git: remove dead branch after a rebase?

Question

I have the following git branches: master and dev:

   *master*
      |
A--B--C
   \--D--E
         |
       *dev*

After rebasing dev on master, I have:

   *master*
      |
A--B--C--D'--E'
   \--D--E   |
             |
           *dev*

So D and E remain in a 'dead branch'. How can I remove them?

Answer 1

You don't need to: because commits D and E have no¹ name pointing to them, they're eligible for "garbage collection". Eventually git will run a git gc and throw them away for you.

If you want to speed this up, you can run git gc yourself, but then footnote 1 comes into play. :-)

---

¹This is not quite true. While the branch-name dev now contains the ID of commit E' (the copy), there are two reflog entries, one for dev and one for HEAD, that allow you (and git) to find commit E. There is also a semi-special name, ORIG_HEAD, which lasts until something else replaces its contents (another rebase, or a git merge, for instance).

By default, most reflog entries persist for either 30 days or 90 days, depending on whether the commit is reachable from the current branch head. Once the reflog entry expires, then the object is a candidate for garbage collection.

As yet another precaution, git gc leaves "loose objects" alone unless they are at least two weeks old (by default—this is configurable, and there is a --prune= option to override this as well). So in general, for a nominally-abandoned object to go away, it:

• must be at least two weeks old;
• must have had any reflog entries expire (generally 30 days); and
• must not have any alternatives names keeping it available.

(And of course, git gc must run, although git automatically runs this whenever it "looks promising".)