setting background image by shell command?

[houghi]

On Tue, Apr 30, 2013 at 06:41:53PM +0200, Ulli Horlacher wrote:
> On Tue 2013-04-30 (18:17), Ulli Horlacher wrote:
> > Is it possible to change the background image by a shell command?
> 
> Found it by myself:
> 
> xfconf-query -c xfce4-desktop -p /backdrop/screen0/monitor0/image-path -s $IMAGE

Just for information, if you have more then one screen, the second screen,
just change screen0 to screen1. I go till screen3.

Question: I know how many screens I have, but how could I find this out
with a bash script? I do not mean running `echo $DISPLAY` on each and then
looking what the highest number is. I mean that I should get an output of
4 (or 3 if it starts counting from 0) with my 4 screens and if I have 2 on
another system, I get the answer 2 with the same script/command.

houghi
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