man: refer to snapshot man page from unit man page

[thirdparty/systemd.git] / man / systemd.unit.xml

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<refentry id="systemd.unit">

        <refentryinfo>
                <title>systemd.unit</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

        <refmeta>
                <refentrytitle>systemd.unit</refentrytitle>
                <manvolnum>5</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>systemd.unit</refname>
                <refpurpose>systemd unit configuration files</refpurpose>
        </refnamediv>

        <refsynopsisdiv>
                <para><filename>systemd.service</filename>,
                <filename>systemd.socket</filename>,
                <filename>systemd.device</filename>,
                <filename>systemd.mount</filename>,
                <filename>systemd.automount</filename>,
                <filename>systemd.swap</filename>,
                <filename>systemd.target</filename>,
                <filename>systemd.path</filename>,
                <filename>systemd.timer</filename>,
                <filename>systemd.snapshot</filename></para>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para>A unit configuration file encodes information
                about a service, a socket, a device, a mount point, an
                automount point, a swap file or partition, a start-up
                target, a file system path or a timer controlled and
                supervised by
                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>. The
                syntax is inspired by <ulink
                url="http://standards.freedesktop.org/desktop-entry-spec/latest/">XDG
                Desktop Entry Specification</ulink> <filename>.desktop</filename> files, which are in turn
                inspired by Microsoft Windows
                <filename>.ini</filename> files.</para>

                <para>This man pages lists the common configuration
                options of the all unit types. These options need to
                be configured in the [Unit] resp. [Install]
                section of the unit files.</para>

                <para>In addition to the generic [Unit] and [Install]
                sections described here each unit should have a
                type-specific section, e.g. [Service] for a service
                unit. See the respective man pages for more
                information.</para>

                <para>Unit files may contain additional options on top
                of those listed here. If systemd encounters an unknown
                option it will write a warning log message but
                continue loading the unit. If an option is prefixed
                with <option>X-</option> it is ignored completely by
                systemd. Applications may use this to include
                additional information in the unit files.</para>

                <para>Boolean arguments used in unit files can be
                written in various formats. For positive settings the
                strings <option>1</option>, <option>yes</option>,
                <option>true</option> and <option>on</option> are
                equivalent. For negative settings the strings
                <option>0</option>, <option>no</option>,
                <option>false</option> and <option>off</option> are
                equivalent.</para>

                <para>Time span values encoded in unit files can be
                written in various formats. A stand-alone number
                specifies a time in seconds. If suffixed with a time
                unit, the unit is honored. A concatenation of
                multiple value with units is supported, in which case
                the values are added up. Example: "50" refers to 50
                seconds; "2min 200ms" refers to 2 minutes plus 200
                milliseconds, i.e. 120200ms. The following time units
                are understood: s, min, h, d, w, ms, us.</para>

                <para>Empty lines and lines starting with # or ; are
                ignored. This may be used for commenting.</para>

                <para>If a line starts with <option>.include</option>
                followed by a file name the specified file will be
                read as if its contents where listed in place of the
                <option>.include</option> directive.</para>

                <para>Along with a unit file
                <filename>foo.service</filename> a directory
                <filename>foo.service.wants/</filename> may exist. All
                units symlinked from such a directory are implicitly
                added as dependencies of type
                <varname>Wanted=</varname> to the unit. This is useful
                to hook units into the start-up of other units,
                without having to modify their unit configuration
                files. For details about the semantics of
                <varname>Wanted=</varname> see below. The preferred
                way to create symlinks in the
                <filename>.wants/</filename> directory of a service is
                with the
                <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                tool which reads information from the [Install]
                section of unit files. (See below.)</para>

                <para>Note that while systemd offers a flexible
                dependency system between units it is recommended to
                use this functionality only sparsely and instead rely
                on techniques such as bus-based or socket-based
                activation which makes dependencies implicit, which
                both results in a simpler and more flexible
                system.</para>

                <para>Some unit names reflect paths existing in the
                file system name space. Example: a device unit
                <filename>dev-sda.device</filename> refers to a device
                with the device node <filename>/dev/sda</filename> in
                the file system namespace. If this applies a special
                way to escape the path name is used, so that it is
                usable as part of a file name. Basically, given a
                path, "/" is replaced by "-", and all unprintable
                characters and the "-" are replaced by C-style "\x20"
                escapes. The root directory "/" is encoded as single
                dash, while otherwise the initial and ending "/" is
                removed from all paths during transformation. This
                escaping is reversible.</para>

