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229 lines
11 KiB
XML
229 lines
11 KiB
XML
<?xml version="1.0"?>
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<!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd.preset">
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<refentryinfo>
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<title>systemd.preset</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.preset</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.preset</refname>
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<refpurpose>Service enablement presets</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><simplelist>
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<member><filename>/etc/systemd/system-preset/*.preset</filename></member>
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<member><filename>/run/systemd/system-preset/*.preset</filename></member>
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<member><filename>/usr/local/lib/systemd/system-preset/*.preset</filename></member>
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<member><filename>/usr/lib/systemd/system-preset/*.preset</filename></member>
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<member><filename>/etc/systemd/user-preset/*.preset</filename></member>
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<member><filename>/run/systemd/user-preset/*.preset</filename></member>
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<member><filename>/usr/local/lib/systemd/user-preset/*.preset</filename></member>
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<member><filename>/usr/lib/systemd/user-preset/*.preset</filename></member>
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</simplelist></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>Preset files may be used to encode policy which units shall be enabled by default and which ones
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shall be disabled. They are read by <command>systemctl preset</command> which uses this information to
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enable or disable a unit. Depending on that policy, <command>systemctl preset</command> is identical to
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<command>systemctl enable</command> or <command>systemctl disable</command>.
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<command>systemctl preset</command> is used by the post install scriptlets of rpm packages (or other OS
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package formats), to enable/disable specific units by default on package installation, enforcing
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distribution, spin, or administrator preset policy. This allows choosing a certain set of units to be
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enabled/disabled even before installing the actual package. For more information, see
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
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<para>It is not recommended to ship preset files within the respective software packages implementing the
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units, but rather centralize them in a distribution or spin default policy, which can be amended by
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administrator policy, see below.</para>
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<para>If no preset files exist, preset operations will enable all units that are installed by default. If
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this is not desired and all units shall rather be disabled, it is necessary to ship a preset file with a
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single, catchall "<filename>disable *</filename>" line. (See example 1, below.)</para>
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<para>When the machine is booted for the first time,
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will
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enable/disable all units according to preset policy, similarly to <command>systemctl
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preset-all</command>. Also see <varname>ConditionFirstBoot=</varname> in
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
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"First Boot Semantics" in
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<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para>
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</refsect1>
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<refsect1>
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<title>Preset File Format</title>
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<para>The preset files contain a list of directives, one per line. Empty lines and lines whose first
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non-whitespace character is <literal>#</literal> or <literal>;</literal> are ignored. Each directive
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consists of one of the words <literal>enable</literal>, <literal>disable</literal>, or
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<literal>ignore</literal>, followed by whitespace and a unit name. The unit name may contain shell-style
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wildcards.</para>
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<para>For the enable directive for template units, one or more instance names may be specified as a
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space-separated list after the unit name. In this case, those instances will be enabled instead of the
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instance specified via DefaultInstance= in the unit.</para>
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<para>Presets must refer to the "real" unit file, and not to any aliases. See
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for a description of unit aliasing.</para>
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<para>Three different directives are understood: <literal>enable</literal> may be used to enable units by
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default, <literal>disable</literal> to disable units by default, and <literal>ignore</literal> to ignore
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units and leave existing configuration intact.</para>
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<para>If multiple lines apply to a unit name, the first matching
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one takes precedence over all others.</para>
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<para>Each preset file shall be named in the style of
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<filename><priority>-<policy-name>.preset</filename>. Files
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in <filename>/etc/</filename> override files with the same name in
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<filename>/usr/lib/</filename> and <filename>/run/</filename>.
