<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 maintenance. The majority of units are configured
- in unit configuration files, whose syntax and basic
- set of options is described in
+ various entities called "units" of 12 different
+ types. Units encapsulate various objects that are
+ relevant for system boot-up and maintenance. 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, see below), or
- 'inactive' (meaning stopped, unbound, unplugged, ...),
- as well as in the process of being activated or
- deactivated, i.e. between the two states (these states
- are called 'activating', 'deactivating'). A special
- 'failed' state is available as well which is very
- similar to 'inactive' and is entered when the service
- failed in some way (process returned error code on
- exit, or crashed, or an operation timed out). If this
- state is entered the cause will be logged, for later
+ configuration, dynamically from system state or
+ programmatically at runtime. Units may be "active"
+ (meaning started, bound, plugged in, ..., depending on
+ the unit type, see below), or "inactive" (meaning
+ stopped, unbound, unplugged, ...), as well as in the
+ process of being activated or deactivated,
+ i.e. between the two states (these states are called
+ "activating", "deactivating"). A special "failed"
+ state is available as well, which is very similar to
+ "inactive" and is entered when the service failed in
+ some way (process returned error code on exit, or
+ crashed, or an operation timed out). If this state is
+ entered, the cause will be logged, for later
reference. Note that the various unit types may have a
number of additional substates, which are mapped to
the five generalized unit states described
<para>The following unit types are available:</para>
<orderedlist>
- <listitem><para>Service units, which control
+ <listitem><para>Service units, which start and control
daemons and the processes they consist of. For
details see
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
objects change or are modified. See
<citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+ <listitem><para>Slice units may be used to
+ group units which manage system processes
+ (such as service and scope units) in a
+ hierarchical tree for resource management
+ purposes. See
+ <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>Scope units are similar to
+ service units, but manage foreign processes
+ instead of starting them as well. See
+ <citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
</orderedlist>
<para>Units are named as their configuration
option prefixed with
<literal>rd.</literal> is honored
only in the initial RAM disk (initrd),
- while the one that isn't prefixed only
+ while the one that is not prefixed only
in the main system.</para></listitem>
</varlistentry>
<term><varname>systemd.setenv=</varname></term>
<listitem><para>Takes a string
- argument in the form
- VARIABLE=VALUE. May be used to set
- environment variables for the init
- process and all its children at boot
- time. May be used more than once to
- set multiple variables. If the equal
- sign and variable are missing it unsets
- an environment variable which might be
- passed in from the initial ram
- disk.</para></listitem>
+ argument in the form VARIABLE=VALUE.
+ May be used to set default environment
+ variables to add to forked child processes.
+ May be used more than once to set multiple
+ variables.</para></listitem>
</varlistentry>
<varlistentry>
~ianmdlvl / elogind.git / blobdiff