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On my Pi I am using cron to run a shell script, which in turn runs a python script on startup using the @reboot flag. This python script sets up a GPIO pin to listen for a button push, and for some reason about 2/3 of the time (but not all the time) I get a RuntimeError: Failed to add edge detection. This has never happened to me when running the script in the terminal, only on reboot.

Here's my crontab entry:

@reboot sh /home/pi/Desktop/startup.sh

The startup.sh script:

#!/bin/sh

python3 /home/pi/Desktop/hdrShooter.py

Within hdrShooter.py there is a lot going on, but here is the line that crashes:

GPIO.add_event_detect(17, GPIO.RISING, callback=self.on_button_press)
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  • It is futile trying to run a GUI script on startup because it runs before the GUI exists. There are hundreds of similar questions on this site.
    – Milliways
    Nov 18 '21 at 21:00
  • @Milliways Doesn't need to be a GUI script. I also tried not running it via lxterminal, same result.
    – Spencer
    Nov 18 '21 at 21:46
  • You have changed the Question - which ORIGINALLY used lxterminal
    – Milliways
    Nov 18 '21 at 22:35
  • @Milliways I removed that to prevent potential confusion. My question is not how to run a terminal at startup, but rather how to fix the GPIO RuntimeError, which I don't think is related. As mentioned it failed both ways.
    – Spencer
    Nov 18 '21 at 22:46
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This is a brief answer; I won't try to address potential issues in Python, but I will make one suggestion wrt your cron job running @reboot.

Many of the cron issues reported here have to do with the fact that cron does not monitor availability of resources prior to running an @reboot job. In other words, the system (or specifically systemd in most modern Linux distros) starts cron before it starts a service/daemon that may be needed. When this happens, cron simply cannot deal with it, and throws an error that can be misleading.

A solution that often helps (but not always) is to run the sleep command before starting the job you want to run. In your case, the crontab entry would be this:

@reboot sleep 15; sh /home/pi/Desktop/startup.sh  

15 means 15 seconds; you may want to try a few different values...

Finally, it's sometimes useful to redirect the stderr output from a script to a file to aid in trouble shooting:

@reboot sleep 15; sh /home/pi/Desktop/startup.sh 2> /home/pi/Desktop/stderr.txt 
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  • Thanks Seamus, this is helpful. I was manually writing the traceback to a txt file in python, but didn't realize you can do that from the shell script. I hate the idea of sleeping for a somewhat arbitrary number of seconds, is there a way to know what service is handling the GPIOs and waiting for that to start? Or is there a better option than cron? I tried rc.local, but that seemed like a generally worse idea...
    – Spencer
    Nov 18 '21 at 23:11
  • If nothing else I guess I could try/except python to just keep trying to setup the GPIO callback for a few seconds...
    – Spencer
    Nov 18 '21 at 23:15
  • @Spencer: You may not want to use it as a permanent solution, but you should try it to verify that it resolves the issue - that is to say, is the issue created by resource availability/timing? Why don't you resolve your initial question first, and then we can move on to discuss alternatives?
    – Seamus
    Nov 18 '21 at 23:16

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