How to get the new changes from another branch with git

Question

I merged branch B to my branch C a week ago and did some updates on my branch C.

The same time, my colleague, did some updates on his branch B.

I want to get his updates on my branch C.

I am really confused, because he tells me to just rebase on his branch - but that keeps deleting all my new updates / added files.

I tried to merge the two branches again - but somehow I don't get his updates this time.

Our project looks kind of like this:

Origin---o---o---o---o---o---      main
 \
  o---o---o---o---o---o---o---o---o B
   \               \(merged)       \(rebase here?)
    o---o---o---o---o---o---o---o---o C

Answer 1

Rebasing is complicated, and was probably a bad idea in this situation. Let's adopt your topology:

o---o---o---o---C1---o---o---o---o friend
 \               \
  o---o---o---o---M---o---o---o me

To pick up your friend's changes, the right and simplest thing to do was (on me):

git merge friend

That would give this:

o---o---o---o---C1---o---o---o---o friend
 \               \                \
  o---o---o---o---M---o---o---o---M2 me

You cannot coherently rebase at this point, despite what your friend said, because me already includes a merge commit (M) from friend. Rebasing can't easily cope with that; it will ignore the merge commit, and thus the effect of the merge will be undone in your branch. There are ways around this, but it's not worth going into them here.

The key question now, however, is how to get out of the mess you've gotten yourself into with a bad rebase followed by a bad merge. The answer is to use the reflog, which makes much of what you do in Git undoable. When you say git reflog you will be shown a list of all previous states of your HEAD. One of these will be the state just before you started the rebase. You want to go back to that by saying git reset --hard <SHA> (as I explain here). Now do the merge that you should have done, and move on.