Provided by: systemd_252.5-2ubuntu3_amd64 bug


       systemd.preset - Service enablement presets









       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

       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 "First Boot
       Semantics" in machine-id(5).


       The preset files contain a list of directives consisting of either the word "enable" or
       "disable" 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 "#"
       or ";" 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 "@" and
       the unit suffix.

       Presets must refer to the "real" unit file, and not to any aliases. See systemd.unit(5)
       for a description of unit aliasing.

       Two different directives are understood: "enable" may be used to enable units by default,
       "disable" to disable units by default.

       If multiple lines apply to a unit name, the first matching one takes precedence over all

       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 /lib/. Packages should install their preset files in /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.


       Example 1. Default to off

           # /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

           # /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

           # /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


       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.


       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].


        1. Features/PackagePresets