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diff --git a/man/systemd.service.xml b/man/systemd.service.xml index 4c890dfb7b..f33e8df056 100644 --- a/man/systemd.service.xml +++ b/man/systemd.service.xml @@ -1358,6 +1358,302 @@ ExecStart=/bin/echo $ONE $TWO $THREE</programlisting> </refsect1> <refsect1> + <title>Examples</title> + + <example> + <title>Simple service</title> + + <para>The following unit file creates a service + that will execute + <filename>/usr/sbin/foo-daemon</filename>. + Since no <varname>Type=</varname> is specified, + the default + <varname>Type=</varname><option>simple</option> + will be assumed. systemd will assume the unit + to be started immediately after the program has + begun executing.</para> + + <programlisting>[Unit] +Description=Foo + +[Service] +ExecStart=/usr/sbin/foo-daemon + +[Install] +WantedBy=multi-user.target</programlisting> + + <para>Note that systemd assumes here that the + process started by systemd will continue + running until the service terminates. If the + program daemonizes itself (i.e. forks), please + use + <varname>Type=</varname><option>forking</option> + instead.</para> + + <para>Since no <varname>ExecStop=</varname> was + specified, systemd will send SIGTERM to all + processes started from this service, and after + a timeout also SIGKILL. This behavior can be + modified, see + <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry> + for details.</para> + + <para>Note that this unit type does not include + any type of notification when a service has + completed initialization. For this, you should + use other unit types, such as + <varname>Type=</varname><option>notify</option> + if the service understands systemd's + notification protocol, + <varname>Type=</varname><option>forking</option> + if the service can background itself or + <varname>Type=</varname><option>dbus</option> + if the unit acquires a DBus name once + initialization is complete. See below.</para> + </example> + + <example> + <title>Oneshot service</title> + + <para>Sometimes units should just execute an + action without keeping active processes, such + as a filesystem check or a cleanup action on + boot. For this, + <varname>Type=</varname><option>oneshot</option> + exists. Units of this type will wait until the + process specified terminates and then fall back + to being inactive. The following unit will + perform a clenaup action:</para> + + <programlisting>[Unit] +Description=Cleanup old Foo data + +[Service] +Type=oneshot +ExecStart=/usr/sbin/foo-cleanup + +[Install] +WantedBy=multi-user.target</programlisting> + + <para>Note that systemd will consider the unit + to be in the state 'starting' until the program + has terminated, so ordered dependencies will + wait for the program to finish before starting + themselves. The unit will revert to the + 'inactive' state after the execution is + done, never reaching the 'active' state. That + means another request to start the unit will + perform the action again.</para> + + <para><varname>Type=</varname><option>oneshot</option> + are the only service units that may have more + than one <varname>ExecStart=</varname> + specified. They will be executed in order until + either they are all successful or one of them + fails.</para> + </example> + + <example> + <title>Stoppable oneshot service</title> + + <para>Similarly to the oneshot services, there + are sometimes units that need to execute a + program to set up something and then execute + another to shut it down, but no process remains + active while they are considered + 'started'. Network configuration can sometimes + fall into this category. Another use case is if + a oneshot service shall not be executed a + each time when they are pulled in as a + dependency, but only the first time.</para> + + <para>For this, systemd knows the setting + <varname>RemainAfterExit=</varname><option>yes</option>, + which causes systemd to consider the unit to be + active if the start action exited successfully. + This directive can be used with all types, but + is most useful with + <varname>Type=</varname><option>oneshot</option> + and + <varname>Type=</varname><option>simple</option>. + With + <varname>Type=</varname><option>oneshot</option> + systemd waits until the start action has + completed before it considers the unit to be + active, so dependencies start only after the + start action has succeeded. With + <varname>Type=</varname><option>simple</option> + dependencies will start immediately after the + start action has been dispatched. The following + unit provides an example for a simple static + firewall.</para> + + <programlisting>[Unit] +Description=Simple firewall + +[Service] +Type=oneshot +RemainAfterExit=yes +ExecStart=/usr/local/sbin/simple-firewall-start +ExecStop=/usr/local/sbin/simple-firewall-stop + +[Install] +WantedBy=multi-user.target</programlisting> + + <para>Since the unit is considered to be + running after the start action has exited, + invoking <command>systemctl start</command> on + that unit again will cause no action to be + taken.