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diff --git a/man/systemd.preset.xml b/man/systemd.preset.xml new file mode 100644 index 0000000..5d46a88 --- /dev/null +++ b/man/systemd.preset.xml @@ -0,0 +1,222 @@ +<?xml version="1.0"?> +<!--*-nxml-*--> +<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" + "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> +<!-- SPDX-License-Identifier: LGPL-2.1-or-later --> +<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, preset operations 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> + + <para>When the machine is booted for the first time, + <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will + enable/disable all units according to preset policy, similarly to <command>systemctl + preset-all</command>. Also see "First Boot Semantics" in + <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>. + </para> + </refsect1> + + <refsect1> + <title>Preset File Format</title> + + <para>The preset files contain a list of directives, one per line. Empty lines and lines whose first + non-whitespace character is <literal>#</literal> or <literal>;</literal> are ignored. Each directive + consists of one of the words <literal>enable</literal>, <literal>disable</literal>, or + <literal>ignore</literal>, followed by whitespace and a unit name. The unit name may contain shell-style + wildcards.</para> + + <para>For the enable directive for template units, one or more instance names may be specified as a + space-separated list after the unit name. In this case, those instances will be enabled instead of the + instance specified via DefaultInstance= in the unit.</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>Three different directives are understood: <literal>enable</literal> may be used to enable units by + default, <literal>disable</literal> to disable units by default, and <literal>ignore</literal> to ignore + units and leave existing configuration intact.</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><priority>-<policy-name>.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> |