Replaying changes from one git branch onto another
Or rebasing without rebasing
Where I work, we use git (like everyone else), and we follow this common pattern for development:
- Create a feature branch off of master
- Work on your feature in the branch
- When getting ready to submit a Pull Request, squash your commits
- Rebase master against your branch
- Open PR
- Get feedback
- After feedback corrected (if present), merge branch into master
Pretty standard practice. 99% of the time, this is a frictionless process.
The other day I picked up a story which, while small in scope, touched a ton of files.
The path to failure
I created a branch, did the work, but never squashed my commits - I basically skipped step 3 - and tried to rebase master against my branch. I got the common enemy of every PR - merge conflicts.
I had 5+ commits in the branch. The idea of wading through multiple failing rebase steps left me queasy, so I abandoned the rebase and went to merge master.
So to add to my failure at step 3, now I had skipped step 4 as well.
After getting the code working I pushed my PR. I received some minor feedback, updated my code, and went to merge master again. More merge conflicts.
I fixed those conflicts, but now I had a convoluted history of un-squashed commits and two merges. This was both out of practice and felt sloppy.
Finding a solution
When you have a messed up git history, you normally do a
git rebase --interactive, fix your commit history, and move on.
But with the merges scattered between commits, an interactive rebase would be difficult.
It also won't give you what you want - a single commit.
Enter git symbolic-ref.
This command, while well documented, doesn't give away the true beauty of this command.
Hiding on this line is the heart of the command:
Given two arguments, creates or updates a symbolic ref
<name>to point at the given branch.
What does this do? Let's say you're working on a sloppy branch. You want to carry over those changes onto another branch (like master), without changing the history of the that branch.
git symbolic-ref HEAD refs/heads/master sets the state of the current branch onto master. You haven't changed the history - git modifies the files on master to match the state of your branch. The commit history doesn't change. What this means is that doing something like this:
git checkout super-sloppy-work-branch //this causes you to 'checkout' master, but with your current file changes from the sloppy branch git symbolic-ref HEAD refs/heads/master git checkout -b new-branch-that-is-great git add -A git commit -m 'I totally did this work in one pass'
Will give you a one-commit branch with all the work you did in the sloppy branch, but against the latest version of master. Pretty cool!