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Automatically Reformat all Commits on a Branch

· 4 min read
Nils Wireklint
Nils Wireklint

If you have a formatter tool that can rewrite your code you can run it automatically on all unmerged commits. This will show you how to script git-rebase to do so without any conflicts.

There are two ways to do it manually, forward or backward. The forward pass amends each commit and deals with the conflicts when stepping to the next commit. In contrast the backwards pass, formats each commit from the end, which will avoid conflicts but for long commit chains it can be almost as boring.

This pattern comes up when working with long-lived feature branches, or tasks that were almost done, and then pre-empted by other prioritized work. Here are a few oneliners you can run to tidy up your commits.

See also the full technical guide for developing this git-rebase workflow in our documentation. Which contains more details on rebasing with git, using a scriptable editor to automate the git-rebase todo-list, as well as the squashed commit messages.

Example commits​

Say you have three unmerged commits:

21cc7b5 My amazing feature e05fd9f Other complimentary work acb9fae Fix annoying bug

They contain important work, but you forgot to run some linters, or the main branch added more lint requirements after the feature work was started. This will run linters that can automatically fix issues on each commit through a scripted git-rebase.

Rebase algorithm​

We have a three-step process to update each commit.

  • 1: Create a fixup commit with the applied lint suggestions, which we immediately revert so the next commit still applies

    #!/bin/sh

    # Formatters and fixers go here.
    # Replace with your tools of choice! rustfmt, gofmt, black, ...
    ./run-all-linters-and-autofixers.sh

    # Add a new commit with the changes and revert it again.
    git add -u
    git commit --allow-empty --fixup HEAD
    # 'git-revert' does not support '--allow-empty'.
    git revert --no-commit HEAD
    git commit --allow-empty --no-edit

  • 2: Squash the fixup commit into the original feature commit

  • 3: Squash the revert down into the next feature commit

These tabs show how the commits evolve and are squashed, the extra commits are grouped to indicate the target commit. The revert of the first commit is grouped with the second feature commit, and so on. We discard the final revert.

21cc7b5 My amazing feature
01900c5 fixup! My amazing feature

55feaba Revert "fixup! My amazing feature"
e05fd9f Other complimentary work
d122da7 fixup! Other complimentary work

249b0d3 Revert "fixup! Other complimentary work"
acb9fae Fix annoying bug
50e426a fixup! Fix annoying bug

7e84259 Revert "fixup! Fix annoying bug"

Oneliners​

git allows us to set any editor to edit the todo-list, $GIT_SEQUENCE_EDITOR, and the commit message, $EDITOR. We choose vim as it is often available, and easier to use than sed and awk. It is nice to have a scriptable interactive editor to make changes to the workflow and try out the commands.

See the full technical guide for details and more tips on git-rebase and vim.

Reformat:

$ env                          \
    GIT_SEQUENCE_EDITOR="true" \
    git rebase -i --exec ./reformat.sh origin/main

Fixup (autosquash):

# More robust autosquash, that handles duplicated commit messages.
# If your commit messages are all unique you can use '--autosquash' instead.
# See the technical guide for more details.
$ env                                                             \
    GIT_SEQUENCE_EDITOR="vim +'g/^\w* \w* fixup!/s/^pick/fixup/'" \
    git rebase -i origin/main

Squash:

$ env                                                                                               \
    EDITOR="sed -i '1,9d'"                                                                          \
    GIT_SEQUENCE_EDITOR="vim +'g/^#/d' +'normal! Gdk' +'g/^pick \w* Revert \"fixup!/normal! j0ces'" \
    git rebase -i origin/main
info

We have not developed the incantation, git-rebase command, to preserve the author date from the original commits. We will address that next!