Avoid boot time stall due to udev rules
The script deployed by the displaylink installer at /opt/displaylink/udev.sh is triggered by udev rules and will run:
systemctl start displaylink-driver
Under certain conditions this will trigger "udevadm settle" to block forever as it leaved processes left and can't return.
I found https://github.com/NixOS/nixpkgs/pull/51760/commits/c72bb28b9cb5138d39647f8fa12394443fea682d which fixes the issue for them.
Applying the same to /opt/displaylink/udev.sh in Ubuntu 20.04 works just fine, the boot is fast again and functionality had no negative impact.
So please could you change this line in your code to be like:
systemctl start --no-block displaylink-driver