units: for all unit settings that take lists, allow the empty string for resetting...

[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>Unit configuration</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 page lists the common configuration
                options of all the unit types. These options need to
                be configured in the [Unit] or [Install]
                sections of the unit files.</para>

                <para>In addition to the generic [Unit] and [Install]
                sections described here, each unit may 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
                values 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. For details
                see
                <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

                <para>Empty lines and lines starting with # or ; are
                ignored. This may be used for commenting. Lines ending
                in a backslash are concatenated with the following
                line while reading and the backslash is replaced by a
                space character. This may be used to wrap long lines.</para>

                <para>Along with a unit file
                <filename>foo.service</filename> the directory
                <filename>foo.service.wants/</filename> may exist. All
                unit files 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 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 unit file
                is with the <command>enable</command> command of the
                <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                tool which reads information from the [Install]
                section of unit files (see below). A similar
                functionality exists for <varname>Requires=</varname>
                type dependencies as well, the directory suffix is
                <filename>.requires/</filename> in this case.</para>

                <para>Along with a unit file
                <filename>foo.service</filename> a directory
                <filename>foo.service.d/</filename> may exist. All
                files with the suffix <filename>.conf</filename> from
                this directory will be parsed after the file itself is
                parsed. This is useful to alter or add configuration
                settings to a unit, without having to modify their
                unit files. Make sure that the file that is included
                has the appropriate section headers before any
                directive.</para>

                <para>If a line starts with <option>.include</option>
                followed by a file name, the specified file will be
                parsed at this point. Make sure that the file that is
                included has the appropriate section headers before
                any directives.</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 the
                result 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.</para>

                <para>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. See below for details.</para>

                <para>If a unit file is empty (i.e. has the file size
                0) or is symlinked to <filename>/dev/null</filename>
                its configuration will not be loaded and it appears
                with a load state of <literal>masked</literal>, and
                cannot be activated. Use this as an effective way to
                fully disable a unit, making it impossible to start it
                even manually.</para>

                <para>The unit file format is covered by the
                <ulink
                url="http://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
                Stability Promise</ulink>.</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>Description=</varname></term>
                                <listitem><para>A free-form string
                                describing the unit. This is intended
                                for use in UIs to show descriptive
                                information along with the unit
                                name.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Documentation=</varname></term>
                                <listitem><para>A space separated list
                                of URIs referencing documentation for
                                this unit or its
                                configuration. Accepted are only URIs
                                of the types
                                <literal>http://</literal>,
                                <literal>https://</literal>,
                                <literal>file:</literal>,
                                <literal>info:</literal>,
                                <literal>man:</literal>. For more
                                information about the syntax of these
                                URIs see
                                <citerefentry><refentrytitle>uri</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
                                URIs should be listed in order of
                                relevance, starting with the most
                                relevant. It is a good idea to first
                                reference documentation that explains
                                what the unit's purpose is, followed
                                by how it is configured, followed by
                                any other related documentation. This
                                option may be specified more than once
                                in which case the specified list of
                                URIs is merged. If the empty string is
                                assigned to this option the list is
                                reset and all prior assignments will
                                have no effect.</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Configures requirement
                                dependencies on other units. If this
                                unit gets 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>

                                <para>Note that dependencies of this
                                type may also be configured outside of
                                the unit configuration file by
                                adding a symlink to a
                                <filename>.requires/</filename> directory
                                accompanying the unit file. For
                                details see above.</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 if 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>
                                and <varname>RequiresOverridable=</varname>, respectively. 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,
                                if 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.</para>

                                <para>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>BindsTo=</varname></term>

                                <listitem><para>Configures requirement
                                dependencies, very similar in style to
                                <varname>Requires=</varname>, however
                                in addition to this behavior it also
                                declares that this unit is stopped
                                when any of the units listed suddenly
                                disappears. Units can suddenly,
                                unexpectedly disappear if a service
                                terminates on its own choice, a device
                                is unplugged or a mount point
                                unmounted without involvement of
                                systemd.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>PartOf=</varname></term>

                                <listitem><para>Configures dependencies
                                similar to <varname>Requires=</varname>,
                                but limited to stopping and restarting
                                of units. When systemd stops or restarts
                                the units listed here, the action is
                                propagated to this unit.
                                Note that this is a one way dependency -
                                changes to this unit do not affect the
                                listed units.
                                </para></listitem>
                        </varlistentry>

