very thanks 4 the answers.
I solved doing this:
I opened the xscreensaver control-gui, looked 4 a screensaver entry unusable; selected it and in its (advanced) properties, deleted it name and put in its place the command I needed.
this way works just fine ..but blanks the screen! -nothing is perfect, uh?
think it's the easiest way to do the job.
(about the "at" command:
right, there's something called "batch"..
but I've never been able to make the 'at' command work-I give up using it long time ago)