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From d77d5a9399e393734fe8c8a5ad085036775854a7 Mon Sep 17 00:00:00 2001
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From: Lennart Poettering <lennart@poettering.net>
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Date: Tue, 17 Jul 2018 12:01:26 +0200
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Subject: [PATCH] man: document the new Type=exec type
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And while we are at it, let's rearrange and extend the Type=
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documentation a bit. Let's make it an itemized list, and let's add a
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paragraph explaining which type best to use.
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(cherry picked from commit 79905a246d645d21633f09f564b3672d5085a85c)
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Resolves: #1683334
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---
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man/systemd-run.xml | 10 +++
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man/systemd.service.xml | 158 ++++++++++++++++++++++------------------
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2 files changed, 97 insertions(+), 71 deletions(-)
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diff --git a/man/systemd-run.xml b/man/systemd-run.xml
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index 1c254afae3..a134b2c0dc 100644
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--- a/man/systemd-run.xml
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+++ b/man/systemd-run.xml
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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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+
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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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diff --git a/man/systemd.service.xml b/man/systemd.service.xml
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index add54524ce..315b80e704 100644
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--- a/man/systemd.service.xml
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+++ b/man/systemd.service.xml
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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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-
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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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-
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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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-
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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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-
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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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-
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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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-
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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>
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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