Question

let’s say if I have 2 branches
topic and master
if I am on topic branch, and run git rebase master does it rebase master or rebase topic branch?

doing git rebase --help gave me this

Assume the following history exists and the current branch is “topic”:

             A---B---C topic
            /
       D---E---F---G master

From this point, the result of either of the following commands:

       git rebase master
       git rebase master topic

would be:

                         A'--B'--C' topic
                        /
           D---E---F---G master

NOTE: The latter form is just a short-hand of git checkout topic followed by git rebase master. When rebase exits topic will remain the checked-out branch.

And if I want to achieve this D---E---F---G---A---B---C master, should I switch to master and run git rebase -i topic?

And if I run git rebase -i topic try to squash A,B,C to C to become D---E---F---G---C master, I get noop

Answer 1

Assuming you start out with a tree that looks like this

         A---B---C topic
        /
   D---E---F---G master

You’ll want to run 2 commands git rebase master to make it look like this

                     A'--B'--C' topic
                    /
   D---E---F---G master

then from master run git merge topic, which will do a fast forward merge, and you’ll end up with a tree that looks like this

                             topic
                            /
   D---E---F---G---A---B---C -- master

---

Can I just checkout master and run git rebase topic??

Running that command would result in a tree that looks like this

                             topic
                            /
   D---A---B---C---E---F---G -- master

---

This is the (somewhat edited) git documentation for rebase, hopefully this will help:

All changes made by commits in the current branch but that are not in upstream are saved to a temporary area.

If you are on topic and run git rebase master, that means commits A, B and C are going into a temporary area.

The current branch is reset to upstream. This has the exact same effect as git reset –hard upstream.

At this point the temporary state of the tree looks like this

                 topic
                /
   D---E---F---G -- master

The commits that were previously saved into the temporary area are then reapplied to the current branch, one by one, in order.

Commits A, B, C that were saved into the temporary area are “recommitted” to the branch you are currently on topic.

                             topic
                            /
   D---E---F---G---A---B---C -- master

Running git rebase topic from master would perform the exact same steps, except instead of putting commits A, B, C into the temporary area, it would put commits E, F G into the temporary area.