                <para>Optionally, units may be instantiated from a
                template file at runtime. This allows creation of
                multiple units from a single configuration file. If
                systemd looks for a unit configuration file it will
                first search for the literal unit name in the
                filesystem. If that yields no success and the unit
                name contains an @ character, systemd will look for a
                unit template that shares the same name but with the
                instance string (i.e. the part between the @ character
                and the suffix) removed. Example: if a service
                <filename>getty@tty3.service</filename> is requested
                and no file by that name is found, systemd will look
                for <filename>getty@.service</filename> and
                instantiate a service from that configuration file if
                it is found. To refer to the instance string from
                within the configuration file you may use the special
                <literal>%i</literal> specifier in many of the
                configuration options. Other specifiers that may be
                used are <literal>%n</literal>, <literal>%N</literal>,
                <literal>%p</literal>, <literal>%P</literal> and
                <literal>%I</literal>, for the full unit name, the
                unescaped unit name, the prefix name, the unescaped
                prefix name and the unescaped instance name,
                respectively. The prefix name here refers to the
                string before the @, i.e. "getty" in the example
                above, where "tty3" is the instance name.</para>
        </refsect1>

        <refsect1>
                <title>Options</title>

                <para>Unit file may include a [Unit] section, which
                carries generic information about the unit that is not
                dependent on the type of unit:</para>

                <variablelist>
                        <varlistentry>
                                <term><varname>Names=</varname></term>

                                <listitem><para>Additional names for
                                this unit. The names listed here must
                                have the same suffix (i.e. type) as
                                the unit file name. This option may be
                                specified more than once, in which
                                case all listed names are used. Note
                                that this option is different from the
                                <varname>Alias=</varname> option from
                                the [Install] section mentioned
                                below. See below for details.</para>
                                </listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Description=</varname></term>
                                <listitem><para>A free-form string
                                describing the unit. This is intended for use
                                in UIs wanting to show
                                descriptive information along with the
                                unit name.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Requires=</varname></term>

                                <listitem><para>Configures requirement
                                dependencies on other units. If this
                                units get activated the units listed
                                here will be activated as well. If one
                                of the other units gets deactivated or
                                its activation fails, this unit will
                                be deactivated. This option may be
                                specified more than once, in which
                                case requirement dependencies for all
                                listed names are created. Note that
                                requirement dependencies do not
                                influence the order in which services
                                are started or stopped. This has to be
                                configured independently with the
                                <varname>After=</varname> or
                                <varname>Before=</varname> options. If
                                a unit
                                <filename>foo.service</filename>
                                requires a unit
                                <filename>bar.service</filename> as
                                configured with
                                <varname>Requires=</varname> and no
                                ordering is configured with
                                <varname>After=</varname> or
                                <varname>Before=</varname>, then both
                                units will be started simultaneously
                                and without any delay between them if
                                <filename>foo.service</filename> is
                                activated. Often it is a better choice
                                to use <varname>Wants=</varname>
                                instead of
                                <varname>Requires=</varname> in order
                                to achieve a system that is more
                                robust when dealing with failing
                                services.</para></listitem>
                        </varlistentry>


                        <varlistentry>
                                <term><varname>RequiresOverridable=</varname></term>

                                <listitem><para>Similar to
                                <varname>Requires=</varname>.
                                Dependencies listed in
                                <varname>RequiresOverridable=</varname>
                                which cannot be fulfilled or fail to
                                start are ignored iff the startup was
                                explicitly requested by the user. If
                                the start-up was pulled in indirectly
                                by some dependency or automatic
                                start-up of units that is not
                                requested by the user this dependency
                                must be fulfilled and otherwise the
                                transaction fails. Hence, this option
                                may be used to configure dependencies
                                that are normally honored unless the
                                user explicitly starts up the unit, in
                                which case whether they failed or not
                                is irrelevant.</para></listitem>

                        </varlistentry>
                        <varlistentry>
                                <term><varname>Requisite=</varname></term>
                                <term><varname>RequisiteOverridable=</varname></term>

                                <listitem><para>Similar to
                                <varname>Requires=</varname>
                                resp. <varname>RequiresOverridable=</varname>. However,
                                if a unit listed here is not started
                                already it will not be started and the
                                transaction fails
                                immediately.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Wants=</varname></term>

                                <listitem><para>A weaker version of
                                <varname>Requires=</varname>. A unit
                                listed in this option will be started
                                if the configuring unit is. However,
                                it the listed unit fails to start up
                                or cannot be added to the transaction
                                this has no impact on the validity of
                                the transaction as a whole. This is
                                the recommended way to hook start-up
                                of one unit to the start-up of another
                                unit. Note that dependencies of this
                                type may also be configured outside of
                                the unit configuration file by
                                adding a symlink to a
                                <filename>.wants/</filename> directory
                                accompanying the unit file. For
                                details see above.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Conflicts=</varname></term>

                                <listitem><para>Configures negative
                                requirement dependencies. If a unit
                                that has a
                                <varname>Conflicts=</varname> setting
                                on another unit starting the former
                                will stop the latter and vice
                                versa. Note that this setting is
                                independent of and orthogonal to the
                                <varname>After=</varname> and
                                <varname>Before=</varname> ordering
                                dependencies.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Before=</varname></term>
                                <term><varname>After=</varname></term>

