man: document the new Type=exec type
And while we are at it, let's rearrange and extend the Type= documentation a bit. Let's make it an itemized list, and let's add a paragraph explaining which type best to use.
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Lennart Poettering
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5686391b00
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@ -83,6 +83,16 @@
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<replaceable>COMMAND</replaceable> may be omitted. In this case, <command>systemd-run</command> creates only a
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<filename>.path</filename>, <filename>.socket</filename>, or <filename>.timer</filename> unit that triggers the
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specified unit.</para>
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<para>By default, services created with <command>systemd-run</command> default to the <option>simple</option> type,
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see the description of <varname>Type=</varname> in
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<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
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details. Note that when this type is used the service manager (and thus the <command>systemd-run</command> command)
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considers service start-up successful as soon as the <function>fork()</function> for the main service process
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succeeded, i.e. before the <function>execve()</function> is invoked, and thus even if the specified command cannot
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be started. Consider using the <option>exec</option> service type (i.e. <option>--property=Type=exec</option>) to
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ensure that <command>systemd-run</command> returns successfully only if the specified command line has been
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successfully started.</para>
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</refsect1>
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<refsect1>
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@ -153,77 +153,93 @@
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<varlistentry>
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<term><varname>Type=</varname></term>
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<listitem><para>Configures the process start-up type for this
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service unit. One of
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<option>simple</option>,
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<option>forking</option>,
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<option>oneshot</option>,
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<option>dbus</option>,
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<option>notify</option> or
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<option>idle</option>.</para>
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<listitem>
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<para>Configures the process start-up type for this service unit. One of <option>simple</option>,
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<option>exec</option>, <option>forking</option>, <option>oneshot</option>, <option>dbus</option>,
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<option>notify</option> or <option>idle</option>:</para>
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<para>If set to <option>simple</option> (the default if
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neither <varname>Type=</varname> nor
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<varname>BusName=</varname>, but <varname>ExecStart=</varname>
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are specified), it is expected that the process configured
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with <varname>ExecStart=</varname> is the main process of the
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service. In this mode, if the process offers functionality to
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other processes on the system, its communication channels
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should be installed before the daemon is started up (e.g.
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sockets set up by systemd, via socket activation), as systemd
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will immediately proceed starting follow-up units.</para>
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<itemizedlist>
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<listitem><para>If set to <option>simple</option> (the default if <varname>ExecStart=</varname> is
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specified but neither <varname>Type=</varname> nor <varname>BusName=</varname> are), the service manager
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will consider the unit started immediately after the main service process has been forked off. It is
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expected that the process configured with <varname>ExecStart=</varname> is the main process of the
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service. In this mode, if the process offers functionality to other processes on the system, its
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communication channels should be installed before the service is started up (e.g. sockets set up by
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systemd, via socket activation), as the service manager will immediately proceed starting follow-up units,
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right after creating the main service process, and before executing the service's binary. Note that this
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means <command>systemctl start</command> command lines for <option>simple</option> services will report
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success even if the service's binary cannot be invoked successfully (for example because the selected
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<varname>User=</varname> doesn't exist, or the service binary is missing).</para></listitem>
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<para>If set to <option>forking</option>, it is expected that
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the process configured with <varname>ExecStart=</varname> will
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call <function>fork()</function> as part of its start-up. The
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parent process is expected to exit when start-up is complete
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and all communication channels are set up. The child continues
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to run as the main daemon process. This is the behavior of
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traditional UNIX daemons. If this setting is used, it is
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recommended to also use the <varname>PIDFile=</varname>
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option, so that systemd can identify the main process of the
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daemon. systemd will proceed with starting follow-up units as
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soon as the parent process exits.</para>
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<listitem><para>The <option>exec</option> type is similar to <option>simple</option>, but the service
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manager will consider the unit started immediately after the main service binary has been executed. The service
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manager will delay starting of follow-up units until that point. (Or in other words:
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<option>simple</option> proceeds with further jobs right after <function>fork()</function> returns, while
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<option>exec</option> will not proceed before both <function>fork()</function> and
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<function>execve()</function> in the service process succeeded.) Note that this means <command>systemctl
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start</command> command lines for <option>exec</option> services will report failure when the service's
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binary cannot be invoked successfully (for example because the selected <varname>User=</varname> doesn't
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exist, or the service binary is missing).</para></listitem>
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<para>Behavior of <option>oneshot</option> is similar to
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<option>simple</option>; however, it is expected that the
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process has to exit before systemd starts follow-up units.
