Getting a hook from shell mode to do motion detection?

I want to set up a wildlife camera, so I've read a few articles about
it.  I've been practicing with an old Canon PowerShot S70 and it seems
suitable.  What I'd like to try to do is use the preview images for
motion detection.  I need to mask out some areas to avoid false
activations from clouds passing over, that sort of stuff, and probably
some averaging to account for the sun rising and setting.

For speed it seems advisable to work in Gphoto2's shell mode but I'm
trying to figure out a way to provide input from an external program.
I want to do capture-preview in a loop: get a preview, run my compare
program, get another preview.  If the compare sees something go into
another loop grabbing big images for some amount of time, then back to
previews.

I did help in shell mode but I don't see anything listed that looks
like it would work, like being able to detect the presence of some
file I could use a a semaphore.  There aren't any branch (or loop)
instructions either.

  Alan

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