From: Robert Klemme <shortcutter@...>
Date: 2009-05-07T15:56:10+09:00
Subject: Re: system call not working under cron

2009/5/7 Michael Satterwhite <michael@weblore.com>:
> Robert Klemme wrote:
>> On 06.05.2009 23:10, Michael Satterwhite wrote:
>>>
>>> Please accept that the "new_picture" variable has a good value pointing
>>> to a new background. There are traces in the program that write values
>>> to a trace file that I can examine after the fact. The fact that the
>>> trace file is updated also shows the script is running.
>>>
>>> Does anyone know why this command would work from a console, but not
>>> from a cron job?
>>
>> This is likely due to X11's handling of permissions.  Your cron job
>> simply does not have access rights to the X session.  I do not know the
>> details but the man page of "xhost" may be a good starting point.  Error
>> message that you get might also be a good indication.
>
> I didn't even know about xhost before this. According to the
> documentation, the command "xhost" without arguments will list those who
> have authorization for the xserver. My user id was authorized already -
> still won't run from cron.

If you executed the xhost command from a terminal started from your X
session this comes as no surprise.  You should see different results
when executing from the cron job.

The point is that since cron is not a child of the X server process
your job does not automatically inherit certain permissions.

The point is, a cron job is usually not the proper thing to update an
X session because cron is something permanent while X sessions are
transitional.  Rather you want to start your screen updater as part of
the X session initialization (~/.xinitrc IIRC).  Then your process
will inherit all the necessary permissions and DISPLAY will also be
set properly.

Kind regards

robert


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