A Python script to run as a service on a Raspberry Pi, powering off the machine on a GPIO button push.
I run a "headless" Pi (no keyboard or monitor, and not necessarily any network access, either, so maybe no ssh) as an adaptor of sorts, interfacing an electronic drum kit to a drum sound module. (See my older miditrans project). There is software that does the interfacing (along with hardware of course) that starts automatically when the Pi is powered up, but how do I shut it down? It's not a good idea to just "pull the plug" on a running Linux system. So I added a pushbutton switch onto the Pi, and now, with this software, all I have to do is hold the button down for 1 second, and the Pi automatically and safely shuts down.
This should run on any Raspberry Pi - even a Pi Zero, I think (but I don't have one of those).
I'm using an old RPi B 2 with the smaller 26-pin GPIO header, and using pin 26 ("GPIO7") to short to ground (pin 25) with a momentary-contact switch, because those two pins are next to each other and I can use a little 2-contact header.
You can of course change the code to use whatever GPIO pin you want (as long as it's an input!).
This was originally written on Raspian version 8 ("Jessie"), but is now running on version 10, "Buster". I don't think it matters. It was developed under Python 3.7.3, but any Python 3.x should work.
It also uses the GPIOZero package, which makes GPIO programming much easier.
This code can be run stand-alone, in test mode, or installed as a Linux service; to run as a service, use the included service file (you may need to edit it slightly for your install).
The program was pretty straightforward, although of course I made it a little more complicated than necessary. :-) I added a "test" mode so that I could test the software without actually shutting the system down every time. I added logging so I could see what it was doing.
One complication: the code uses signal.pause()
to allow the code to wait for the button push.
(I could perhaps have used a loop around sleep(), but this is what signal.pause is for!)
In the real use-case of this service, eveything works as desired because the system shutdown kills the Python process.
But in test mode, the code never exits - even when I put a sys.exit() call in there!
But, as noted elsewhere on the web (here for instance)
the signal.pause() won't exit until you do .... something.
The docs say "Cause the process to sleep until a signal is received" which was a little mysterious to me.
But after handling the button push (in test mode) we can send ourselves a SIGUSER1 signal, and that causes signal.pause() to finish.
And as shown in the code, you can either catch this signal, or not - either way serves to let the code exit.
If you don't handle the signal, you will see "User defined signal 1" printed on the console at the end of the run. Fine.