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

                                <listitem><para>Configures negative
                                requirement dependencies. If a unit
                                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>

                                <para>If a unit A that conflicts with
                                a unit B is scheduled to be started at
                                the same time as B, the transaction
                                will either fail (in case both are
                                required part of the transaction) or
                                be modified to be fixed (in case one
                                or both jobs are not a required part
                                of the transaction). In the latter
                                case the job that is not the required
                                will be removed, or in case both are
                                not required the unit that conflicts
                                will be started and the unit that is
                                conflicted is
                                stopped.</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 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
                                or started up simultaneously, and
                                no ordering takes
                                place. </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>OnFailure=</varname></term>

                                <listitem><para>Lists one or more
                                units that are activated when this
                                unit enters the
                                '<literal>failed</literal>'
                                state.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>PropagatesReloadTo=</varname></term>
                                <term><varname>ReloadPropagatedFrom=</varname></term>

                                <listitem><para>Lists one or more
                                units where reload requests on the
                                unit will be propagated to/on the
                                other unit will be propagated
                                from. Issuing a reload request on a
                                unit will automatically also enqueue a
                                reload request on all units that the
                                reload request shall be propagated to
                                via these two
                                settings.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>RequiresMountsFor=</varname></term>

                                <listitem><para>Takes a space
                                separated list of absolute paths. Automatically
                                adds dependencies of type
                                <varname>Requires=</varname> and
                                <varname>After=</varname> for all
                                mount units required to access the
                                specified path.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>OnFailureIsolate=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option> the
                                unit listed in
                                <varname>OnFailure=</varname> will be
                                enqueued in isolation mode, i.e. all
                                units that are not its dependency will
                                be stopped. If this is set only a
                                single unit may be listed in
                                <varname>OnFailure=</varname>. Defaults
                                to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>IgnoreOnIsolate=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                this unit will not be stopped when
                                isolating another unit. Defaults to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>IgnoreOnSnapshot=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                this unit will not be included in
                                snapshots. Defaults to
                                <option>true</option> for device and
                                snapshot units, <option>false</option>
                                for the others.</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 not stop units by default
                                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>RefuseManualStart=</varname></term>
                                <term><varname>RefuseManualStop=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                this unit can only be activated
                                or deactivated indirectly. In
                                this case explicit start-up
                                or termination requested by the
                                user is denied, however if it is
                                started or stopped as a
                                dependency of another unit, start-up
                                or termination 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, and not
                                accidentally deactivate units that are
                                not intended to be deactivated.
                                These options default to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>AllowIsolate=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                this unit may be used with the
                                <command>systemctl isolate</command>
                                command. Otherwise this will be
                                refused. It probably is a good idea to
                                leave this disabled except for target
                                units that shall be used similar to
                                runlevels in SysV init systems, just
                                as a precaution to avoid unusable
                                system states. This option defaults to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>DefaultDependencies=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                (the default), a few default
                                dependencies will implicitly be
                                created for the unit. The actual
                                dependencies created depend on the
                                unit type. For example, for service
                                units, these dependencies ensure that
                                the service is started only after
                                basic system initialization is
                                completed and is properly terminated on
                                system shutdown. See the respective
                                man pages for details. Generally, only
                                services involved with early boot or
                                late shutdown should set this option
                                to <option>false</option>. It is
                                highly recommended to leave this
                                option enabled for the majority of
                                common units. If set to
                                <option>false</option> this option
                                does not disable all implicit
                                dependencies, just non-essential
                                ones.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>JobTimeoutSec=</varname></term>