                                <listitem><para>Configures ordering
                                dependencies between units. If a unit
                                <filename>foo.service</filename>
                                contains a setting
                                <option>Before=bar.service</option>
                                and both units are being started
                                <filename>bar.service</filename>'s
                                start-up is delayed until
                                <filename>foo.service</filename> is
                                started up. Note that this setting is
                                independent of and orthogonal to the
                                requirement dependencies as configured
                                by <varname>Requires=</varname>. It is
                                a common pattern to include a unit
                                name in both the
                                <varname>After=</varname> and
                                <varname>Requires=</varname> option in
                                which case the unit listed will be
                                started before the unit that is
                                configured with these options. This
                                option may be specified more than
                                once, in which case ordering
                                dependencies for all listed names are
                                created. <varname>After=</varname> is
                                the inverse of
                                <varname>Before=</varname>, i.e. while
                                <varname>After=</varname> ensures that
                                the configured unit is started after
                                the listed unit finished starting up,
                                <varname>Before=</varname> ensures the
                                opposite, i.e.  that the configured
                                unit is fully started up before the
                                listed unit is started. Note that when
                                two units with an ordering dependency
                                between them are shut down, the
                                inverse of of the start-up order is
                                applied. i.e. if a unit is configured
                                with <varname>After=</varname> on
                                another unit, the former is stopped
                                before the latter if both are shut
                                down. If one unit with an ordering
                                dependency on another unit is shut
                                down while the latter is started up,
                                the shut down is ordered before the
                                start-up regardless whether the
                                ordering dependency is actually of
                                type <varname>After=</varname> or
                                <varname>Before=</varname>. If two
                                units have no ordering dependencies
                                between them they are shut down
                                resp. started up simultaneously, and
                                no ordering takes
                                place. </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>RecursiveStop=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option> and
                                the unit stops without this being
                                requested by the user all units
                                depending on it will be stopped as
                                well. (e.g. if a service exits or
                                crashes on its own behalf, units using
                                it will be stopped) Note that normally
                                if a unit stops without user request
                                units depending on it will not be
                                terminated. Only if the user requested
                                shutdown of a unit all units depending
                                on the unit will be shut down as well
                                and at the same time. Defaults to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>StopWhenUnneeded=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                this unit will be stopped when it is
                                no longer used. Note that in order to
                                minimize the work to be executed
                                systemd will by default not stop units
                                unless they are conflicting with other
                                units, or the user explicitly
                                requested their shut down. If this
                                option is set a unit will be
                                automatically cleaned up if no other
                                active unit requires it. Defaults to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>OnlyByDependency=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                this unit may only be activated
                                indirectly. In this case explicit
                                start-up requested by the user is
                                denied, however if it is started as
                                dependency of another unit start-up
                                will succeed. This is mostly a safety
                                feature to ensure that the user does
                                not accidentally activate units that are
                                not intended to be activated
                                explicitly. This option defaults to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

                </variablelist>

                <para>Unit file may include a [Install] section, which
                carries installation information for the unit. This
                section is not interpreted by
                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                during runtime. It is used exclusively by the
                <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                tool during installation of a unit:</para>

                <variablelist>
                        <varlistentry>
                                <term><varname>Alias=</varname></term>

                                <listitem><para>Additional names, this
                                unit shall be installed under. The
                                names listed here must have the same
                                suffix (i.e. type) as the unit file
                                name. This option may be specified
                                more than once, in which case all
                                listed names are used. At installation
                                time,
                                <command>systemd-install</command>
                                will create symlinks from these names
                                to the unit file name. Note that this
                                is different from the
                                <varname>Names=</varname> option from
                                the [Unit] section mentioned above:
                                The names from
                                <varname>Names=</varname> apply
                                unconditionally if the unit is
                                loaded. The names from
                                <varname>Alias=</varname> apply only
                                if the unit has actually been
                                installed with the
                                <command>systemd-install</command>
                                tool.  Also, if systemd searches for a
                                unit, it will discover symlinked alias
                                names as configured with
                                <varname>Alias=</varname>, but not
                                names configured with
                                <varname>Names=</varname> only. It is
                                a common pattern to list a name in
                                both options. In this case, a unit
                                will be active under all names if
                                installed, but also if not installed
                                but requested explicitly under its
                                main name.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>WantedBy=</varname></term>

                                <listitem><para>Installs a symlink in
                                the <filename>.wants/</filename>
                                subdirectory for a unit. This has the
                                effect that when the listed unit name
                                is activated the unit listing it is
                                activated
                                too. <command>WantedBy=foo.service</command>
                                in a service
                                <filename>bar.service</filename> is
                                mostly equivalent to
                                <command>Alias=foo.service.wants/bar.service</command>
                                in the same file.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Also=</varname></term>

                                <listitem><para>Additional units to
                                install when this unit is
                                installed. If the user requests
                                installation of a unit with this
                                option configured
                                <command>systemd-install</command>
                                will automatically install units
                                listed in this option as
                                well.</para></listitem>
                        </varlistentry>
                </variablelist>

        </refsect1>

        <refsect1>
                <title>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.snapshot</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>