System.cmd(0'|zenity --info --text="Goodbye, World!"|); // GTK+ pop up
System.cmd(0'|notify-send "Goodbye, World!"|); // desktop notification
System.cmd(0'|xmessage -buttons Ok:0,"Not sure":1,Cancel:2 -default Ok -nearmouse "Goodbye, World!" -timeout 10|); // X Windows dialog