Comments on: Dealing With Stupid Programs That Think They Need X

By: Huw Lynes

Huw Lynes — Fri, 18 Sep 2009 17:38:14 +0000

Roman,

I like your solution better. I’ll do some testing this weekend and update the post appropriately.

Thanks,
Huw

By: Roman Zenka

Roman Zenka — Wed, 19 Aug 2009 15:45:07 +0000

This script has a race condition – you are testing for the id of last running Xvfb to change. Somebody else can start another Xvfb after you test but before you start your Xvfb, therefore fooling your script.

Proposed fix:

create_xvfb () {
DISPLAYNO=1
while [ -z $xvfb_success ]
do
Xvfb :${DISPLAYNO} -screen 0 1024x1024x8 >& /dev/null &
XVFB_PID = $!
sleep 1
if ps –pid $XFVB_PID
then
echo “Started XVFB on display $DISPLAYNO process $XVFB_PID”
xvfb_success=1
else
echo “Failed to run Xvfb on display $DISPLAYNO”
DISPLAYNO=$(($DISPLAYNO + 1))
fi
fi
done
export XVFB_PID
export DISPLAY=:${DISPLAYNO}
}

kill_xvfb () {
kill $XVFB_PID
}

It simplifies your code, you are not firing grep upon grep upon grep and I think it is more robust too. E.g. what if I had a user named “Xvfb” that would show up in ps?

By: Ceri Davies

Ceri Davies — Wed, 10 Sep 2008 08:12:59 +0000

Process IDs will wrap, which is my main concern. The awk pattern could be refined to filter out toothpicks, perhaps.

More importantly, I fed you a duff command line – -e overrides -U in most implementations, so you’d probably need to drop it (I’m pleased to comply with the rules that state when being a smartarse, it’s compulsory to make at least one mistake).

By: huw

huw — Tue, 09 Sep 2008 22:13:20 +0000

ps on linux seems to sort by PID which given the way linux behaves seems to equate to latest process last. Hence the use of tail -n 1 which effectively gives us the PID of the last Xvfb to be spawned.

It didn’t occur to me to use the -U flag of ps, I shall use it next time I need to grep $USERNAME it’s clearly the better way. The awk trick doesn’t work because the /Xvfb/ pattern matches the awk command and you end up with the PID of awk not Xvfb.

By: Ceri Davies

Ceri Davies — Tue, 09 Sep 2008 20:07:57 +0000

I wasn’t aware of Xvfb, thanks Huw.

With regard to get_xvfb_pid, may I respectfully refer you to the -U option to ps(1) and awk pattern matches?
The function could be written with less overhead as:

get_xvfb_pid () {
XVFB_PID=`ps -efwwU $USERNAME | awk ‘/Xvfb/ {print $2}’`
}

The presence of tail in your pipeline looks wrong too, I think ps(1) sorts by controlling terminal then process id, neither of which are likely to be useful!