Question

I have branch say “experiment” and it was pushed a few days back. Now, the master has been updated and I need to push experiment again after rebasing it to master. But when I rebase it to master I don’t see the patch of experiment on top of master. My experiment branch points to master.

What could be wrong? Could it be because this experiment branch has been already merger with the master or something else?

Git status

$ git status
On branch experiment
Untracked files:
(use “git add …” to include in what will be committed)
a.txt
nothing added to commit but untracked files present (use “git add” to track)

Git log

$ git log experiment
commit bc6c71d5ec6d8798aa283c9feaec844cd03edc60
Author: ssrivastava
Date: Tue Nov 27 12:15:37 2012 +0530
[experiment] Added index to players status

On rebase to master

First, rewinding head to replay your work on top of it…
Fast-forwarded experiment to master.

Generally on applying rebase it applies the latest change on the current branch on top of the base branch but I don’t see anything here.

Answer 1

In general, when rebasing, Git is smart enough to detect which commits do nothing and leaves them out. So when you have changes in master that do the same things as a commit in the experiment branch, that commit is skipped because it would change nothing. While this behaviour can be confusing when reviewing a rebase, it is not what happened in your case.

A rebase works by finding the common ancestor of both branches; that is then used as the old base. In your case, as you have merged the changes of the experiment branch into the master branch, the common ancenstor is that merge commit. Now that commit is the same what master points at at the moment, and it is actually a child of the experiment branch (which points at the parent in the experiment branch before the merge):

B --- * --- O --- M       B ~ old base, O ~ old master
 \               /        M ~ current master (the merge commit)
  * --- * ----- E         E ~ experiment branch

So master already contains the full experiment branch, and M is actually ahead of the experiment branch by one commit. If Git were to replay the commits in the experiment branch now on top of master, it would just skip those commits because they are empty—all the changes are inside master already. So essentially it would end up in a fast-forward. Now, as Git is smart, it does not need to try to apply all those commits on top of master to see that they would all be skipped. As master is ahead of the experiment branch, this is always the case. So it can just fast-forward the experiment branch to master directly.