Question

I don’t really understand what happens if I check out an old commit, do some modifications, and commit using git commit --amend.

Will that change automatically propagate to future commits? How does it work?

Answer 1

In git, commits are just objects. When you git commit --amend, you’re just creating a new commit with the same parent. Initially that looks like this:

                  {HEAD}
                  {master}
---[A]---[B]---[C]

Now you amend C, creating a new commit D:

                  {HEAD}
                  {master}
---[A]---[B]---[D]
            \
             \
              [C]

The old C is still there for the moment. However, it is no longer referenced by any branch, so the next time a garbage collection occurs, it’s going to be swept away.