git erase

If you write software, then you should be using version control tools such as git. However, following git best practices frequently leaves you with a messy commit history. How can one clean it up?

You might find yourself in a situation like this:

λ> git log --oneline
gshk3j2 (HEAD -> feature-X-branch) update README to document feature X
wxyz123 final clean-up. 
opqrstu fix random other thing (we hope!).
ghijklm refactored, but random other thing broke
k8l9m0n oh I see, I can refactor this into...
g5h6i7j clean up
c2d3e4f ahh it's working now but it's messy
y9z0a1b why is this still broken? try alternate fix
q3r4s5t fix bug edge-case
e4f5g6h fix bug
a1b2c3d add new feature X

It is hard to follow along with the history of the project, especially when several such histories accumulate over a project's lifetime. All the commits were building toward a single feature, and it would be nice to have a more coherent history of that feature.

You should be comfortable with what a rebase is. It also helps if you understand merges and how merges and rebases differ. You should also be comfortable with a console-based text editor such as vim or emacs — but if you're not, don't worry about it.

The idea is to use an interactive rebase to squash select commits into a single commit. We also need to specify the earliest commit we wish to include in the interactive rebase.

λ> git rebase -i <commit>

It is possible to specify a commit by its count from the HEAD, e.g. HEAD~5 for the fifth commit before the current state of the repository.

Running git rebase -i will launch a console text editor with the specified commit and all commits made after it, listed in chronological order. The commits are prefixed with the word pick, the default action, which means that the commit will be included in the rebase.

pick a1b2c3d add new feature
pick e4f5g6h fix bug
pick q3r4s5t fix bug edge-case
pick y9z0a1b why is this still broken? try alternate fix
pick c2d3e4f ahh it's working now but it's messy
pick g5h6i7j clean up
pick k8l9m0n oh I see, I can refactor this into...
pick ghijklm refactored, but random other thing broke
pick opqrstu fix random other thing (we hope!)
pick wxyz123 final clean-up
pick gshk3j2 update README to document feature X

To specify that a commit be squashed into the previous commit, change the prefixed pick to squash, or s for short.

pick a1b2c3d add new feature X
squash e4f5g6h fix bug
squash q3r4s5t fix bug edge-case
squash y9z0a1b why is this still broken? try alternate fix
squash c2d3e4f ahh it's working now but it's messy
squash g5h6i7j clean up
squash k8l9m0n oh I see, I can refactor this into...
squash ghijklm refactored, but random other thing broke
squash opqrstu fix random other thing (we hope!)
squash wxyz123 final clean-up
pick gshk3j2 update README to document feature X

If working with a lot of commits, you may find it useful to read these references on find and replace in vim or find and replace in emacs.

Once you have finished editing the list of commits and specified which to squash, save the document and exit. If you find yourself stuck in the editor, you are not alone.

• vim: press esc to enter command mode, then:wq followed by Enter. w saves the document ("writes") and q closes the document ("quits").
• emacs: type Ctrl-x Ctrl-s folowed by Ctrl-x Ctrl-c. Ctrl-x Ctrl-s saves the document, and Ctrl-x Ctrl-c exits the editor.

After saving and exiting the editor, the rebase will begin. git will revert the state of the repository to the specified starting commit and walk through each commit in the list, applying it to the repository then either saving the state as a commit (if pick) or skipping it (if squash). After walking through all the commits, git will open the text editor and prompt you to specify a commit message for the new commits where multiple commits were squashed into one.

The state of the repository is now identical to before the squash, but the history is much cleaner.

λ> git log --oneline
gshk3j2 (HEAD -> feature-X-branch) update README to document feature X
ghj23h3 fixes and clean-up (squashed)
a1b2c3d add new feature X

Next, we wish to publish our new history to the remote repository. However, the remote will not accept our new history — it's essentially different from its current version of the history. In other words, the two histories have diverged. We must tell the upstream repository that we wish to overwrite its existing history — with whatever history we have locally by running git push --force.

Phew! Your git history is now less cringe.