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<varname>RemainAfterExit=</varname> is particularly useful for
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this type of service. This is the implied default if neither
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<varname>Type=</varname> nor <varname>ExecStart=</varname> are
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specified.</para>
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<listitem><para>If set to <option>forking</option>, it is expected that the process configured with
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<varname>ExecStart=</varname> will call <function>fork()</function> as part of its start-up. The parent
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process is expected to exit when start-up is complete and all communication channels are set up. The child
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continues to run as the main service process, and the service manager will consider the unit started when
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the parent process exits. This is the behavior of traditional UNIX services. If this setting is used, it is
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recommended to also use the <varname>PIDFile=</varname> option, so that systemd can reliably identify the
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main process of the service. systemd will proceed with starting follow-up units as soon as the parent
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process exits.</para></listitem>
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<para>Behavior of <option>dbus</option> is similar to
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<option>simple</option>; however, it is expected that the
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daemon acquires a name on the D-Bus bus, as configured by
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<varname>BusName=</varname>. systemd will proceed with
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starting follow-up units after the D-Bus bus name has been
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acquired. Service units with this option configured implicitly
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gain dependencies on the <filename>dbus.socket</filename>
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unit. This type is the default if <varname>BusName=</varname>
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is specified.</para>
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<listitem><para>Behavior of <option>oneshot</option> is similar to <option>simple</option>; however, the
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service manager will consider the unit started after the main process exits. It will then start follow-up
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units. <varname>RemainAfterExit=</varname> is particularly useful for this type of
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service. <varname>Type=</varname><option>oneshot</option> is the implied default if neither
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<varname>Type=</varname> nor <varname>ExecStart=</varname> are specified.</para></listitem>
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<para>Behavior of <option>notify</option> is similar to
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<option>simple</option>; however, it is expected that the
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daemon sends a notification message via
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<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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or an equivalent call when it has finished starting up.
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systemd will proceed with starting follow-up units after this
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notification message has been sent. If this option is used,
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<varname>NotifyAccess=</varname> (see below) should be set to
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open access to the notification socket provided by systemd. If
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<varname>NotifyAccess=</varname> is missing or set to
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<option>none</option>, it will be forcibly set to
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<option>main</option>. Note that currently
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<varname>Type=</varname><option>notify</option> will not work
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if used in combination with
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<varname>PrivateNetwork=</varname><option>yes</option>.</para>
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<listitem><para>Behavior of <option>dbus</option> is similar to <option>simple</option>; however, it is
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expected that the service acquires a name on the D-Bus bus, as configured by
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<varname>BusName=</varname>. systemd will proceed with starting follow-up units after the D-Bus bus name
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has been acquired. Service units with this option configured implicitly gain dependencies on the
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<filename>dbus.socket</filename> unit. This type is the default if <varname>BusName=</varname> is
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specified.</para></listitem>
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<para>Behavior of <option>idle</option> is very similar to <option>simple</option>; however, actual execution
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of the service program is delayed until all active jobs are dispatched. This may be used to avoid interleaving
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of output of shell services with the status output on the console. Note that this type is useful only to
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improve console output, it is not useful as a general unit ordering tool, and the effect of this service type
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is subject to a 5s time-out, after which the service program is invoked anyway.</para>
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<listitem><para>Behavior of <option>notify</option> is similar to <option>exec</option>; however, it is
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expected that the service sends a notification message via
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<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> or an
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equivalent call when it has finished starting up. systemd will proceed with starting follow-up units after
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this notification message has been sent. If this option is used, <varname>NotifyAccess=</varname> (see
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below) should be set to open access to the notification socket provided by systemd. If
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<varname>NotifyAccess=</varname> is missing or set to <option>none</option>, it will be forcibly set to
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<option>main</option>. Note that currently <varname>Type=</varname><option>notify</option> will not work if
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used in combination with <varname>PrivateNetwork=</varname><option>yes</option>.</para></listitem>
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<listitem><para>Behavior of <option>idle</option> is very similar to <option>simple</option>; however,
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actual execution of the service program is delayed until all active jobs are dispatched. This may be used
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to avoid interleaving of output of shell services with the status output on the console. Note that this
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type is useful only to improve console output, it is not useful as a general unit ordering tool, and the
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effect of this service type is subject to a 5s time-out, after which the service program is invoked
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anyway.</para></listitem>
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</itemizedlist>
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<para>It is generally recommended to use <varname>Type=</varname><option>simple</option> for long-running
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services whenever possible, as it is the simplest and fastest option. However, as this service type won't
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propagate service start-up failures and doesn't allow ordering of other units against completion of
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initialization of the service (which for example is useful if clients need to connect to the service through
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some form of IPC, and the IPC channel is only established by the service itself — in contrast to doing this
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ahead of time through socket or bus activation or similar), it might not be sufficient for many cases. If so,
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<option>notify</option> or <option>dbus</option> (the latter only in case the service provides a D-Bus
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interface) are the preferred options as they allow service program code to precisely schedule when to
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consider the service started up successfully and when to proceed with follow-up units. The
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<option>notify</option> service type requires explicit support in the service codebase (as
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<function>sd_notify()</function> or an equivalent API needs to be invoked by the service at the appropriate
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time) — if it's not supported, then <option>forking</option> is an alternative: it supports the traditional
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UNIX service start-up protocol. Finally, <option>exec</option> might be an option for cases where it is
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enough to ensure the service binary is invoked, and where the service binary itself executes no or little
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initialization on its own (and its initialization is unlikely to fail). Note that using any type other than
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<option>simple</option> possibly delays the boot process, as the service manager needs to wait for service
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initialization to complete. It is hence recommended not to needlessly use any types other than
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<option>simple</option>. (Also note it is generally not recommended to use <option>idle</option> or
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<option>oneshot</option> for long-running services.)</para>
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</listitem>
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</varlistentry>
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