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Comment by sophacles | Hacker News Reader

Do you have an actual example of this? I've never encountered a situation where those three points are incorrect.

If you're in a detached head state and create a new commit, you have a commit that isn't on a branch. The commit still has a parent, so it's not the branch that's tracking the sequence, it's the commit. Branches are just pointers that get updated as you add commits.

This feels like declaring squares aren't rectangles because you can make rectangles that aren't a square.

Git is only commits. Branches and tags are references to the commits; one moves while the other one doesn’t. That’s it.

Commits link to their parent(s) until the initial commit. You can visualize a “branch” as a series of commits, but the same thing can exist without calling it a branch:

    A -> B -> C —> Initial

Is that a branch? No. How about:

    /refs/heads/main = A

That’s a branch, and it looks exactly the same as a tag:

    /refs/tags/v1 = A

Neither a branch or a tag is “a series of commits” even if you think it does.

> You can visualize a “branch” as a series of commits, but the same thing can exist without calling it a branch

You can visualize a "square" as a rectangle with equal sides, but the same thing can exist without calling it a square.

2. is true or untrue depending on how deeply you interpret it.

A branch is named, points to a commit and under normal circumstances will track the sequence of commits made via itself (i.e. when you commit and HEAD points to branch b, b starts pointing to the child commit). So you could say it's a named sequence of commits.

On the other hand you can reset the branch to any commit in the tree, even ones which aren't even on the current sequence of parents. It is still technically a named sequence of commits, just a totally different one.

Git branch is actually very similar to git tag[1]. For example, there are commands to point branch to completely different commit in different section of the commit graph, just like you can do it with tags.

[1] The main difference perhaps is that, if you create a new commit, the branch pointer will actually be moved to point to that latest commit (while tag stays fixed).

Git supports history rewriting so 1 isn't true. Git uses hashes for "unique ids" hashes aren't unique just low probability of collision, so 3 is also not true.

Having run into issues that appear to be caused by 3 not being true, I don't see that as a theoretical issue.

1 is true. When git is “rewriting” history it’s actually creating a new commits and moving the branch pointer over.

Until the gc reaps them, you can absolutely git checkout the hash of any of the rewritten history and it’s still there, same as you left it. You can even move the branch pointer back, undoing the history rewrite.

So when I do a pull I get their un-rewritten history? Just because they have a short undo buffer doesn't make modifiable history immutable.

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