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The Python script works perfectly fine when ran from the command line while signed in as the "pi" user, with this command:

python3 /home/pi/www/main.py &

But from the crontab, it just doesn't work at all. Logging errors reveals that OpenCV4 cannot find a shared object, because obviously, it's not running in the correct environment it was installed for. I've tried several things to schedule it to run at boot time and as much as possible as the "pi" user, namely:

1) Under the "pi" user: crontab -e

2) Under the root user: sudo crontab -e

3) By directly editing the crontab (sudo nano /etc/crontab) and specifying the user "pi": @reboot pi python3 /home/pi/www/main.py &

4) By sourcing the "pi" user's bashrc: @reboot source /home/pi/.bashrc && python3 /home/pi/www/main.py &

5) By calling a .sh file containing the python3 command

6) By sudo'ing the pi user to run the python command but as you can see, OpenCV is not happy, because shared objects are missing: (Under: sudo crontab -e) @reboot sudo -u pi /usr/bin/python3 /home/pi/www/main.py >>/home/pi/www/logs/cron_log.txt 2>&1

The cron_log.txt file now contains:

Traceback (most recent call last):
  File "/home/pi/www/main.py", line 4, in <module>
    import cv2
  File "/usr/lib/python3/dist-packages/cv2/__init__.py", line 89, in <module>
    bootstrap()
  File "/usr/lib/python3/dist-packages/cv2/__init__.py", line 79, in bootstrap
    import cv2
ImportError: libopencv_quality.so.4.1: cannot open shared object file: No such file or directory

Kind of out of ideas...

EDIT: OK, so by adding this single command to the chain (as per seen at the bottom of /home/pi/.bashrc) it resolved the whole thing: export LD_LIBRARY_PATH=/opt/opencv-4.1.0/lib:$LD_LIBRARY_PATH

So basically, source /home/pi/.bashrc does not seem to work in providing the necessary environment variable for OpenCV 4 to run from cron. But why? Is there a different command that would work instead of source /home/pi/.bashrc ?

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  • Does this answer your question? python script not launching when put into crontab
    – Milliways
    Commented Dec 18, 2019 at 4:22
  • @Milliways Unfortunately, no. I have edited my question with more details...
    – that-ben
    Commented Dec 18, 2019 at 4:33
  • You should use a systemd Unit file for a service to execute a script/program at startup. There you have all possibilities to define the environment.
    – Ingo
    Commented Dec 18, 2019 at 11:56
  • OK, but shouldn't [source /home/pi/.bashrc] suffice to provide cron with the proper environment tough?
    – that-ben
    Commented Dec 18, 2019 at 15:45
  • Good job solving it. Make sure to answer your question thatway people know it's been answered.
    – user96931
    Commented Dec 18, 2019 at 16:50

1 Answer 1

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By adding this single command to the chain (as per seen at the bottom of /home/pi/.bashrc) it resolved the whole thing: export LD_LIBRARY_PATH=/opt/opencv-4.1.0/lib:$LD_LIBRARY_PATH

So basically, source /home/pi/.bashrc does not seem to work in providing the necessary environment variable for OpenCV 4 to run from cron. But why? Is there a different command that would work instead of source /home/pi/.bashrc to include the appropriate bash environment that the user "pi" has from the user shell?

Improve this answer

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