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Lecture 7
Rebasing
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2. Today Review of merging Rebasing instead of merging
Interactive rebases

3. Review: Merging
To combine two branches, with a merge, we do something like
git merge branch B
to bring all of branch B’s changes into master via a merge commit (the commit is necessary to record the changes being brought in, especially if there are conflicts).
branchA,
HEAD
branchB E F C D B A

4. Review: Merging
To combine two branches, with a merge, we do something like
git merge branch B
to bring all of branch B’s changes into master via a merge commit (the commit is necessary to record the changes being brought in, especially if there are conflicts).
branchA,
HEAD G
branchB E F
This is kinda ugly to have in our git history, especially if the two branches being combined make very similar changes.
Wouldn’t it be nice to combine branches in a linear fashion? C D B A

5. Review: Merging
branchA,
HEAD E C
branchA,
HEAD
branchB
branchB E F F
This is kinda ugly to have in our git history, especially if the two branches being combined make very similar changes.
Wouldn’t it be nice to combine branches in a linear fashion? D C D B B A A

6. Review: Merging
branchA,
HEAD E C
branchA,
HEAD
branchB
branchB E F F
This is kinda ugly to have in our git history, especially if the two branches being combined make very similar changes.
Wouldn’t it be nice to combine branches in a linear fashion?
Enter git rebase. D C D B B A A

7. git rebase <branch-to-rebase-onto>
Example use:
git rebase master
Finds the common ancestor of the given branch and the current branch, then “replays” the changes of all commits of the current branch up to the ancestor onto the given branch.

8. Rebasing (assume we’re on branchA) git rebase branchB
Find the common ancestor.
Replay the changes of all commits up to the ancestor on top of the given branch.
Move the branch to the new top.
branchA,
HEAD E C
branchA,
HEAD
branchB
branchB E F F D C D B B
Key idea: git rebase combines two branches while ensuring that the history looks linear. A A

9. Another rebasing problem (adapted from midterm)
What happens if I do the following?
(on master) git rebase experiment
master B A C D F
develop
experiment
base

10. Another rebasing problem (adapted from midterm)
What happens if I do the following?
(on base) git rebase experiment
master B A C D F
develop
experiment
base

11. Another rebasing problem (adapted from midterm)
What happens if I do the following?
(on experiment) git rebase base
master B A C D F
develop
experiment
base

12. Another rebasing problem (adapted from midterm)
What happens if I do the following?
(on master) git rebase develop
master B A C D F
develop
experiment
base

13. Git rebasing does not delete commits
When you rebase, you create new commits, and your branch moves to the new commits you’ve made.
The old commit history still exists, it’s just inaccessible now that the branch has moved.
If you really want to, you can checkout to those commits if you know the hash.

14. Git rebasing does not delete commits
branchA,
HEAD C
branchA,
HEAD
branchB
branchB E F E F C D C D B B A A

15. Merge vs Rebase
Both are ways of combining changes from two different branches.
Generally, I prefer merges for combining branches that are distinct in their purpose (feature branches, branches for different parts of a project) and rebases for branches that are really similar (i.e. two people working on the same part of a project but create a conflict).

16. Interactive Rebase
Normal rebase only allows you to “replay” changes of commits onto another branch.
Interactive rebase gives you a lot more power on how this “replay” happens.

17. git rebase –i <branch-to-rebase-onto>
Example use:
git rebase –i master
Finds the common ancestor of the given branch and the current branch, then “replays” changes of all commits of the current branch up to the ancestor onto the given branch, but opens an editor for you to specify how the “replay” occurs.

18. Rebase Options

19. Git rebasing, but in reverse order
How do I do this?
We need to apply D, then B…
master, HEAD B A C D F
develop
experiment
master, HEAD B A C D F
develop
experiment
base
base

20. Git rebasing, but deleting commits
How do I do this?
We need to apply D, but not B…
master, HEAD B A C D F
develop
experiment A C D F
develop
experiment
master, HEAD
base
base

21. Git rebasing, but squashing commits
How do I do this?
We need to apply B, but apply D in the same commit…
master, HEAD B A C D F
develop
experiment
master, HEAD A C
D, B F
develop
experiment
base
base

22. Summary
Git rebase allows you to combine branches, but maintain a linear history.
Git rebase –i (interactive rebase) allows you to control precisely how commits in a linear history take place, including deleting history!
Rebasing works by reapplying unshared commits onto the given branch and moving your branch there.