Should be given a notice when metadata is overwritten

[ww408]

When I download an important image I like to at least tag it with the page URL from which it came (and if anyone has an idea on how images saved from a browser could be automatically tagged as such, let me know). After tagging an image just now, I said to myself, a person like this might have added metadata to her image, and sure enough, it was filled with redundant metadata, including the tag that I wrote. So, that made me realize that I should have been alerted to the fact that I had just erased metadata. When writing to a tag that isn't empty, one could receive a notice, like, "xmp-dc:source data replaced". And there could also be more involved options that one can configure: (1) A prompt to overwrite the metadata, (2) A prompt to append/prepend the new data to the existing metadata.

[StarGeek]

(and if anyone has an idea on how images saved from a browser could be automatically tagged as such, let me know)

On Windows, you could use the zone_Identifier.config and use the ZoneIDHostUrl tag from that. Something like
exiftool -config zone_Identifier.config -E "-Description<ZoneIDHostUrl" "-Description<$Description&#x0a;$ZoneIDHostUrl" /path/to/files/

This attempts to append a new line and the ZoneIDHostUrl to an existing Description. If there isn't already one, then it will copy it directly. See Note #1 under the -TAG[+-^]=[VALUE] option as for why this works.

There's probably a file system tag for the Mac that does something similar. See the MacOS tags page.

Note that the ZoneID Alternate Data Stream is only created by browsers, AFAIK. Wget, cURL, various download managers, etc., don't create this from what I've seen.

I doubt that there would be a change to make exiftool more interactive, as that would straight up break scripts. There are plenty of options to avoid overwriting data, such as the -if option and the -wm (-writeMode) option, as well as the command structure I listed above.