git-rewrite

A script that helps you rewrite git history (but doesn't do it for you!)

Disclaimer

Use this at your own risk

(but is should be rather safe to do so)

Introduction

Editing GIT_AUTHOR and GIT_COMMITTER data for multiple commits can be a pain. Editing dates even more so.

This script tries to make the process somewhat easier. It works by creating a new branch, applying the changes there and letting the user to actually decide if he wants to keep the branch or throw it away.

The supported fields are:

• AUTHOR_NAME
• AUTHOR_EMAIL
• AUTHOR_DATE
• COMMITTER_NAME
• COMMITTER_EMAIL
• COMMITTER_DATE

Example

Let's assume that we have the following repo and that we want to change the AUTHOR data for commits C and D

                                  HEAD
                                   |
                                   *
A ---> B ---> C ---> D ---> E ---> F

Running this script will:

1. checkout B
2. create a new branch
3. cherry-pick commits: C, D, E and F
4. edit AUTHOR and COMMITER data on each commit with git commit --amend

So, after running the script our repo should look like this:

A ---> B ---> C ---> D ---> E ---> F  (original-branch)
       |
       -----> C' --> D' --> E' --> F' (new-branch)

It is now up to us to ensure that everything is looking OK and actually overwrite history with e.g.:

git checkout original-branch
git reset --hard new-branch
git push

Installation

The recommended installation method is pipx. More specifically, you can install git-rewrite for your user with:

pipx install --spec https://github.com/pmav99/git-rewrite/archive/master.zip git-rewrite

The above command will create a virtual environment in ~/.local/pipx/venvs/git-rewrite and add the git-rewrite script in ~/.local/bin.

Command overview

The script has two subcommands:

• git-rewrite dump
• git-rewrite apply

Both are required.

The dump step saves the commit data of the current history to a JSON file. You are supposed to manually edit the file and make any changes you need (e.g. using sed). After you are done, you just apply the changes to the repo.

How the dump works

The dump script

1. generates a list of commit hashes since the root commit using:

git rev-list --ancestry-path a84477f..HEAD

2. parses the commit data of each commit using git show --format=fuller and saves them to a JSON file.

The end result is something like this:

{
  "root_hash": "a84477f",
  "commits": [
    {
      "commit_hash": "6726ad10be585c21c38726630f7dadf85833f10f",
      "author_name": "John Doe",
      "author_email": "john_doe@gmail.com",
      "author_date": "Thu Jun 6 15:51:30 2019 +0300",
      "committer_name": "john Doe",
      "committer_email": "john_doe@gmail.com",
      "committer_date": "Thu Jun 6 15:53:56 2019 +0300"
    },
    {
      "commit_hash": "4bf077761d21090b46f178b5da534fb9a9da0ca9",
      "author_name": "John Doe",
      "author_email": "john_doe@gmail.com",
      "author_date": "Thu Jun 6 18:12:34 2019 +0300",
      "committer_name": "John Doe",
      "committer_email": "john_doe@gmail.com",
      "committer_date": "Thu Jun 6 18:12:34 2019 +0300"
    }
  ]
}

How the apply works

1. the script checkouts the root commit and creates a new branch.

2. For each commit in the JSON file the script runs:

   1. git cherry-pick it in order to bring the changeset into the new branch
   2. git commit --amend ... in order to update the AUTHOR and the COMMITTER data.