I have now written a shell script and after much trouble (running a GNU shell script on Windows) got it working. I realized while doing this that I have two wishes if I may for exiftool.

I think a syntax exiftool -tag1=$tag2 a.jpg would be much more intuitive that exiftool -tag1<$tag2 (which is by the way difficult to find in the option description).

I think it would be terrific if exiftool had built string manipulation functions such as substitute and substring. With those, one could work without shell or perl scripts and with just one command.