Hi Heiko,
The -= argument expects a tag value on the RHS, but
you want to use a tag name, so you need to use the
-tagsfromfile feature that allows you to copy
(or with "-", delete) a tag value by specifying
the tag name:
exiftool "-iptc:keywords-<iptc:city" *.jpg
See the -tagsFromFile documentation for details. In
the above command, -tagsFromFile @ is assumed.
- Phil