Why is cron not showing a scheduled task when the cron file has the line shown below

Question

I am new to Linux and experimenting with scheduling a task on a Raspberry Pi B. I can not get the scheduled task to run nor do I see the scheduled task when I use crontab -l. When I run crontab -l I get the message no crontab for pi.

I have opened cron using the command

sudo crontab -e

I want to schedule a python script to run every minute so I have this line in the crontab file:

* * * * * /home/pi/range_sensor.py

My python file's first line contains #!/usr/bin/env python and I have used chmod +x /home/pi/range_sensor.py to change the execute permissions. Why is my scheduled task not running every minute? Where did I go wrong?

Answer 1

Your first issue is with understanding sudo.

Your command crontab -l shows you scheduled crons for current user: "pi" according to the error message.

Your second command sudo crontab -e says: as the super user (root) edit the crons.

So sudo crontab -l should show you crons for the super user.

Second part: Cron does not run under the X.org server (a graphical environment) therefore it cannot know the environmental variable necessary to be able to start an X.org server application so they will have to be defined.

You can Google for various solutions. One very good explanation is available on the ArchLinux wiki.

Answer 2

Crontab output has to go somewhere. On some systems, it's wrapped in an e-mail. On the Raspberry Pi, root crontab output seems to be quietly dropped. So if you want to log something, change the crontab line to

* * * * * /home/pi/range_sensor.py >> /home/pi/range_sensor-output.txt

This will trickle out new readings every minute. I might suggest printing a timestamp in your Python output so you can track what's changing.

(I'd also invite you to research whether you have to use sudo at all. Many GPIO libraries for Raspberry Pi no longer need root access, and can be run as a regular user. If you're following a tutorial that's from before mid-2015 or so, it may be suggesting sudo when you really should no longer use it.)