I have over 80,000 photos that have been given proper EXIF keywords but Google Drive requires data in the Description for it to be searchable in their online app. I need to copy the contents of the Keywords entry to the description on this mass of photos that goes deep into sub directories.
snowliondev's solution pointed me in the right place. It works on jpg's using the legacy IPTC:Keywords tag, but the following command will apply to jpg, png and other files with XMP tags:
exiftool −r −overwrite_original -TagsFromFile @ '-XMP:Subject>Description' *
See this thread, which also shows how to include the directory path in the description metadata which is helpful for Google Photos which does not preserve folder hierarchies.