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As originally detailed for https://github.com/networkupstools/nut/issues/2721#issuecomment-2545360867 NUT since v2.8.0 significantly revised the nut-driver start-up on systemd-enabled operating systems, thanks to nut-driver-enumerator (NDE) handling individual driver wrappings.

Some distributions, manual installations e.g. when Building NUT for in‐place upgrades or non‐disruptive tests, and/or end-user craftiness can leave some important units disabled from auto-start. You can see site-local configuration under your /etc/systemd/system/ directory (there are more locations for packaged /usr/lib/systemd, core /lib/systemd/, or run-time /run/systemd/ which are not of interest at the moment).

Since PR #2746 (to be released in NUT v2.8.3) you can also sudo systemctl preset-all but beware -- this instills an opinionated selection of enabled and disabled units, so can override whatever you have customized locally. See the nut-systemd.preset file for more details. That PR also adds nut-logger (wrapping of upslog daemon as a service, disabled by default).

Generally note that in systemd:

• the unitname.d directories allow "drop-in" customizations over the default unit as delivered by packaging and/or allowing instantiation (e.g. nut-driver@.service definition);
• links to /dev/null mark masked services that should never get enabled or started;
• enablement of targets (and other units) for auto-start should have been part of packaging, or can be done manually as systemctl enable UNITNAME.

Overall, a mesh of systemd units can look like the following:

:; ls -ld `find /etc/systemd | grep -w nut | sort

lrwxrwxrwx 1 root root  30 Dec  5 03:32 /etc/systemd/system/multi-user.target.wants/nut.target -> /lib/systemd/system/nut.target
drwxr-xr-x 2 root root   4 Apr 19  2024 /etc/systemd/system/nut-driver.target.wants
lrwxrwxrwx 1 root root  39 Jan  2  2023 /etc/systemd/system/nut-driver.target.wants/nut-driver@dummy.service -> /lib/systemd/system/nut-driver@.service
lrwxrwxrwx 1 root root  39 Apr 19  2024 /etc/systemd/system/nut-driver.target.wants/nut-driver@eco650.service -> /lib/systemd/system/nut-driver@.service
drwxr-xr-x 2 root root   3 Nov  1  2022 /etc/systemd/system/nut-driver@.service.d
-rw-r--r-- 1 root root  81 Apr 19  2023 /etc/systemd/system/nut-driver@.service.d/nut-driver-enumerator-generated-checksum.conf
lrwxrwxrwx 1 root root   9 Jan  2  2023 /etc/systemd/system/nut-driver@dummy.service -> /dev/null
drwxr-xr-x 2 root root   4 Apr 19  2024 /etc/systemd/system/nut-driver@dummy.service.d
-rw-r--r-- 1 root root  74 Jul 23 00:47 /etc/systemd/system/nut-driver@dummy.service.d/nut-driver-enumerator-generated-checksum.conf
-rw-r--r-- 1 root root  39 Apr 19  2024 /etc/systemd/system/nut-driver@dummy.service.d/nut-driver-enumerator-generated-devicename.conf
drwxr-xr-x 2 root root   5 Apr 19  2024 /etc/systemd/system/nut-driver@eco650.service.d
-rw-r--r-- 1 root root  74 Apr 19  2024 /etc/systemd/system/nut-driver@eco650.service.d/nut-driver-enumerator-generated-checksum.conf
-rw-r--r-- 1 root root  40 Apr 19  2024 /etc/systemd/system/nut-driver@eco650.service.d/nut-driver-enumerator-generated-devicename.conf
-rw-r--r-- 1 root root 364 Apr 19  2024 /etc/systemd/system/nut-driver@eco650.service.d/nut-driver-enumerator-generated.conf
drwxr-xr-x 2 root root   7 Dec  5 03:32 /etc/systemd/system/nut.target.wants
lrwxrwxrwx 1 root root  46 Dec  5 03:32 /etc/systemd/system/nut.target.wants/nut-driver-enumerator.path -> /lib/systemd/system/nut-driver-enumerator.path
lrwxrwxrwx 1 root root  49 Dec  5 03:32 /etc/systemd/system/nut.target.wants/nut-driver-enumerator.service -> /lib/systemd/system/nut-driver-enumerator.service
lrwxrwxrwx 1 root root  37 Dec  5 03:32 /etc/systemd/system/nut.target.wants/nut-driver.target -> /lib/systemd/system/nut-driver.target
lrwxrwxrwx 1 root root  39 Dec  5 03:32 /etc/systemd/system/nut.target.wants/nut-monitor.service -> /lib/systemd/system/nut-monitor.service
lrwxrwxrwx 1 root root  38 Dec  5 03:32 /etc/systemd/system/nut.target.wants/nut-server.service -> /lib/systemd/system/nut-server.service

