Cannot delete bash associative array element

Question

Consider an associative array in bash (versions 5.2.15(1)-release and 5.2.21(1)-release):

declare -A ufs=(); ufs["one"]=1; ufs["two"]=2; ufs["three"]=3
printf '> %s\n' "${!ufs[@]}"
> two
> three
> one

unset ufs["one"]
printf '> %s\n' "${!ufs[@]}"
> two
> three

All is as you'd expect.

Now let's try again, this time with nullglob enabled:

shopt -s nullglob
declare -A ufs=(); ufs["one"]=1; ufs["two"]=2; ufs["three"]=3
printf '> %s\n' "${!ufs[@]}"
> two
> three
> one

unset ufs["one"]
printf '> %s\n' "${!ufs[@]}"
> two
> three
> one

Why does the unset fail to delete the element in this case?

Answer 1

ufs["one"] is globbed first. If nullglob isn’t set, and no file matches in the current directory, it is left as-is, and unset clears it as expected. However with nullglob set, if no file matches, it is cleared:

$ set -x
$ unset ufs["one"]
+ unset

unset called with no arguments doesn’t complain, so the change isn’t obvious, unless failglob is set.

If you prevent globbing, everything works in all situations:

$ unset 'ufs["one"]'
$ printf '> %s\n' "${!ufs[@]}"
> two
> three

or, more robustly:

$ unset 'ufs[one]'

(this works even if assoc_expand_once is enabled).