User Crontab + Python + Random wallpapers = Not working?

Question

I have a python script that correctly sets the desktop wallpaper via gconf to a random picture in a given folder.

I then have the following entry in my crontab

* * * * * python /home/bolster/bin/change-background.py

And syslog correctly reports execution

Apr 26 14:11:01 bolster-desktop CRON[9751]: (bolster) CMD (python /home/bolster/bin/change-background.py)
Apr 26 14:12:01 bolster-desktop CRON[9836]: (bolster) CMD (python /home/bolster/bin/change-background.py)
Apr 26 14:13:01 bolster-desktop CRON[9860]: (bolster) CMD (python /home/bolster/bin/change-background.py)
Apr 26 14:14:01 bolster-desktop CRON[9905]: (bolster) CMD (python /home/bolster/bin/change-background.py)
Apr 26 14:15:01 bolster-desktop CRON[9948]: (bolster) CMD (python /home/bolster/bin/change-background.py)
Apr 26 14:16:01 bolster-desktop CRON[9983]: (bolster) CMD (python /home/bolster/bin/change-background.py)

But no desktopy changey, Any ideas?

Answer 1

Your script depends on the DISPLAY environment variable, which is set when you execute the script from the shell in an X session, but unset when the script is run from cron.

Answer 2

As per Bolo's observation, I forgot about building in the DISPLAY into either the script or the crontab.

Easiest solution is to prepend the crontab with env DISPLAY=:0.0

so:

* * * * * env DISPLAY=:0.0 python /home/bolster/bin/change-background.py

Answer 3

To set the DISPLAY environment variable, I would put it directly in the crontab. Also, I would make the script executable and give it a proper header (#!/usr/bin/env python) so that it can be executed directly. Additionally, you can rely on the PWD being set to HOME when the crontab runs.

My crontab would look like this:

DISPLAY=:0.0
* * * * * bin/change-background.py

You can also set the PATH (in the same manner as DISPLAY) so that the bin/ is not even needed.

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The main gotcha for setting environment in the crontab is that values are not variable-interpolated. For example, this not give the expected results:

PATH=$HOME/bin:$PATH