Jake Worth

Note: any code contained in this post is more than a year old. Please use at your own risk.

What does it mean to 'squash' a Git branch?

Disclaimer: This process alters history, so do it on a feature branch, not the default branch. If it's part of a pull request, wait until the project maintainer asks you to do it– changing history on an open pull request can make an ongoing discussion difficult to follow.

Most pull requests go through a loop like this:

  1. Open pull request
  2. Maintainer gives feedback
  3. Make changes
  4. Repeat #2 and #3 until pull request is ready to merge
  5. Maintainer merges pull request

Prior to the merge, the pull request branch can be pretty messy. We can fix this by 'squashing' the branch.

Check out this branch:

* 724d574 (HEAD -> issue/38) Good to merge!
* e2af5ac Refactoring the previous commit
* 3e36475 Test are passing here
* ea479cb Revert "Ooops"
* 77aee1d Ooops
* 4c7ead2 Test breaker

The final commit, 724d574, has been approved to merge by the repo maintainer, but it's a mess. We have a commit that's just refactoring (e2af5ac), two commits that negate each other (ea479cb and 77aee1d), and one that has failing tests (4c7ead2).

Let's squash it!

The Squash

We'll use git rebase, interactive mode, going back to the start of our branch (six commits in the past):

$ git rebase -i HEAD~6

This opens our git-rebase-todo:

pick 4c7ead2 Test breaker
pick 77aee1d Ooops
pick ea479cb Revert "Ooops"
pick 3e36475 Test are passing here
pick e2af5ac Refactoring the previous commit
pick 724d574 Good to merge!

# Rebase dd6bb12..724d574 onto dd6bb12 (6 command(s))
#
# Commands:
# p, pick = use commit
# r, reword = use commit, but edit the commit message
# e, edit = use commit, but stop for amending
# s, squash = use commit, but meld into previous commit
# f, fixup = like "squash", but discard this commit's log message
# x, exec = run command (the rest of the line) using shell
# d, drop = remove commit
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out

Our goal is to make the log atomic. In short, we want each commit to contain distinct changes, that pass all tests, summarized by a simple message.

Here's some notes on these commits:

  • 4c7ead2 Test breaker: this contributes to the solution, but it breaks tests. We want to keep it, but it can't stand on it's own as a single commit
  • pick 77aee1d Ooops: this is useless
  • pick ea479cb Revert "Ooops": this reverts the previous commit
  • pick 3e36475 Test are passing here: this is important!
  • pick e2af5ac Refactoring the previous commit: same as above
  • pick 724d574 Good to merge!: same as above

With this in mind, I'd edit this file as such:

pick 4c7ead2 Test breaker
d 77aee1d Ooops
d ea479cb Revert "Ooops"
s 3e36475 Test are passing here
s e2af5ac Refactoring the previous commit
s 724d574 Good to merge!

This will delete (d) our two useless commits, and squash (s) all four useful commits together. An alternative to squash is fixup, f, which squashes and picks the first commit message in the list to cover them all. That's bad here, because the first commit's message (Test breaker) isn't very good.

Save and close the file, and you'll be in a new commit message editing window, with all four messages available:

# This is a combination of 4 commits.
# The first commit's message is:
Test breaker

# This is the 2nd commit message:

Test are passing here

# This is the 3rd commit message:

Refactoring the previous commit

# This is the 4th commit message:

Good to merge!

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Sun Jul 3 14:31:38 2016 -0500
#
# interactive rebase in progress; onto dd6bb12
# Last commands done (6 commands done):
#    s e2af5ac Refactoring the previous commit
#    s 724d574 Good to merge!
# No commands remaining.
# You are currently editing a commit while rebasing branch 'master' on 'dd6bb12'.

Time to write the squash message. If the issue at hand is issue #38, 'Back button is broken', then I'd change the first line of this file to this:

Fixes the back button

Close #38

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#

Close #38 is a Github feature; when the branch is merged into the default branch, Github issue #38 will be closed automatically.

Here's our revised history:

* 4e763d3 (HEAD -> issue/38) Fixes the back button

Much better! Force push it to Github, which we can do because we're on our own feature branch:

$ git push -f

Now we have a clean pull request that's ready to merge.

Fixing Mistakes

Is your rebase going wrong? Don't panic. You can abort the rebase with:

$ git rebase --abort

Even with the rebase is complete, you can travel go back in time with the git reflog.

Conclusion

Keeping the history of a big project tidy is a never-ending process. As an open-source project maintainer, I'm always trying to make commits smaller, better organized, more coherent. Do your favorite OSS maintainer and your own projects a favor and do your work on a feature branch, and when finished, squash your commits as logically as you can. It will make your code and the project better.

Jul 3, 2016

Hi! I'm Jake Worth, a developer at Hashrocket, based in Chicago. I co-organize Vim Chicago. Read my blog, learn about my work, follow me on Twitter and Github, get in touch.