Detailing the lines as they go:

• Result of systemctl enable nut.target, so the system milestone multi-user.target "wants" (tries, does not fail if unsuccessful) to start NUT (whatever nut.target would pull in):

/etc/systemd/system/multi-user.target.wants/nut.target -> /lib/systemd/system/nut.target

• Result of systemctl enable nut-driver@{dummy,eco650}.service, done by NDE based on ups.conf contents:

/etc/systemd/system/nut-driver.target.wants
/etc/systemd/system/nut-driver.target.wants/nut-driver@dummy.service -> /lib/systemd/system/nut-driver@.service
/etc/systemd/system/nut-driver.target.wants/nut-driver@eco650.service -> /lib/systemd/system/nut-driver@.service

• Nominal customization of nut-driver@.service.d template, generated by NDE from ups.conf contents for tracking of "global" part of configuration:

/etc/systemd/system/nut-driver@.service.d
/etc/systemd/system/nut-driver@.service.d/nut-driver-enumerator-generated-checksum.conf

• The nut-driver@dummy.service never auto-starts; I can manage that (virtual) NUT driver as a program in testing whichever way I like (e.g. with command-line configuration):

/etc/systemd/system/nut-driver@dummy.service -> /dev/null

• Customizations for unit of a dummy UPS, generated by NDE from ups.conf contents:

/etc/systemd/system/nut-driver@dummy.service.d
/etc/systemd/system/nut-driver@dummy.service.d/nut-driver-enumerator-generated-checksum.conf
/etc/systemd/system/nut-driver@dummy.service.d/nut-driver-enumerator-generated-devicename.conf

• Customizations for unit of a for eco650 UPS, generated by NDE from ups.conf contents:

/etc/systemd/system/nut-driver@eco650.service.d
/etc/systemd/system/nut-driver@eco650.service.d/nut-driver-enumerator-generated-checksum.conf
/etc/systemd/system/nut-driver@eco650.service.d/nut-driver-enumerator-generated-devicename.conf
/etc/systemd/system/nut-driver@eco650.service.d/nut-driver-enumerator-generated.conf

• Result of systemctl enable nut-driver-enumerator.{path,service} nut-driver.target nut-monitor.service nut-server.service which "wants" those units to auto-start as part of nut.target which in turn would be "wanted" by the system milestone multi-user.target as seen above (note there are other NUT units, especially around NDE implementation variants, that may be or not be useful in a particular deployment):

/etc/systemd/system/nut.target.wants
/etc/systemd/system/nut.target.wants/nut-driver-enumerator.path -> /lib/systemd/system/nut-driver-enumerator.path
/etc/systemd/system/nut.target.wants/nut-driver-enumerator.service -> /lib/systemd/system/nut-driver-enumerator.service
/etc/systemd/system/nut.target.wants/nut-driver.target -> /lib/systemd/system/nut-driver.target
/etc/systemd/system/nut.target.wants/nut-monitor.service -> /lib/systemd/system/nut-monitor.service
/etc/systemd/system/nut.target.wants/nut-server.service -> /lib/systemd/system/nut-server.service

Also note that systemd drop-in files for site-specific nuances can be created by e.g. ansible recipes that manage a machine, an example of the YAML is posted at https://github.com/networkupstools/nut/issues/1983#issuecomment-3752530376