SYSTEMD.PRESET(5) systemd.preset SYSTEMD.PRESET(5)
NAME
systemd.preset - Service enablement presets
SYNOPSIS
/etc/systemd/system-preset/*.preset
/run/systemd/system-preset/*.preset
/usr/local/lib/systemd/system-preset/*.preset
/usr/lib/systemd/system-preset/*.preset
/etc/systemd/user-preset/*.preset
/run/systemd/user-preset/*.preset
/usr/local/lib/systemd/user-preset/*.preset
/usr/lib/systemd/user-preset/*.preset
DESCRIPTION
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 systemctl
preset which uses this information to enable or disable a unit.
Depending on that policy, systemctl preset is identical to systemctl
enable or systemctl disable. systemctl preset 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
systemctl(1).
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.
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 "disable *" line. (See example 1, below.)
When the machine is booted for the first time, systemd(1) will
enable/disable all units according to preset policy, similarly to
systemctl preset-all. Also see ConditionFirstBoot= in systemd.unit(5)
and "First Boot Semantics" in machine-id(5).
PRESET FILE FORMAT
The preset files contain a list of directives, one per line. Empty lines
and lines whose first non-whitespace character is "#" or ";" are
ignored. Each directive consists of one of the words "enable",
"disable", or "ignore", followed by whitespace and a unit name. The unit
name may contain shell-style wildcards.
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.
Presets must refer to the "real" unit file, and not to any aliases. See
systemd.unit(5) for a description of unit aliasing.
Three different directives are understood: "enable" may be used to
enable units by default, "disable" to disable units by default, and
"ignore" to ignore units and leave existing configuration intact.
If multiple lines apply to a unit name, the first matching one takes
precedence over all others.
Each preset file shall be named in the style of
<priority>-<policy-name>.preset. Files in /etc/ override files with the
same name in /usr/lib/ and /run/. Files in /run/ override files with the
same name in /usr/lib/. Packages should install their preset files in
/usr/lib/. Files in /etc/ 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.
If the administrator wants to disable a preset file supplied by the
vendor, the recommended way is to place a symlink to /dev/null in
/etc/systemd/system-preset/ bearing the same filename.
EXAMPLES
Example 1. Default to off
# /usr/lib/systemd/system-preset/99-default.preset
disable *
This disables all units. Due to the filename prefix "99-", it will be
read last and hence can easily be overridden by spin or administrator
preset policy.
Example 2. Enable multiple template instances
# /usr/lib/systemd/system-preset/80-dirsrv.preset
enable dirsrv@.service foo bar baz
This enables all three of dirsrv@foo.service, dirsrv@bar.service and
dirsrv@baz.service.
Example 3. A GNOME spin
# /usr/lib/systemd/system-preset/50-gnome.preset
enable gdm.service
enable colord.service
enable accounts-daemon.service
enable avahi-daemon.*
This enables the three mentioned units, plus all avahi-daemon 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.
Example 4. Administrator policy
# /etc/systemd/system-preset/00-lennart.preset
enable httpd.service
enable sshd.service
enable postfix.service
disable *
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 "00-" it will be read
early and override all other preset policy files.
MOTIVATION FOR THE PRESET LOGIC
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.
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 gdm as display
manager by default, while the Fedora KDE spin will enable sddm instead.
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 "sshd should be always on, but
everything else off", while another one might say "snmpd always on, and
for everything else use the distribution policy defaults".
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.
The preset mechanism allows clean separation of the enablement mechanism
(inside the package scriptlets, by invoking systemctl preset) 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.
SEE ALSO
systemd(1), systemctl(1), systemd-delta(1)
daemon(7) has a discussion of packaging scriptlets.
Fedora page introducing the use of presets: Features/PackagePresets[1].
NOTES
1. Features/PackagePresets
https://fedoraproject.org/wiki/Features/PackagePresets
systemd 257.9 SYSTEMD.PRESET(5)
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