Move JPG metadata keywords into description field and then strip keywords. For importing to iPhoto. Preserves the info in the keywords but doesn't introduce them into your own keyword space.

README.md

If an image has Caption-Abstract, ImageDescription and Description, it seems iPhoto (and the cmd+i window in Finder) prefers them in that order.

exiftool_command.sh

# Pick values from other desc fields if only those are present.
exiftool '-ImageDescription<Description' -if 'not $ImageDescription' -overwrite_original .
exiftool '-Caption-Abstract<ImageDescription' -if 'not $Caption-Abstract' -overwrite_original .

# append keywords to desc
exiftool -if '$keywords' -overwrite_original -f '-Caption-Abstract<${Caption-Abstract} (Original keywords: ${keywords})' -keywords='' .

# list various descs
exiftool -keywords -description -ImageDescription -Caption-Abstract *.jpg