Checking whether a script can open a display?

[William O'Higgins]

On Tue, Mar 01, 2005 at 05:36:31PM -0500, William Park wrote:
>On Tue, Mar 01, 2005 at 05:03:04PM -0500, Lennart Sorensen wrote:
>> On Tue, Mar 01, 2005 at 04:52:29PM -0500, William O'Higgins wrote:
>> > I use a cron job to update the backdrop (root window) of my window
>> > manager.  Sometimes I am not running X, however, and then my mail box
>> > fills up with cron telling me that it cannot open display :0.0.  Is
>> > there a way I can test whether I can open that display so I can quit
>> > cleanly?
>> > Any pointers would be appreciated.  Thanks.
>> 
>> if [ -z "$DISPLAY" ]; then
>> 	echo "No display";
>> else
>> 	echo "Do something";
>> fi
>
>Hmm...  Checking DISPLAY works if you're in Xterm shell session, say
>from ~/.profile.  But, I don't think Cron sees or care about DISPLAY
>environment variable.  In fact, Cron by design runs with minimum set of
>environment variables.
>
>Try 'ps -C X' or something.

This is, indeed, the problem - cron runs with a very limited
$ENVIRONMENT.  If not for that I'm sure that it would work.

>By the way, why would Cron cause reboot of your computer if X is not
>running?

I think you misunderstand - I have no idea what causes the reboots, but
I am pretty sure it isn't cron.  X is running when I leave the house,
but after a reboot, it isn't (because I have not run startx).  The fact
that X is not running is the clue that the machine rebooted (I would not
necessarily notice otherwise - I am not a slave to "uptime").
-- 

yours,

William

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