systemd/man/systemd.preset.xml

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<refentry id="systemd.preset">

  <refentryinfo>
    <title>systemd.preset</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.preset</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.preset</refname>
    <refpurpose>Service enablement presets</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>/etc/systemd/system-preset/*.preset</filename></para>
    <para><filename>/run/systemd/system-preset/*.preset</filename></para>
    <para><filename>/usr/lib/systemd/system-preset/*.preset</filename></para>
    <para><filename>/etc/systemd/user-preset/*.preset</filename></para>
    <para><filename>/run/systemd/user-preset/*.preset</filename></para>
    <para><filename>/usr/lib/systemd/user-preset/*.preset</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>Preset files may be used to encode policy which units shall be enabled by default and which ones
    shall be disabled. They are read by <command>systemctl preset</command> which uses this information to
    enable or disable a unit. Depending on that policy, <command>systemctl preset</command> is identical to
    <command>systemctl enable</command> or <command>systemctl disable</command>.

    <command>systemctl preset</command> is used by the post install scriptlets of rpm packages (or other OS
    package formats), to enable/disable specific units by default on package installation, enforcing
    distribution, spin or administrator preset policy. This allows choosing a certain set of units to be
    enabled/disabled even before installing the actual package. For more information, see
    <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>

    <para>It is not recommended to ship preset files within the respective software packages implementing the
    units, but rather centralize them in a distribution or spin default policy, which can be amended by
    administrator policy, see below.</para>

    <para>If no preset files exist, <command>systemctl
    preset</command> will enable all units that are installed by
    default. If this is not desired and all units shall rather be
    disabled, it is necessary to ship a preset file with a single,
    catchall "<filename>disable *</filename>" line. (See example 1,
    below.)</para>
  </refsect1>

  <refsect1>
    <title>Preset File Format</title>

    <para>The preset files contain a list of directives consisting of
    either the word <literal>enable</literal> or
    <literal>disable</literal> followed by a space and a unit name
    (possibly with shell style wildcards), separated by newlines.
    Empty lines and lines whose first non-whitespace character is <literal>#</literal> or
    <literal>;</literal> are ignored. Multiple instance names for unit
    templates may be specified as a space separated list at the end of
    the line instead of the customary position between <literal>@</literal>
    and the unit suffix.</para>

    <para>Presets must refer to the "real" unit file, and not to any aliases. See
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for a description of unit aliasing.</para>

    <para>Two different directives are understood:
    <literal>enable</literal> may be used to enable units by default,
    <literal>disable</literal> to disable units by default.</para>

    <para>If multiple lines apply to a unit name, the first matching
    one takes precedence over all others.</para>

    <para>Each preset file shall be named in the style of
    <filename>&lt;priority&gt;-&lt;policy-name&gt;.preset</filename>. Files
    in <filename>/etc/</filename> override files with the same name in
    <filename>/usr/lib/</filename> and <filename>/run/</filename>.
    Files in <filename>/run/</filename> override files with the same
    name in <filename>/usr/lib/</filename>. Packages should install
    their preset files in <filename>/usr/lib/</filename>. Files in
    <filename>/etc/</filename> are reserved for the local
    administrator, who may use this logic to override the preset files
    installed by vendor packages. All preset files are sorted by their
    filename in lexicographic order, regardless of which of the
    directories they reside in. If multiple files specify the same
    unit name, the entry in the file with the lexicographically
    earliest name will be applied. It is recommended to prefix all
    filenames with a two-digit number and a dash, to simplify the
    ordering of the files.</para>

    <para>If the administrator wants to disable a preset file supplied
    by the vendor, the recommended way is to place a symlink to
    <filename>/dev/null</filename> in
    <filename>/etc/systemd/system-preset/</filename> bearing the same
    filename.</para>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Default to off</title>

      <programlisting># /usr/lib/systemd/system-preset/99-default.preset

disable *</programlisting>
    </example>

    <para>This disables all units. Due to the filename prefix
    <literal>99-</literal>, it will be read last and hence can easily
    be overridden by spin or administrator preset policy.</para>

    <example>
      <title>Enable multiple template instances</title>

      <programlisting># /usr/lib/systemd/system-preset/80-dirsrv.preset

enable dirsrv@.service foo bar baz</programlisting>
    </example>

    <para>This enables all three of <filename>dirsrv@foo.service</filename>,
    <filename>dirsrv@bar.service</filename> and <filename>dirsrv@baz.service</filename>.</para>

    <example>
      <title>A GNOME spin</title>

      <programlisting># /usr/lib/systemd/system-preset/50-gnome.preset

enable gdm.service
enable colord.service
enable accounts-daemon.service
enable avahi-daemon.*</programlisting>

    </example>

    <para>This enables the three mentioned units, plus all
    <filename>avahi-daemon</filename> regardless of which unit type. A
    file like this could be useful for inclusion in a GNOME spin of a
    distribution. It will ensure that the units necessary for GNOME
    are properly enabled as they are installed. It leaves all other
    units untouched, and subject to other (later) preset files, for
    example like the one from the first example above.</para>

    <example>
      <title>Administrator policy</title>

      <programlisting># /etc/systemd/system-preset/00-lennart.preset

enable httpd.service
enable sshd.service
enable postfix.service
disable *</programlisting>
    </example>

    <para>This enables three specific services and disables all
    others. This is useful for administrators to specifically select
    the units to enable, and disable all others. Due to the filename
    prefix <literal>00-</literal> it will be read early and
    override all other preset policy files.</para>
  </refsect1>

  <refsect1>
    <title>Motivation for the preset logic</title>

    <para>Different distributions have different policies on which services shall be enabled by default when
    the package they are shipped in is installed. On Fedora all services stay off by default, so that
    installing a package will not cause a service to be enabled (with some exceptions). On Debian all
    services are immediately enabled by default, so that installing a package will cause its services to be
    enabled right-away.</para>

    <para>Even within a single distribution, different spins (flavours, remixes, whatever you might want to
    call them) of a distribution also have different policies on what services to enable, and what services
    to leave off. For example, Fedora Workstation will enable <command>gdm</command> as display manager by
    default, while the Fedora KDE spin will enable <command>sddm</command> instead.</para>

    <para>Different sites might also have different policies what to turn on by default and what to turn
    off. For example, one administrator would prefer to enforce the policy of "<command>sshd</command> should
    be always on, but everything else off", while another one might say "<command>snmpd</command> always on,
    and for everything else use the distribution policy defaults".</para>

    <para>Traditionally, policy about which services shall be enabled were implemented in each package
    individually. This made it cumbersome to implement different policies per spin or per site, or to create
    software packages that do the right thing on more than one distribution. The enablement mechanism was
    also encoding the enablement policy.</para>

    <para>The preset mechanism allows clean separation of the enablement mechanism (inside the package
    scriptlets, by invoking <command>systemctl preset</command>) and enablement policy (centralized in the
    preset files), and lifts the configuration out of individual packages. Preset files may be written for
    specific distributions, for specific spins or for specific sites, in order to enforce different policies
    as needed. It is recommended to apply the policy encoded in preset files in package installation
    scriptlets.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-delta</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>

    <para><citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    has a discussion of packaging scriptlets.</para>

    <para>Fedora page introducing the use of presets:
    <ulink url="https://fedoraproject.org/wiki/Features/PackagePresets">Features/PackagePresets</ulink>.
    </para>
  </refsect1>

</refentry>