</para> + </example> + + <example> + <title>Traditional forking services</title> + + <para>Many traditional daemons/services + background (i.e. fork, daemonize) themselves + when starting. Set + <varname>Type=</varname><option>forking</option> + in the service's unit file to support this mode + of operation. systemd will consider the service + to be in the process of initialization while + the original program is still running. Once + it exits successfully and at least a process + remains (and + <varname>RemainAfterExit=</varname><option>no</option>), + the service is considered started.</para> + + <para>Often a traditional daemon only consists + of one process. Therefore, if only one process + is left after the original process terminates, + systemd will consider that process the main + process of the service. In that case, the + <varname>$MAINPID</varname> variable will be + available in <varname>ExecReload=</varname>, + <varname>ExecStop=</varname>, etc.</para> + + <para>In case more than one process remains, + systemd will be unable to determine the main + process, so it will not assume there is one. + In that case, <varname>$MAINPID</varname> will + not expand to anything. However, if the process + decides to write a traditional PID file, + systemd will be able to read the main PID from + there. Please set <varname>PIDFile=</varname> + accordingly. Note that the daemon should write + that file before finishing with its + initialization, otherwise systemd might try to + read the file before it exists.</para> + + <para>The following example shows a simple + daemon that forks and just starts one process + in the background:</para> + + <programlisting>[Unit] +Description=Some simple daemon + +[Service] +Type=forking +ExecStart=/usr/sbin/my-simple-daemon -d + +[Install] +WantedBy=multi-user.target</programlisting> + + <para>Please see + <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry> + for details on how you can influence the way + systemd terminates the service.</para> + </example> + + <example> + <title>DBus services</title> + + <para>For services that acquire a name on the + DBus system bus, use + <varname>Type=</varname><option>dbus</option> + and set <varname>BusName=</varname> + accordingly. The service should not fork + (daemonize). systemd will consider the service + to be initialized once the name has been + acquired on the system bus. The following + example shows a typical DBus service:</para> + + <programlisting>[Unit] +Description=Simple DBus service + +[Service] +Type=dbus +BusName=org.example.simple-dbus-service +ExecStart=/usr/sbin/simple-dbus-service + +[Install] +WantedBy=multi-user.target</programlisting> + + <para>For <emphasis>bus-activatable</emphasis> + services, don't include a + <literal>[Install]</literal> section in the + systemd service file, but use the + <varname>SystemdService=</varname> option in + the corresponding DBus service file, for + example + (<filename>/usr/share/dbus-1/system-services/org.example.simple-dbus-service.service</filename>):</para> + + <programlisting>[D-BUS Service] +Name=org.example.simple-dbus-service +Exec=/usr/sbin/simple-dbus-service +User=root +SystemdService=simple-dbus-service.service</programlisting> + + <para>Please see + <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry> + for details on how you can influence the way + systemd terminates the service.</para> + </example> + + <example> + <title>Services that notify systemd about their initialization</title> + + <para><varname>Type=</varname><option>simple</option> + services are really easy to write, but have the + major disadvantage of systemd not being able to + tell when initialization of the given service + is complete. For this reason, systemd supports + a simple notification protocol that allows + daemons to make systemd aware that they are + done initializing. Use + <varname>Type=</varname><option>notify</option> + for this. A typical service file for such a + daemon would look like this:</para> + + <programlisting>[Unit] +Description=Simple notifying service + +[Service] +Type=notify +ExecStart=/usr/sbin/simple-notifying-service + +[Install] +WantedBy=multi-user.target</programlisting> + + <para>Note that the daemon has to support + systemd's notification protocol, else systemd + will think the service hasn't started yet and + kill it after a timeout. For an example of how + to update daemons to support this protocol + transparently, take a look at + <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>. + systemd will consider the unit to be in the + 'starting' state until a readiness notification + has arrived.</para> + + <para>Please see + <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry> + for details on how you can influence the way + systemd terminates the service.</para> + </example> + </refsect1> + + <refsect1> <title>See Also</title> <para> <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |