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User Crontab + Python + Random wallpapers = Not working?

Answer 1

+6 A:

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.

Bolo 2010-04-26 13:27:09

Please state your answer in the form of an answer :)

Andrew Bolster 2010-04-26 13:31:57

@Andrew I though I did. Your question was "Any ideas?", I gave you the most probable reason given the limited information that you've presented. I've correctly guessed that the problem is not with cron or gconf, it is with a particular environment variable. Seeing that you're apparently fluent with *NIX and Python, I didn't feel the need to teach you how to set environment variables (for the record, it's `os.environ["DISPLAY"] = ...` in Python).

Bolo 2010-04-26 21:15:51

Answer 2

+2 A:

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

Andrew Bolster 2010-04-26 13:35:07

Then accept his answer?

manifest 2010-04-26 14:15:48

I will, in 44 hours :)

Andrew Bolster 2010-04-26 18:33:50

Errr, unless some comment was deleted: *his*, not *this*, so no need to wait 44 hours? (But well, that's [a discussion on Meta](http://meta.stackoverflow.com/questions/47906/etiquette-for-dealing-with-hint-answers).)

Arjan 2010-04-26 19:19:18

No, sorry, I think you miss understand, or i was not clear enough, I'm waiting for the community to decide; whichever has more upvotes by the time i *could* accept my own answer, will be accepted. Its only fair.

Andrew Bolster 2010-04-26 19:42:50

Answer 3

+1 A:

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.

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

bukzor 2010-04-26 16:06:29

Script is executable, has proper header, and Path is correctly set ( I have alot of things in ~/bin) but I just like explicit stating of whats going on, so having the 'export DISPLAY:0.0' inline in the crontab will remind me why its there in 6 months time :D Thank you for your interest.

Andrew Bolster 2010-04-26 18:33:26

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User Crontab + Python + Random wallpapers = Not working?

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