction that might remove the loop. Also, systemd
+ tries to suppress non-essential jobs in the
+ transaction that would stop a running service. Finally
+ it is checked whether the jobs of the transaction
+ contradict jobs that have already been queued, and
+ optionally the transaction is aborted then. If all
+ worked out and the transaction is consistent and
+ minimized in its impact it is merged with all already
+ outstanding jobs and added to the run
+ queue. Effectively this means that before executing a
+ requested operation, systemd will verify that it makes
+ sense, fixing it if possible, and only failing if it
+ really cannot work.</para>
+
+ <para>Systemd contains native implementations of
+ various tasks that need to be executed as part of the
+ boot process. For example, it sets the host name or
+ configures the loopback network device. It also sets
+ up and mounts various API file systems, such as
+ <filename>/sys</filename> or
+ <filename>/proc</filename>.</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
+ Design Document</ulink>.</para>
+ </refsect1>
+
<refsect1>
<title>Directories</title>
--variable=systemdsystemconfdir</command>
returns the path of the system
configuration directory. Packages
- should alter this directory only with
- the
+ should alter the content of these directories
+ only with the
<citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
tool.</para></listitem>
</varlistentry>
SysV init script directory varies
between distributions. If systemd
cannot find a native unit file for a
- requested service it will look for a
+ requested service, it will look for a
SysV init script of the same name
(with the
<filename>.service</filename> suffix
~ianmdlvl / elogind.git / blobdiff