Checking whether a script can open a display?

[William Park]

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?
> > 
> > The reason that this has come up is that I have had a few reboots in the
> > past couple of months - always when I am not at home.  The only thing in
> > /var/log/messages is this:
> > 
> > Mar  1 14:41:56 hostname -- MARK -- 
> > 
> > # these happen every twenty minutes, like clockwork ;-) until I get a
> > # message like this:
> > 
> > Mar  1 14:59:09 hostname syslogd 1.4.1#16: restart.
> > 
> > And then there are all the ramblings of a reboot.  The first I hear
> > about it is if I was logged on via ssh and my terminal hangs, or I
> > see the emails from cron about being unable to open the display ('cause
> > X doesn't run by default).
> > 
> > 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.

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

-- 
William Park <opengeometry-FFYn/CNdgSA at public.gmane.org>, Toronto, Canada
Slackware Linux -- because I can type.
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