https://freedesktop.org/wiki/Software/systemd/Debugging/

2022
https://www.psdn.io/posts/systemd-shutdown-unit/
    https://habr.com/ru/company/skillfactory/blog/697152/
2017
https://askubuntu.com/questions/764568/ubuntu-16-04-hangs-on-shutdown-restart
    sudo -H vim /etc/default/grub

diagnostics
https://forum.siduction.org/index.php?topic=5989.0
    journalctl -b -1 -n250