Julia Evans

The "current branch" in git

Hello! I know I just wrote a blog post about HEAD in git, but I’ve been thinking more about what the term “current branch” means in git and it’s a little weirder than I thought.

four possible definitions for “current branch”

  1. It’s what’s in the file .git/HEAD. This is how the git glossary defines it.
  2. It’s what git status says on the first line
  3. It’s what you most recently checked out with git checkout or git switch
  4. It’s what’s in your shell’s git prompt. I use fish_git_prompt so that’s what I’ll be talking about.

I originally thought that these 4 definitions were all more or less the same, but after chatting with some people on Mastodon, I realized that they’re more different from each other than I thought.

So let’s talk about a few git scenarios and how each of these definitions plays out in each of them. I used git version 2.39.2 (Apple Git-143) for all of these experiments.

scenario 1: right after git checkout main

Here’s the most normal situation: you check out a branch.

  1. .git/HEAD contains ref: refs/heads/main
  2. git status says On branch main
  3. The thing I most recently checked out was: main
  4. My shell’s git prompt says: (main)

In this case the 4 definitions all match up: they’re all main. Simple enough.

scenario 2: right after git checkout 775b2b399

Now let’s imagine I check out a specific commit ID (so that we’re in “detached HEAD state”).

  1. .git/HEAD contains 775b2b399fb8b13ee3341e819f2aaa024a37fa92
  2. git status says HEAD detached at 775b2b39
  3. The thing I most recently checked out was 775b2b399
  4. My shell’s git prompt says ((775b2b39))

Again, these all basically match up – some of them have truncated the commit ID and some haven’t, but that’s it. Let’s move on.

scenario 3: right after git checkout v1.0.13

What if we’ve checked out a tag, instead of a branch or commit ID?

  1. .git/HEAD contains ca182053c7710a286d72102f4576cf32e0dafcfb
  2. git status says HEAD detached at v1.0.13
  3. The thing I most recently checked out was v1.0.13
  4. My shell’s git prompt says ((v1.0.13))

Now things start to get a bit weirder! .git/HEAD disagrees with the other 3 indicators: git status, the git prompt, and what I checked out are all the same (v1.0.13), but .git/HEAD contains a commit ID.

The reason for this is that git is trying to help us out: commit IDs are kind of opaque, so if there’s a tag that corresponds to the current commit, git status will show us that instead.

Some notes about this:

  • If we check out the commit by its ID (git checkout ca182053c7710a286d72) instead of by its tag, what shows up in git status and in my shell prompt are exactly the same – git doesn’t actually “know” that we checked out a tag.
  • it looks like you can find the tags matching HEAD by running git describe HEAD --tags --exact-match (here’s the fish git prompt code)
  • You can see where git-prompt.sh added support for describing a commit by a tag in this way in commit 27c578885 in 2008.
  • I don’t know if it makes a difference whether the tag is annotated or not.
  • If there are 2 tags with the same commit ID, it gets a little weird. For example, if I add the tag v1.0.12 to this commit so that it’s with both v1.0.12 and v1.0.13, you can see here that my git prompt changes, and then the prompt and git status disagree about which tag to display:
bork@grapefruit ~/w/int-exposed ((v1.0.12))> git status
HEAD detached at v1.0.13

(my prompt shows v1.0.12 and git status shows v1.0.13)

scenario 4: in the middle of a rebase

Now: what if I check out the main branch, do a rebase, but then there was a merge conflict in the middle of the rebase? Here’s the situation:

  1. .git/HEAD contains c694cf8aabe2148b2299a988406f3395c0461742 (the commit ID of the commit that I’m rebasing onto, origin/main in this case)
  2. git status says interactive rebase in progress; onto c694cf8
  3. The thing I most recently checked out was main
  4. My shell’s git prompt says (main|REBASE-i 1/1)

Some notes about this:

  • I think that in some sense the “current branch” is main here – it’s what I most recently checked out, it’s what we’ll go back to after the rebase is done, and it’s where we’d go back to if I run git rebase --abort
  • in another sense, we’re in a detached HEAD state at c694cf8aabe2. But it doesn’t have the usual implications of being in “detached HEAD state” – if you make a commit, it won’t get orphaned! Instead, assuming you finish the rebase, it’ll get absorbed into the rebase and put somewhere in the middle of your branch.
  • it looks like during the rebase, the old “current branch” (main) is stored in .git/rebase-merge/head-name. Not totally sure about this though.

scenario 5: right after git init

What about when we create an empty repository with git init?

