This little program watches the power status of a HDMI CEC-capable device on the HDMI port for changes and executes a shell script upon change.
Background: I recently got a RasPi4 with official 7" touch screen in a case and wanted to make this my living-room media control center. I attached a video beamer to its HDMI port (would also work for a TV set) and installed KODI. The aim is to automatically restart Kodi and execute it on the video projector screen, as soon as I turn on the device and restart it on the internal touch screen when the device gets turned off again.
The program has one major advantage over libcec-daemon, which can do the same thing: The libcec-daemon relies on libcec, which is only capable of accessing the cec interface with one application, so as soon as the libcec-daemon monitors the CEC-interface, no other applications (like KODI) can use the CEC interface anymore, which is not desirable, as i.e. KODI also implements remote control capabilities in order to be able to control KODI with the remote that comes with the HDMI device.
I documented the whole project here
CEC kernel support i.e., on raspi, the KMS/DRM driver driver (vc4-kms-v3d) is capable of this, the firmware kms driver (vc4-fkms-v3d) is NOT!
Simply use make.
If make is not available, you can just compile it with GCC:
gcc -o cecpowmon cecpowmon.c
cecpowmon <Script to execute>
Program watches CEC traffic and if TV is turned on or off, scripts gets executed. Parameter passed on activation: CEC_MSG_REQUEST_ACTIVE_SOURCE Parameter passed on standby: CEC_MSG_STANDBY