AS an Archer
I WANT to plug my external screen via USB-C
SO THAT I can compute on a single screen

Udev

Udev is the place to put rules so that when hardware events can be acted upon.

sudo udevadm monitor

However there are issues.

No ID for the external monitor

sudo udevadm monitor | grep drm
KERNEL[6296.403550] change   /devices/pci0000:00/0000:00:08.1/0000:04:00.0/drm/card0 (drm)
UDEV  [6296.409078] change   /devices/pci0000:00/0000:00:08.1/0000:04:00.0/drm/card0 (drm)
KERNEL[6303.624819] change   /devices/pci0000:00/0000:00:08.1/0000:04:00.0/drm/card0 (drm)
UDEV  [6303.630592] change   /devices/pci0000:00/0000:00:08.1/0000:04:00.0/drm/card0 (drm)

The device doesn’t appear to change whether I’m using the external USB-C monitor. So you don’t know if you are connected or not!

Udev isn’t user based

Udev will run the script as root, so it makes working with a user’s X session awkward.

Tricky to work with rules

cat /etc/udev/rules.d/95-monitor-hotplug.rules
KERNEL=="card0", ACTION=="change", SUBSYSTEM=="drm", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/hendry/.Xauthority", RUN+="/home/hendry/runme.sh"

If you change the rules, don’t forget to run:

udevadm control --reload-rules && udevadm trigger

Logging via journalctl -b -u systemd-udevd isn’t much help!

/sys/class/drm/ - everything is a file on Unix!

We don’t need udev! We have the /dev, sorry /sys file interface to check on our devices!

external_monitor=/sys/class/drm/card0-eDP-2/status
read STATUS < $external_monitor
if test $STATUS == "connected"
then

This will not work as expected due to fact that /sys/class/drm/card0-eDP-2/status takes seconds to update, so if you run the script, you will see wrongly see the state disconnected, when in fact you have connected it!

This is why you might see in scripts:

sleep $START_DELAY

inotifywait will fix it!

sadly inotify doesnt work on most /sys procfs etc files

inotifywait -m /sys/class/drm/card0-DP-2/status will not work or worse will create spurious events.

Now what?

You have to use some program like autorandr or xlayoutdisplay.