Info about active window

[Naira Kaieski <naira faccat br>]

Hi,

I want monitoring the users that login em a workstation with Ubuntu 9.04.
I want monitor this with SNMP and a external script. This monitoring 
will be remote.

I want to monitor only those applications that are being used, 
especially the active application.

By gnome terminal i can get this information with:
WINDOWID=$(/usr/bin/xprop -root | grep "_NET_ACTIVE_WINDOW(WINDOW)"| cut 
-d ' ' -f 5)
/usr/bin/xwininfo -id $WINDOWID | awk 'BEGIN {FS="\""}/xwininfo: Window 
id/{print $2}' | sed 's/-[^-]*$//g'

But i want do a remote monitoring and the above commands don work, i 
have a message:
/usr/bin/xprop:  unable to open display ''
/usr/bin/xwininfo:  unable to open display ''

Someone can help?

Thanks,
Naira