10 second Git tip: Rebase since master

When I was new to Git, I saw editing the commit history as some sort of heinous sin. Git was about recording the exact steps that it took to create a piece of software, and to edit, resequence, or “squash” those steps was deceptive and dangerous.

Ah, youth.

Now that I more frequently end up reviewing other colleagues’ pull requests, or stepping through commits in poorly-documented open source modules, I’ve come to appreciate the value of some Git history housekeeping. Software development is messy. Tidying up your path to a solution, if it helps other people understand your working, can surely be no bad thing.

xkcd comic about git commit messages

The workhorse of most Git history editing is git rebase -i – which shows you a list of all commits in a certain tree, and lets you reword, edit, resequence, or combine them.

All the work we do at mySociety is constructed in feature branches, until it has been reviewed and merged into master. It’s not uncommon for me to want to quickly squash or resequence commits in a feature branch before it goes up for review.

If you know how many commits back you want to start your rebase, you can specify it:

$ git rebase -i HEAD~3

But keeping track of how many commits back you need to start is a pain. Much easier to just say “rebase everything since this branch diverged from master”:

For this, I have an alias in my ~/.gitconfig file, like this:

  rebase-since-master = !git rebase -i `git merge-base HEAD master`

Which can be run from the command line like this:

$ git rebase-since-master

Git works out the exact point at which the current feature branch (HEAD) diverged from master, and presents an overview of all the commits, ready for editing. It’s a super handy way to check your commits are nice and tidy before pushing to Github for code review.