                                <listitem><para>When clients are
                                waiting for a job of this unit to
                                complete, time out after the specified
                                time. If this time limit is reached
                                the job will be cancelled, the unit
                                however will not change state or even
                                enter the '<literal>failed</literal>'
                                mode. This value defaults to 0 (job
                                timeouts disabled), except for device
                                units. NB: this timeout is independent
                                from any unit-specific timeout (for
                                example, the timeout set with
                                <varname>Timeout=</varname> in service
                                units) as the job timeout has no
                                effect on the unit itself, only on the
                                job that might be pending for it. Or
                                in other words: unit-specific timeouts
                                are useful to abort unit state
                                changes, and revert them. The job
                                timeout set with this option however
                                is useful to abort only the job
                                waiting for the unit state to
                                change.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>ConditionPathExists=</varname></term>
                                <term><varname>ConditionPathExistsGlob=</varname></term>
                                <term><varname>ConditionPathIsDirectory=</varname></term>
                                <term><varname>ConditionPathIsSymbolicLink=</varname></term>
                                <term><varname>ConditionPathIsMountPoint=</varname></term>
                                <term><varname>ConditionPathIsReadWrite=</varname></term>
                                <term><varname>ConditionDirectoryNotEmpty=</varname></term>
                                <term><varname>ConditionFileNotEmpty=</varname></term>
                                <term><varname>ConditionFileIsExecutable=</varname></term>
                                <term><varname>ConditionKernelCommandLine=</varname></term>
                                <term><varname>ConditionVirtualization=</varname></term>
                                <term><varname>ConditionSecurity=</varname></term>
                                <term><varname>ConditionCapability=</varname></term>
                                <term><varname>ConditionHost=</varname></term>
                                <term><varname>ConditionACPower=</varname></term>
                                <term><varname>ConditionNull=</varname></term>

                                <listitem><para>Before starting a unit
                                verify that the specified condition is
                                true. If it is not true the starting
                                of the unit will be skipped, however
                                all ordering dependencies of it are
                                still respected. A failing condition
                                will not result in the unit being
                                moved into a failure state. The
                                condition is checked at the time the
                                queued start job is to be
                                executed.</para>

                                <para>With
                                <varname>ConditionPathExists=</varname>
                                a file existence condition is
                                checked before a unit is started. If
                                the specified absolute path name does
                                not exist the condition will
                                fail. If the absolute path name passed
                                to
                                <varname>ConditionPathExists=</varname>
                                is prefixed with an exclamation mark
                                ('!'), the test is negated, and the unit
                                is only started if the path does not
                                exist.</para>

                                <para><varname>ConditionPathExistsGlob=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>,
                                but checks for the existence of at
                                least one file or directory matching
                                the specified globbing pattern.</para>

                                <para><varname>ConditionPathIsDirectory=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>
                                but verifies whether a certain path
                                exists and is a
                                directory.</para>

                                <para><varname>ConditionPathIsSymbolicLink=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>
                                but verifies whether a certain path
                                exists and is a symbolic
                                link.</para>

                                <para><varname>ConditionPathIsMountPoint=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>
                                but verifies whether a certain path
                                exists and is a mount
                                point.</para>

                                <para><varname>ConditionPathIsReadWrite=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>
                                but verifies whether the underlying
                                file system is readable and writable
                                (i.e. not mounted
                                read-only).</para>

                                <para><varname>ConditionDirectoryNotEmpty=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>
                                but verifies whether a certain path
                                exists and is a non-empty
                                directory.</para>

                                <para><varname>ConditionFileNotEmpty=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>
                                but verifies whether a certain path
                                exists and refers to a regular file
                                with a non-zero size.</para>

                                <para><varname>ConditionFileIsExecutable=</varname>
                                is similar to
                                <varname>ConditionPathExists=</varname>
                                but verifies whether a certain path
                                exists, is a regular file and marked
                                executable.</para>

                                <para>Similar,
                                <varname>ConditionKernelCommandLine=</varname>
                                may be used to check whether a
                                specific kernel command line option is
                                set (or if prefixed with the
                                exclamation mark unset). The argument
                                must either be a single word, or an
                                assignment (i.e. two words, separated
                                '='). In the former
                                case the kernel command line is
                                searched for the word appearing as is,
                                or as left hand side of an
                                assignment. In the latter case the
                                exact assignment is looked for with
                                right and left hand side
                                matching.</para>