  1. .git/HEAD contains ref: refs/heads/main
  2. git status says On branch main (and “No commits yet”)
  3. The thing I most recently checked out was, well, nothing
  4. My shell’s git prompt says: (main)

So here everything mostly lines up, except that we’ve never run git checkout or git switch. Basically Git automatically switches to whatever branch was configured in init.defaultBranch.

scenario 6: a bare git repository

What if we clone a bare repository with git clone --bare https://github.com/rbspy/rbspy?

  1. HEAD contains ref: refs/heads/main
  2. git status says fatal: this operation must be run in a work tree
  3. The thing I most recently checked out was, well, nothing, git checkout doesn’t even work in bare repositories
  4. My shell’s git prompt says: (BARE:main)

So #1 and #4 match (they both agree that the current branch is “main”), but git status and git checkout don’t even work.

Some notes about this one:

  • I think HEAD in a bare repository mainly only really affects 1 thing: it’s the branch that gets checked out when you clone the repository. It’s also used when you run git log.
  • if you really want to, you can update HEAD in a bare repository to a different branch with git symbolic-ref HEAD refs/heads/whatever. I’ve never needed to do that though and it seems weird because git symbolic ref doesn’t check if the thing you’re pointing HEAD at is actually a branch that exists. Not sure if there’s a better way.

all the results

Here’s a table with all of the results:

.git/HEAD git status checked out prompt
1. checkout main ref: refs/heads/main On branch main main (main)
2. checkout 775b2b 775b2b399... HEAD detached at 775b2b39 775b2b399 ((775b2b39))
3. checkout v1.0.13 ca182053c... HEAD detached at v1.0.13 v1.0.13 ((v1.0.13))
4. inside rebase c694cf8aa... interactive rebase in progress; onto c694cf8 main (main\|REBASE-i 1/1)
5. after git init ref: refs/heads/main On branch main n/a (main)
6. bare repository ref: refs/heads/main fatal: this operation must be run in a work tree n/a (BARE:main)

“current branch” doesn’t seem completely well defined

My original instinct when talking about git was to agree with the git glossary and say that HEAD and the “current branch” mean the exact same thing.

But this doesn’t seem as ironclad as I used to think anymore! Some thoughts:

  • .git/HEAD is definitely the one with the most consistent format – it’s always either a branch or a commit ID. The others are all much messier
  • I have a lot more sympathy than I used to for the definition “the current branch is whatever you last checked out”. Git does a lot of work to remember which branch you last checked out (even if you’re currently doing a bisect or a merge or something else that temporarily moves HEAD off of that branch) and it feels weird to ignore that.
  • git status gives a lot of helpful context – these 5 status messages say a lot more than just what HEAD is set to currently
    1. on branch main
    2. HEAD detached at 775b2b39
    3. HEAD detached at v1.0.13
    4. interactive rebase in progress; onto c694cf8
    5. on branch main, no commits yet

some more “current branch” definitions

I’m going to try to collect some other definitions of the term current branch that I heard from people on Mastodon here and write some notes on them.

  1. “the branch that would be updated if i made a commit”
    • Most of the time this is the same as .git/HEAD
    • Arguably if you’re in the middle of a rebase, it’s different from HEAD, because ultimately that new commit will end up on the branch in .git/rebase-merge/head-name
  2. “the branch most git operations work against”
    • This is sort of the same as what’s in .git/HEAD, except that some operations (like git status) will behave differently in some situations, like how git status won’t tell you the current branch if you’re in a bare repository

on orphaned commits

One thing I noticed that wasn’t captured in any of this is whether the current commit is orphaned or not – the git status message (HEAD detached from c694cf8) is the same whether or not your current commit is orphaned.

I imagine this is because figuring out whether or not a given commit is orphaned might take a long time in a large repository: you can find out if the current commit is orphaned with git branch --contains HEAD, and that command takes about 500ms in a repository with 70,000 commits.

Git will warn you if the commit is orphaned (“Warning: you are leaving 1 commit behind, not connected to any of your branches…“) when you switch to a different branch though.

that’s all!

I don’t have anything particularly smart to say about any of this. The more I think about git the more I can understand why people get confused.

How HEAD works in git Some Git poll results