This is what I did, based on my experience (not a lot) and research (not enough?). There may be better ways to do it. Feel free to tell me what I'm doing wrong in the comments ;-)

Basics:

• Using Ubuntu MATE 18.04 for RPi, downloaded 22 May 2020 (lost password and hadn't done anything with previous ROS install so didn't mind starting over).
• 16GB Sandisk µSD
• Wired Ethernet connection (already setup). Otherwise, pre-configure WiFi with wpa_supplicant.conf
• KVM switch for easy use of GUI.

Install OS

• Unzip (7-zip)
• Flash .img to SD (balenaEtcher)
• Insert & power-on

Check, set and update via config tool

Boot to GUI or CLI etc…

sudo raspi-config

May as well update everything…

sudo apt-get update && sudo apt-get upgrade

Not necessary, but useful

Edit /boot/config.txt and add/change both of these...

dtparam=pwr_led_trigger=heartbeat   #nice load indicator

dtparam=watchdog=on

Create/copy-in the mUPS.py file

(filenames are case-sensitive!)

#########################################################################################
# mUPS.py handles UPS/main-power switch-over and RPi shutdown for Droidbot.
# P.Crouch
# Rev3
# May 2020
#
# *** Developed for my own use and provided as-is, with no guarantees ***
#
# RPi signals to mUPS when it is up. mUPS tells Pi to shutdown and waits for RPi_UP
# to go low before removing power.
#########################################################################################
RPi_UP=21       # pin 40
RPi_SD=27       # pin 11

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
import time
import sys
from subprocess import call

GPIO.setup(RPi_UP, GPIO.OUT)
GPIO.setup(RPi_SD, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) # active-high from UPS (RPi_SD)

#########################################################################################
try:
        GPIO.output(RPi_UP, GPIO.HIGH)   # signal when PI is up

        while 1:
            if GPIO.input(RPi_SD):
                call(['shutdown', '-h', 'now'], shell=False)
            time.sleep(5)  # sleep to reduce unnecessary CPU usage

#########################################################################################
finally:
        GPIO.cleanup()

Create Bash script

Create new text file called mUPS.sh with the following contents:

#!/bin/sh

cd /home/yourusername/
sudo python mUPS.py

IIRC it wouldn't run before login (no good if running headless and not logging-in during normal operation), so python code needed to run with sudo.

Make both files executable (for all users)…

chmod u+x mPSU.py

chmod u+x mPSU.sh

Test it:

./mUPS.sh

 RPi_UP (pin 21) should go high.

Create Cron job to run script at startup

(on newer distros without the rc.local that I used before - workarounds didn't work for me)

Create a cron job that will wait 10 seconds after system startup, then execute the specified script. Sudo is required again here, else you'll edit the user crontab.

sudo crontab -e

…enter the following:

@reboot (sleep 10; sh /home/yourusername/mUPS.sh)

A reboot is considered to be the point at which the daemon starts, it may be before other system daemons so use/test a suitable sleep period.

sudo crontab -l

 To confirm that the cron job has been inserted.

 Reboot to test it.

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At this point the mUPS works almost flawlessly. Almost. I noticed some relay chatter at Arduino power-on that I need to investigate...