xuserrun is a bash script to detect the first X.org server environmental values, these values are then used to transmit a command to it. The script is primarily useful from another environment: a different user, from the tty console, from cron, a boot script…. xuserrun requires systemd.
Running it is basic:
xuserrun xclock -digital
xuserrun notify-send "Remember to get bread."
I have put it in its own repository.