                                <para><varname>ConditionVirtualization=</varname>
                                may be used to check whether the
                                system is executed in a virtualized
                                environment and optionally test
                                whether it is a specific
                                implementation. Takes either boolean
                                value to check if being executed in
                                any virtualized environment, or one of
                                <varname>vm</varname> and
                                <varname>container</varname> to test
                                against a generic type of
                                virtualization solution, or one of
                                <varname>qemu</varname>,
                                <varname>kvm</varname>,
                                <varname>vmware</varname>,
                                <varname>microsoft</varname>,
                                <varname>oracle</varname>,
                                <varname>xen</varname>,
                                <varname>bochs</varname>,
                                <varname>chroot</varname>,
                                <varname>openvz</varname>,
                                <varname>lxc</varname>,
                                <varname>lxc-libvirt</varname>,
                                <varname>systemd-nspawn</varname> to
                                test against a specific
                                implementation. If multiple
                                virtualization technologies are nested
                                only the innermost is considered. The
                                test may be negated by prepending an
                                exclamation mark.</para>

                                <para><varname>ConditionSecurity=</varname>
                                may be used to check whether the given
                                security module is enabled on the
                                system.  Currently the only recognized
                                value is <varname>selinux</varname>.
                                The test may be negated by prepending
                                an exclamation
                                mark.</para>

                                <para><varname>ConditionCapability=</varname>
                                may be used to check whether the given
                                capability exists in the capability
                                bounding set of the service manager
                                (i.e. this does not check whether
                                capability is actually available in
                                the permitted or effective sets, see
                                <citerefentry><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                for details). Pass a capability name
                                such as <literal>CAP_MKNOD</literal>,
                                possibly prefixed with an exclamation
                                mark to negate the check.</para>

                                <para><varname>ConditionHost=</varname>
                                may be used to match against the
                                host name or machine ID of the
                                host. This either takes a host name
                                string (optionally with shell style
                                globs) which is tested against the
                                locally set host name as returned by
                                <citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
                                or a machine ID formatted as string
                                (see
                                <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
                                The test may be negated by prepending
                                an exclamation mark.</para>

                                <para><varname>ConditionACPower=</varname>
                                may be used to check whether the
                                system has AC power, or is exclusively
                                battery powered at the time of
                                activation of the unit. This takes a
                                boolean argument. If set to
                                <varname>true</varname> the condition
                                will hold only if at least one AC
                                connector of the system is connected
                                to a power source, or if no AC
                                connectors are known. Conversely, if
                                set to <varname>false</varname> the
                                condition will hold only if there is
                                at least one AC connector known and
                                all AC connectors are disconnected
                                from a power source.</para>

                                <para>Finally,
                                <varname>ConditionNull=</varname> may
                                be used to add a constant condition
                                check value to the unit. It takes a
                                boolean argument. If set to
                                <varname>false</varname> the condition
                                will always fail, otherwise
                                succeed.</para>

                                <para>If multiple conditions are
                                specified the unit will be executed if
                                all of them apply (i.e. a logical AND
                                is applied). Condition checks can be
                                prefixed with a pipe symbol (|) in
                                which case a condition becomes a
                                triggering condition. If at least one
                                triggering condition is defined for a
                                unit then the unit will be executed if
                                at least one of the triggering
                                conditions apply and all of the
                                non-triggering conditions. If you
                                prefix an argument with the pipe
                                symbol and an exclamation mark the
                                pipe symbol must be passed first, the
                                exclamation second. Except for
                                <varname>ConditionPathIsSymbolicLink=</varname>,
                                all path checks follow symlinks. If
                                any of these options is assigned the
                                empty string the list of conditions is
                                reset completely, all previous
                                condition settings (of any kind) will
                                have no effect.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SourcePath=</varname></term>
                                <listitem><para>A path to a
                                configuration file this unit has been
                                generated from. This is primarily
                                useful for implementation of generator
                                tools that convert configuration from
                                an external configuration file format
                                into native unit files. Thus
                                functionality should not be used in
                                normal units.</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
                <command>enable</command> and
                <command>disable</command> commands of the
                <citerefentry><refentrytitle>systemctl</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>systemctl enable</command>
                                will create symlinks from these names
                                to the unit file name.</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Installs a symlink in
                                the <filename>.wants/</filename>
                                or <filename>.requires/</filename>
                                subdirectory for a unit, respectively. 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>systemctl enable</command>
                                will automatically install units
                                listed in this option as
                                well.</para></listitem>
                        </varlistentry>
                </variablelist>

