What's the use case for "git branch -d"

Question

I was helping out a co-worker today, and discovered that git branch -D and git branch -d do slightly different things. From the git help documentation:

-d --delete  delete fully merged branch
-D           delete branch (even if not merged)

I can appreciate avoiding deleting branching arbitrarily, but how does git determine when branch -d is allowable? When would someone use branch -d correctly?

Answer 1

Think of -D as a forced branch delete. It will delete the branch even if it has not been merged into the branch you're currently in.

-d however, will warn you and won't delete the branch until its been merged.

For example

You've branched off master branch into branch A. Made commits into A. If you then switched to master branch again and attempted to git branch -d A you'd get a message like so

git branch -d A
error: The branch 'A' is not fully merged.
If you are sure you want to delete it, run 'git branch -D A'.

This is because you have commits in A branch that master does not have and it's making sure you want to delete it before pulling those changes into the current branch.

Answer 2

When a branch is fully merged (i.e. all revisions of this branch are either pushed to it's corresponding remote or the branch is merged to master), -d is sufficient. From docs:

Delete a branch. The branch must be fully merged in its upstream branch, or in HEAD if no upstream was set with --track or --set-upstream.

If you did a merge --squash (i.e. not all revisions of this branch are contained in master) or you didn't merge the branch anywhere because you recognized that you're doing wrong and want to throw this branch away, -D is needed:

Delete a branch irrespective of its merged status.