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moviuro
You could probably watch dmesg and lock instantly. Probably along those lines:

  while IFS= read -r line; do
    if printf '%s\n' "$line" | grep [something here]; then
      # lock screen
    elif printf '%s\n' "$line" | grep [something else here]; then
      # unlock screen
    fi
  done <<< "$(dmesg -w)"
qertoip
Thanks! I wanted to force connecting the smartphone back on the user by making the check every couple of seconds and locking. For that reason event driven approach wasn't the best fit.
kees99
Or better yet, let udev trigger do the hard work of watching for you. Just add one line to /etc/udev/rules.d/99-my.rules file:

  ACTION=="remove", ENV{ID_SERIAL}=="Pixel_3_0a7...", RUN+="/usr/bin/screensaver-lock-script"
To find out your phone's ID_SERIAL string, run this then plug in:

  udevadm monitor -up
parliament32
Cool idea, and a good use for systemd timers.

A few things: 1) your home directory (/home/qertoip/) is hardcoded in the .service file, 2) this assume you're using cinnamon-screensaver, for portability you should probably have a configuration var for "lock command" or similar, and 3) you should avoid the symlink setup for /usr/lib/systemd/system/, user .service and .timer files should be put in (or symlinked into) /etc/systemd/system (see man 5 systemd.unit).