If I understand correctly: you just wish to extract exif data into some external file, which can be later reused (for examining, writing back to image, etc.)?
If so, then I suggest you use MIE file:

exiftool -o exif.mie -exif:all --makernotes photo.jpg

-this will create exif.mie file, which will contain (only) all exif info from photo.jpg file. Think of mie file as being jpg (or any image file) without containing any image. So, you can read/edit mie using exiftool and even write it back into another image file.

Greetings,
Bogdan