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With <para><filename>…</filename></para>, we get a separate "paragraph" for each line, i.e. entries separated by empty lines. This uses up a lot of space and was only done because docbook makes it hard to insert a newline. In some other places, <literallayout> was used, but then we cannot indent the source text (because the whitespace would end up in the final page). We can get the desired result with <simplelist>. With <simplelist> the items are indented in roff output, but not in html output. In some places this looks better then no indentation, and in others it would probably be better to have no indent. But this is a minor issue and we cannot control that. (I didn't convert all spots. There's a bunch of other man pages which have two lines, e.g. an executable and service file, and it doesn't matter there so much.)
225 lines
11 KiB
XML
225 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/4.2/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/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/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 "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>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-delta</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</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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