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man: move content from the wiki to systemd.preset(5)

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The wiki was slightly stale, and almost all the information there
was already present in the man page. I moved the remaing part (discussion)
into the man page and adjusted all links to point to the man page instead.

daemon(7) has a some examples of packaging scriptlets… I don't think it fits
there very well. Most likely they should be moved to systemd.preset(5) or maybe
even removed, but I'm leaving that for later.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2021-01-31 16:48:44 +01:00
parent 5ffa2eaa54
commit 7e215af765
3 changed files with 53 additions and 24 deletions
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2
docs/PORTABILITY_AND_STABILITY.md
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@ -120,7 +120,7 @@ And now, here's the list of (hopefully) all APIs that we have introduced with sy
| `/run` | File hierarchy change | yes | yes | numerous | yes | OpenSUSE, Debian, ArchLinux | no |
| [Generators](https://www.freedesktop.org/software/systemd/man/systemd.generator.html) | Subprocess | yes | yes | - | no | - | no |
| [System Updates](https://www.freedesktop.org/software/systemd/man/systemd.offline-updates.html) | System Mode | yes | yes | - | no | - | no |
| [Presets](https://freedesktop.org/wiki/Software/systemd/Preset) | File format | yes | yes | - | no | - | no |
| [Presets](https://www.freedesktop.org/software/systemd/man/systemd.preset.html) | File format | yes | yes | - | no | - | no |
| Udev rules | File format | yes | yes | numerous | no | no | partially |

4
man/systemctl.xml
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@ -793,9 +793,7 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
<para>For more information on the preset policy format, see
<citerefentry><refentrytitle>systemd.preset</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
For more information on the concept of presets, please consult the
<ulink url="https://www.freedesktop.org/wiki/Software/systemd/Preset">Preset</ulink>
document.</para>
</para>
</listitem>
</varlistentry>

71
man/systemd.preset.xml
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@ -32,28 +32,20 @@
<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> (for more information
see
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
which uses this information to enable or disable a unit according
to preset policy. <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.</para>
<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>.
<para>For more information on the preset logic please have a look
at the <ulink
url="https://www.freedesktop.org/wiki/Software/systemd/Preset">Presets</ulink>
document.</para>
<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.</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
@ -175,6 +167,38 @@ disable *</programlisting>
override all other preset policy files.</para>
</refsect1>
<refsect1>
<title>Motiviation 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>
@ -182,6 +206,13 @@ disable *</programlisting>
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-delta</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
<para><citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>8</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>