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Files in <filename>/run/</filename> override files with the same
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name in <filename>/usr/lib/</filename>. Packages should install
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their preset files in <filename>/usr/lib/</filename>. Files in
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<filename>/etc/</filename> are reserved for the local
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administrator, who may use this logic to override the preset files
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installed by vendor packages. All preset files are sorted by their
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filename in lexicographic order, regardless of which of the
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directories they reside in. If multiple files specify the same
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unit name, the entry in the file with the lexicographically
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earliest name will be applied. It is recommended to prefix all
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filenames with a two-digit number and a dash, to simplify the
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ordering of the files.</para>
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<para>If the administrator wants to disable a preset file supplied
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by the vendor, the recommended way is to place a symlink to
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<filename>/dev/null</filename> in
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<filename>/etc/systemd/system-preset/</filename> bearing the same
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filename.</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<example>
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<title>Default to off</title>
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<programlisting># /usr/lib/systemd/system-preset/99-default.preset
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disable *</programlisting>
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</example>
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<para>This disables all units. Due to the filename prefix
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<literal>99-</literal>, it will be read last and hence can easily
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be overridden by spin or administrator preset policy.</para>
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<example>
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<title>Enable multiple template instances</title>
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<programlisting># /usr/lib/systemd/system-preset/80-dirsrv.preset
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enable dirsrv@.service foo bar baz</programlisting>
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</example>
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<para>This enables all three of <filename>dirsrv@foo.service</filename>,
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<filename>dirsrv@bar.service</filename> and <filename>dirsrv@baz.service</filename>.</para>
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<example>
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<title>A GNOME spin</title>
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<programlisting># /usr/lib/systemd/system-preset/50-gnome.preset
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enable gdm.service
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enable colord.service
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enable accounts-daemon.service
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enable avahi-daemon.*</programlisting>
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</example>
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<para>This enables the three mentioned units, plus all
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<filename>avahi-daemon</filename> regardless of which unit type. A
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file like this could be useful for inclusion in a GNOME spin of a
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distribution. It will ensure that the units necessary for GNOME
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are properly enabled as they are installed. It leaves all other
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units untouched, and subject to other (later) preset files, for
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example like the one from the first example above.</para>
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<example>
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<title>Administrator policy</title>
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<programlisting># /etc/systemd/system-preset/00-lennart.preset
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enable httpd.service
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enable sshd.service
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enable postfix.service
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disable *</programlisting>
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</example>
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<para>This enables three specific services and disables all
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others. This is useful for administrators to specifically select
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the units to enable, and disable all others. Due to the filename
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prefix <literal>00-</literal> it will be read early and
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override all other preset policy files.</para>
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</refsect1>
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<refsect1>
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<title>Motivation for the preset logic</title>
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<para>Different distributions have different policies on which services shall be enabled by default when
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the package they are shipped in is installed. On Fedora all services stay off by default, so that
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installing a package will not cause a service to be enabled (with some exceptions). On Debian all
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services are immediately enabled by default, so that installing a package will cause its services to be
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enabled right-away.</para>
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<para>Even within a single distribution, different spins (flavours, remixes, whatever you might want to
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call them) of a distribution also have different policies on what services to enable, and what services
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to leave off. For example, Fedora Workstation will enable <command>gdm</command> as display manager by
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default, while the Fedora KDE spin will enable <command>sddm</command> instead.</para>
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<para>Different sites might also have different policies what to turn on by default and what to turn
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off. For example, one administrator would prefer to enforce the policy of "<command>sshd</command> should
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be always on, but everything else off", while another one might say "<command>snmpd</command> always on,
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and for everything else use the distribution policy defaults".</para>
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<para>Traditionally, policy about which services shall be enabled were implemented in each package
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individually. This made it cumbersome to implement different policies per spin or per site, or to create
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software packages that do the right thing on more than one distribution. The enablement mechanism was
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also encoding the enablement policy.</para>
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<para>The preset mechanism allows clean separation of the enablement mechanism (inside the package
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scriptlets, by invoking <command>systemctl preset</command>) and enablement policy (centralized in the
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preset files), and lifts the configuration out of individual packages. Preset files may be written for
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specific distributions, for specific spins or for specific sites, in order to enforce different policies
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as needed. It is recommended to apply the policy encoded in preset files in package installation
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scriptlets.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-delta</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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</simplelist></para>
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<para><citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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has a discussion of packaging scriptlets.</para>
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<para>Fedora page introducing the use of presets:
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<ulink url="https://fedoraproject.org/wiki/Features/PackagePresets">Features/PackagePresets</ulink>.
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</para>
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</refsect1>
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</refentry>
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