git commit --amend in detached HEAD state

Question

I understand that the correct way of amending an old Git commit is to use rebase --interactive, but just to get clear on the concepts, I would like to understand what happens when I do

• git checkout <commit>
• change something in a file
• add the changed file to the index
• and then git commit . --amend

When I do this, instead of amending the commit, it branches a new commit off of the PARENT of that same commit.

Is this just Git's way of telling me that I cannot amend a commit that already has children commits?

Answer 1

In Git, once a commit is created, it's set in stone; you cannot change it. All you can do—by amending it, cherry-picking it, etc.—is create a new commit that "resembles" it.

I understand your confusion: "amend" is a bit of a misnomer; it's somewhat misleading, as it suggests modifying something in place. In Git, amending a commit actually consists in creating a brand new commit that has the same parent(s) as the original commit.

As an example, let's assume that, after running git checkout B, you're in the following situation:

(Your HEAD is detached, but that's beside the point.) Whether or not you make and stage changes, running git commit --amend will put you in this situation:

Commit D may be very, very similar to B; in particular, it may have exactly the same patch, exactly the same commit message as B, etc.. However, (commit, author) timestamps will usually differ (unless you can amend a commit under a second!), which means the SHA-1 of D will differ from that of B; and if two commits don't have the same SHA, they're not the same commit.

When we say B is a parent commit of C, we mean commit C references commit B by its SHA. However, commit C has no way of knowing anything about the SHA of commit D, because commit D was created after C. Therefore, D cannot be C's parent. That's why commit D goes off on a tangent and doesn't have any descendants.

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If you want to land in the following state,

where B' is even only slightly different from B, you should use git rebase -i, not git commit --amend.