For quite some time darktable supports tethering your camera. What was missing all the time was live view. This is about to change though. If you use master (and have a camera supporting it) you can now either use the eye button in the “camera settings” module or hit ‘v’ on your keyboard to start live view from within tethering mode. Since the preview is scaled to fit your screen it might be a good idea to hide some of the side panels.