What does git checkout origin do?

    •     git

Suppose you cloned a repository that has a branch named “origin”, and you try to check it out:

$ git checkout origin
Note: switching to 'origin'.
You are in 'detached HEAD' state.

I’m sorry, what? It should’ve created a local “origin” branch, but it detached the HEAD instead. Believe me or not, this is the correct behaviour. To explain it, though, we’ll have to learn a bit about Git.

N.B. I’m assuming that you didn’t change Git’s default names for original remote (origin) and default branch (master). If you did, everything I say here remains valid, but you’ll have to substitute your preferred names for the default ones.

HEAD, and what it means to detach it

HEAD is Git’s notion of your current position in the repo’s history. Most of the time, it points at a branch, e.g. “master”. When you create a new commit, it’s added to that branch, but HEAD itself stays still. When you check out a new branch, HEAD is updated to match.

But you can also check out a tag, or even a random commit. In that case, HEAD would point at a specific commit. That’s what’s called a “detached HEAD” state: the HEAD is not attached to any branch. Committing something now would move the HEAD, but won’t affect any branches. That can be useful for one-off experiments, but can also lead to data loss; please see the “Detached HEAD” section of git-checkout(1).

Remote HEAD

When you’re cloning a repo, the remote’s HEAD takes up a new meaning: it’s not just the “current state” of that repo, it’s the default branch. If you ever changed the default on GitHub or GitLab, and then wondered how the clients know about your decision—that’s how.

Documentation for git remote set-head mentions the following:

Having a default branch for a remote is not required, but allows the name of the remote to be specified in lieu of a specific branch. For example, if the default branch for origin is set to master, then origin may be specified wherever you would normally specify origin/master.

That’s literally what happens in our case: git checkout origin acts like git checkout origin/master. But origin/master is a branch, so why do we end up in the “detached HEAD” state?

Remote and local branches

Why does git checkout origin/master result in a detached HEAD? Because origin/master is a remote branch. It’s your local clone’s idea of where master points to in the remote repository.

A crucial difference between the origin/master and your local master is how they are updated:

As a result, it doesn’t make sense for Git to point HEAD at a remote branch: your local commands like git commit won’t be able to move the branch anyway. That only leaves one option: detach the HEAD and point it at the commit referenced by the remote branch.

“But wait”, I hear you cry, “Why does git checkout feature/123-improve-perf in a fresh clone doesn’t fail, then?” That’s a valid question. The key here is that feature/123-improve-perf is neither a remote branch nor a local one. There is only a remote branch named origin/feature/123-improve-perf—note the prefix. Git recognizes this situation and creates a local branch feature/123-improve-perf that tracks the remote one.

That’s all we need to know right now. For more details, please refer to the relevant chapter of Pro Git.

The answer

Putting all of the above together, we finally understand what git checkout origin does:

  1. origin gets resolved into the default branch, i.e. origin/master;
  2. origin/master is a remote branch, so checking it out results in a detached HEAD that points at the commit that’s at the tip of origin/master.

To achieve what you actually wanted, tell Git to create a branch named “origin” that points at the same commit as origin/origin:

$ git checkout -b origin origin/origin

Drop me a line! (wonder where’s the comments form?)