NoDups(1) deletes tags when no duplicates

[msbc]

I'm seeing that when no duplicates exist then NoDups(1) deletes all entries in the list:
exiftool -keywords a.jpg
Keywords                        : Arctic, Norway, Spitsbergen, Svalbard, bears, ice, mammal, pack ice, polar bear, snow

exiftool -m -q -sep '##' '-keywords<${keywords;NoDups(1)}' a.jpg
exiftool -keywords a.jpg
Keywords                        :

When there is at least one duplicate the command works as expected.

macOS: 10.15.5
exiftool -ver: 11.85

[StarGeek]

From the docs on the -m (ignoreMinorErrors) option
   Note that this causes missing values in -tagsFromFile, -p and -if strings to be set to an empty string rather than an undefined value.

'-keywords<keywords' is an implied -TagsFromFile and when -m is added, the NoDups(1) will be set to a blank string instead of undefined.  Remove it and the command will do what you want.

[msbc]

Teach me to read the fine print in the docs!

Thanks

[herb]

Hello everybody,

sorry to give my 2 cents to this problem.

But I have seen that the tag is not deleted, but an empty list is created/written.
Should the listtype-tag be deleted?

Best regards
herb

[StarGeek]

I'm not sure I follow.  Do you have an example command and tag data?

[herb]

Hello StarGeek,

sorry, that I was not clear enough.
In first post of this topic, msbc gives a good example. The output of his display-command shows that keywords exists and that keywords is empty.
I had expected that keywords is deleted (does not exist) after "NoDups(1)" command.

Best regards
herb

[StarGeek]

Normally, NoDups(1) would return Undef in the event there were not any duplicates.  That way the file wouldn't be rewritten if there wasn't a change. But because of the -m (ignoreMinorErrors) option, it won't return undefined, it returns an empty string, which now overwrites the original keywords.

[herb]

Hello StarGeek,

thanks for your detailed explanation. It is all understood.
Usuallay -keywords=   (which is -keywords=<empty string returned by NoDups(1)> will remove the keywords tag, so that it does not exists anymore.
But why does it exist afterwards as "an empty listtype tag" ?

Best regards
herb

[Phil Harvey]

Writing an empty string to any tag will do this.  The confusion arises because ExifTool uses the -TAG= syntax to indicate that you want to delete the tag.  So it is actually very difficult to write an empty string from the command line.  (That is why I added the -TAG^= syntax.)  But using -m with an advanced formatting expression that returns undef (or just an expression that returns an empty string) is one way to do it.

- Phil

[herb]

Hello Phil,

thanks for your detailed description.

Best regards
herb