git: list dangling tags

Question

Context:

• assume you have some rather tricky CI/CD workflow which relies on git tags
• the feature branches are built and generate some short-lived tags to signify commits which yield the deployable artifacts
• when the feature branch gets squash-merged, usually it's deleted, but the tags, unsurprisingly, survive
• after, say, several months of development the tag listing predictably becomes hairy

Hence, the question:

how would I, using git command line and, optionally, basic bash tooling

1. list all the branches which have given tag reachable (the dual to that is git tag -l --merged ${BRANCH_COMMITTISH}, but I need not tags for the given branch but branches for a given tag)
2. list all the tags which have empty output from point 1 above (obviously this is doable with a for loop (given any terminating implementation for 1), but maybe there's some neat magical git one-liner)?

Answer 1

git log --simplify-by-decoration --tags --not --branches --remotes --pretty=%d

--simplify-by-decoration says only show the bare minimum needed to reveal the ancestry (usually you use this with --graph). --tags --not --branches --remotes says, well, what it says: list the tag history that's not in the branches or remotes history, i.e. tags unreachable from any branch or remote-tracking branch. --pretty=%d says just show the refs.

Answer 2

1. git branch --contains ${TAG}

https://git-scm.com/docs/git-branch#git-branch---containsltcommitgt.