Copy tag only if it does not exist already

[alta]

Hi, I'm using ExifTool to record the orginal filenames of images in the OriginalFilename tag and trying to come up with a command that doesn't change the value of the tag when it's run again on a file that was renamed. This example from the documentation seems perfect for that purpose:

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exiftool -comment-= -comment='new comment' a.jpg
     Write  a  new  comment only if the image doesn't
     have one already.
However, when copying a tag instead of assigning a literal value, the tag gets overwritten if it already exists.

For example:
exiftool -OriginalFilename old.jpg

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exiftool -OriginalFilename-= '-OriginalFilename<Filename' old.jpg
exiftool -OriginalFilename old.jpg

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Original File Name              : old.jpgSo far, so good. But then:
mv old.jpg new.jpg
exiftool -OriginalFilename-= '-OriginalFilename<Filename' new.jpg
exiftool -OriginalFilename new.jpg

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Original File Name              : new.jpg
Is this expected? What am I missing, and how can I keep the tag value unchanged once it exists?

[StarGeek]

Try the -wm (-writeMode) option, specifically -wm cg. That will "Create new tags" (c) and "Create new Groups as necessary" (g) but will not write to existing tags.

[alta]

Thank you, StarGeek. That solved it and it'll be a very useful option to keep in mind.