The Universe of Discourse

Sat, 17 Jun 2017

Git remote branches and Git's missing terminology

Beginning and even intermediate Git users have several common problem areas, and one of these is the relationship between remote and local branches. I think the basic confusion is that it seems like there ought to be two things, the remote branch and the local one, and you copy back and forth between them. But there are not two but three, and the Git documentation does not clearly point this out or adopt clear terminology to distinguish between the three.

Let's suppose we have a remote repository, which could be called anything, but is typically named origin. And we have a local repository which has no name; it's just the local repo. And let's suppose we're working on a branch named master, as one often does.

There are not two but three branches of interest, and they might all be pointing to different commits:

1. The branch named master in the local repo. This is where we do our work and make our commits. This is the local branch. It is at the lower left in the diagram.

2. The branch named master in the remote repo. This is the remote branch, at the top of the diagram. We cannot normally see this at all because it is (typically) on another computer and (typically) requires a network operation to interact with it. So instead, we mainly deal with…

3. The branch named origin/master in the local repo. This is the tracking branch, at the lower right in the diagram.

   We never modify the tracking branch ourselves. It is automatically maintained for us by Git. Whenever Git communicates with the remote repo and learns something about the disposition of the remote master branch, it updates the local branch origin/master to reflect what it has learned.

I think this triangle diagram is the first thing one ought to see when starting to deal with remote repositories and with git-fetch and git-push.

The Git documentation often calls the tracking branch the “remote-tracking branch”. It is important to understand that the remote-tracking branch is a local branch in the local repository. It is called the “remote-tracking” branch because it tracks the state of the remote branch, not because it is itself remote. From now on I will just call it the “tracking branch”.

Now let's consider a typical workflow:

1. We use git fetch origin master. This copies the remote branch master from the remote repo to the tracking branch origin/master in the local repo. This is the green arrow in the diagram.

   If other people have added commits to the remote master branch since our last fetch, now is when we find out what they are. We can compare the local branch master with the tracking branch origin/master to see what is new. We might use git log origin/master to see the new commits, or git diff origin/master to compare the new versions of the files with the ones we had before. These commands do not look at the remote branch! They look at the copy of the remote branch that Git retrieved for us. If a long time elapses between the fetch and the compare, the actual remote branch might be in a completely different place than when we fetched at it.

   (Maybe you use pull instead of fetch. But pull is exactly like fetch except that it does merge or rebase after the fetch completes. So the process is the same; it merely combines this step and the next step into one command. )

2. We decide how to combine our local master with origin/master. We might use git merge origin/master to merge the two branches, or we might use git rebase origin/master to copy our new local commits onto the commits we just fetched. Or we could use git reset --hard origin/master to throw away our local commits (if any) and just take the ones on the tracking branch. There are a lot of things that could happen here, but the blue arrow in the diagram shows the general idea: we see new stuff in origin/master and update the local master to include that new stuff in some way.

3. After doing some more work on the local master, we want to publish the new work. We use git push origin master. This is the red arrow in the diagram. It copies the local master to the remote master, updating the remote master in the process. If it is successful, it also updates the tracking branch origin/master to reflect the new position of the remote master.

In the last step, why is there no slash in git push origin master? Because origin/master is the name of the tracking branch, and the tracking branch is not involved. The push command gets two arguments: the name of the remote (origin) and the branch to push (master) and then it copies the local branch to the remote one of the same name.

Deleting a branch

How do we delete branches? For the local branch, it's easy: git branch -d master does it instantly.

For the tracking branch, we include the -r flag: git branch -d -r origin/master. This deletes the tracking branch, and has no effect whatever on the remote repo. This is a very unusual thing to do.

To delete the remote branch, we have to use git-push because that is the only way to affect the remote repo. We use git push origin :master. As is usual with a push, if this is successful Git also deletes the tracking branch origin/master.

This section has glossed over an important point: git branch -d master does not delete the master branch, It only deletes the ref, which is the name for the branch. The branch itself remains. If there are other refs that refer to it, it will remain as long as they do. If there are no other refs that point to it, it will be deleted in due course, but not immediately. Until the branch is actually deleted, its contents can be recovered.

Hackery

Another way to delete a local ref (whether tracking or not) is just to go into the repository and remove it. The repository is usually in a subdirectory .git of your working tree, and if you cd .git/refs you can see where Git records the branch names and what they refer to. The master branch is nothing more nor less than a file heads/master in this directory, and its contents are the commit ID of the commit to which it refers. If you edit this commit ID, you have pointed the ref at a different commit. If you remove the file, the ref is gone. It is that simple.

Tracking branches are similar. The origin/master ref is in .git/refs/remotes/origin/master.

The remote master branch, of course, is not in your repository at all; it's in the remote repository.

Poking around in Git's repository is fun and rewarding. (If it worries you, make another clone of the repo, poke around in the clone, and throw it away when you are finished poking.) Tinkering with the refs is a good place to start Git repo hacking: create a couple of branches, move them around, examine them, delete them again, all without using git-branch. Git won't know the difference. Bonus fun activity: HEAD is defined by the file .git/HEAD. When you make a new commit, HEAD moves forward. How does that work?

There is a gitrepository-layout manual that says what else you can find in the repository.

Failed pushes

We're now in a good position to understand one of the most common problems that Git beginners face: they have committed some work, and they want to push it to the remote repository, but Git says

      ! [rejected]        master -> master (fetch first)
      error: failed to push some refs to 'remote'
      something something fast-forward, whatever that is

My article explaining this will appear here on Monday. (No, I really mean it.)

Terminology problems

I think one of the reasons this part of Git is so poorly understood is that there's a lack of good terminology in this area. There needs to be a way to say "the local branch named master” and “the branch named master in the remote named origin” without writing a five- or nine-word phrase every time. The name origin/master looks like it might be the second of these, but it isn't. The documentation uses the descriptive but somewhat confusing term “remote-tracking branch” to refer to it. I think abbreviating this to “tracking branch” would tend to clear things up more than otherwise.

I haven't though of a good solution to the rest of it yet. It's tempting to suggest that we should abbreviate “the branch named master in the remote named origin” to something like “origin:master” but I think that would be a disaster. It would be too easy to confuse with origin/master and also with the use of the colon in the refspec arguments to git-push. Maybe something like origin -> master that can't possibly be mistaken for part of a shell command and that looks different enough from origin/master to make clear that it's related but not the same thing.

Git piles yet another confusion on this:

    $ git checkout master 
    Branch master set up to track remote branch master from origin.

This sounds like it has something to with the remote-tracking branch, but it does not! It means that the local branch master has been associated with the remote origin so that fetches and pushes that pertain to it will default to using that remote.

I will think this over and try to come up with something that sucks a little less. Suggestions are welcome.

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