        </refsect1>

        <refsect1>
                <title>Specifiers</title>

                <para>Many settings resolve specifiers which may be
                used to write generic unit files referring to runtime
                or unit parameters that are replaced when the unit
                files are loaded. The following specifiers are
                understood:</para>

                <table>
                  <title>Specifiers available in unit files</title>
                  <tgroup cols='3' align='left' colsep='1' rowsep='1'>
                    <colspec colname="spec" />
                    <colspec colname="mean" />
                    <colspec colname="detail" />
                    <thead>
                      <row>
                        <entry>Specifier</entry>
                        <entry>Meaning</entry>
                        <entry>Details</entry>
                      </row>
                    </thead>
                    <tbody>
                      <row>
                        <entry><literal>%n</literal></entry>
                        <entry>Full unit name</entry>
                        <entry></entry>
                      </row>
                      <row>
                        <entry><literal>%N</literal></entry>
                        <entry>Unescaped full unit name</entry>
                        <entry></entry>
                      </row>
                      <row>
                        <entry><literal>%p</literal></entry>
                        <entry>Prefix name</entry>
                        <entry>For instantiated units this refers to the string before the @. For non-instantiated units this refers to to the name of the unit with the type suffix removed.</entry>
                      </row>
                      <row>
                        <entry><literal>%P</literal></entry>
                        <entry>Unescaped prefix name</entry>
                        <entry></entry>
                      </row>
                      <row>
                        <entry><literal>%i</literal></entry>
                        <entry>Instance name</entry>
                        <entry>For instantiated units: this is the string between the @ character and the suffix.</entry>
                      </row>
                      <row>
                        <entry><literal>%I</literal></entry>
                        <entry>Unescaped instance name</entry>
                        <entry></entry>
                      </row>
                      <row>
                        <entry><literal>%f</literal></entry>
                        <entry>Unescaped file name</entry>
                        <entry>This is either the unescaped instance name (if applicable) with / prepended (if applicable), or the prefix name similarly prepended with /.</entry>
                      </row>
                      <row>
                        <entry><literal>%c</literal></entry>
                        <entry>Control group path of the unit</entry>
                        <entry></entry>
                      </row>
                      <row>
                        <entry><literal>%r</literal></entry>
                        <entry>Root control group path of systemd</entry>
                        <entry></entry>
                      </row>
                      <row>
                        <entry><literal>%R</literal></entry>
                        <entry>Parent directory of the root control group path of systemd</entry>
                        <entry></entry>
                      </row>
                      <row>
                        <entry><literal>%t</literal></entry>
                        <entry>Runtime socket dir</entry>
                        <entry>This is either <filename>/run</filename> (for the system manager) or <literal>$XDG_RUNTIME_DIR</literal> (for user managers).</entry>
                      </row>
                      <row>
                        <entry><literal>%u</literal></entry>
                        <entry>User name</entry>
                        <entry>This is the name of the configured user of the unit, or (if none is set) the user running the systemd instance.</entry>
                      </row>
                      <row>
                        <entry><literal>%U</literal></entry>
                        <entry>User UID</entry>
                        <entry>This is the UID of the configured user of the unit, or (if none is set) the user running the systemd instance.</entry>
                      </row>
                      <row>
                        <entry><literal>%h</literal></entry>
                        <entry>User home directory</entry>
                        <entry>This is the home directory of the configured user of the unit, or (if none is set) the user running the systemd instance.</entry>
                      </row>
                      <row>
                        <entry><literal>%s</literal></entry>
                        <entry>User shell</entry>
                        <entry>This is the shell of the configured user of the unit, or (if none is set) the user running the systemd instance.</entry>
                      </row>
                      <row>
                        <entry><literal>%m</literal></entry>
                        <entry>Machine ID</entry>
                        <entry>The machine ID of the running system, formatted as string. See <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry> for more information.</entry>
                      </row>
                      <row>
                        <entry><literal>%b</literal></entry>
                        <entry>Boot ID</entry>
                        <entry>The boot ID of the running system, formatted as string. See <citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for more information.</entry>
                      </row>
                      <row>
                        <entry><literal>%H</literal></entry>
                        <entry>Host name</entry>
                        <entry>The host name of the running system.</entry>
                      </row>
                    </tbody>
                  </tgroup>
                </table>
        </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.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>,
                        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>