Written by Pete Corey on Sep 12, 2016.

If you’ve been following our blog, you’ll notice that we’ve been writing lots of what we’re calling “literate commit” posts.

The goal of a literate commit style post is to break down each Git commit into a readable, clear explanation of the code change. The idea is that this chronological narrative helps tell the story of how a piece of software came into being.

Combined with tools like git blame and git log you can even generate detailed histories for small, focused sections of the codebase.

But sometimes generating repositories with this level of historical narrative requires something that most Git users warn against: rewriting history.

Why Change the Past

It’s usually considered bad practice to modify a project’s revision history, and in most cases this is true. However, there are certain situations where changing history is the right thing to do.

In our case, the main artifact of each literate commit project is not the software itself; it’s the revision history. The project serves as a lesson or tutorial.

In this situation, it might make sense to revise a commit message for clarity. Maybe we want to break a single, large commit into two separate commits so that each describes a smaller piece of history. Or, maybe while we’re developing we discover a small change that should have been included in a previous commit. Rather than making an “Oops, I should have done this earlier” commit, we can just change our revision history and include the change in the original commit.

It’s important to note that in these situations, it’s assumed that only one person will be working with the repository. If multiple people are contributing, editing revision history is not advised.

In The Beginning…

Imagine that we have some boilerplate that we use as a base for all of our projects. Being good developers, we keep track of its revision history using Git, and possibly host it on an external service like GitHub.

Starting a new project with this base might look something like this:

mkdir my_project
cd my_project
git clone https://github.com/pcorey/base .
git remote remove origin
git remote add origin https://github.com/pcorey/my_project

We’ve cloned base into the my_project directory, removed it’s origin pointer to the base repository, and replaced it with a pointer to a new my_project repository.

Great, but we’re still stuck with whatever commits existed in the base project before we cloned it into my_project. Those commits most likely don’t contribute to the narrative of this specific project and should be changed.

One solution to this problem is to clobber the Git history by removing the .git folder, but this is the nuclear option. There are easier ways of accomplishing our goal.

The --root flag of the git rebase command lets us revise every commit in our project, including the root commit. This means that we can interactively rebase and reword the root commits created in the base project:

git rebase -i --root master

reword f784c6a First commit
# Rebase f784c6a onto 5d85358 (1 command(s))

Using reword tells Git that we’d like to use the commit, but we want to modify its commit message. In our case, we might want to explain the project we’re starting and discuss the base set of files we pulled into the repository.

Splicing in a Commit

Next, let’s imaging that our project has three commits. The first commit sets up our project’s boilerplate. The second commit adds a file called foo.js, and the third commit updates that file:

git log --online

1d5f372 Updated foo.js
873641e Added foo.js
b3065c9 Project setup

What if we forgot to create a file called bar.js after we created foo.js. For maximum clarity, we want this file to be created in a new commit following 873641e. How would we do it?

Once again, interactive rebase comes to the rescue. While doing a root rebase, we can mark 873641e as needing editing:

git rebase -i --root master

pick b3065c9 Project setup
edit 873641e Added foo.js
pick 1d5f372 Updated foo.js

After rebasing, our Git HEAD will point to 873641e. Our git log looks like this:

git log --online

1d5f372 Updated foo.js
873641e Added foo.js

We can now add bar.js and commit the change:

touch bar.js
git add bar.js
git commit -am "Added bar.js"

Reviewing our log, we’ll see a new commit following 873641e:

git log --online

58f31fd Added bar.js
41817a4 Added foo.js
81df941 Project setup

Everything looks good. Now we can continue our rebase and check out our final revision history:

git rebase --continue
git log --oneline

b8b7b18 Updated foo.js
58f31fd Added bar.js
41817a4 Added foo.js
81df941 Project setup

We’ve successfully injected a commit into our revision history!

Revising a Commit

What if we notice a typo in our project that was introduced by our boilerplate? We don’t want to randomly include a typo fix in our Git history; that will distract from the overall narrative. How would we fix this situation?

Once again, we’ll harness the power of our interactive root rebase!

git rebase -i --root master

edit b3065c9 Project setup
pick 873641e Added foo.js
pick 1d5f372 Updated foo.js

After starting the rebase, our HEAD will point to the first commit, b3065c9. From there, we can fix our typo, and then amend the commit:

vim README.md
git add README.md
git commit --amend

Our HEAD is still pointing to the first commit, but now our fixed typo is included in the set of changes!

We can continue our rebase and go about our business, pretending that the typo never existed.

git rebase --continue

With Great Power

Remember young Time Lord, with great power comes great responsibility.

Tampering with revision history can lead to serious losses for your project if done incorrectly. It’s recommended that you practice any changes you plan to make in another branch before attempting them in master. Another fallback is to reset hard to origin/master if all goes wrong:

git reset --hard origin/master

While changing history can be dangerous, it’s a very useful skill to have. When you want your history to be the main artifact of your work, it pays to ensure it’s as polished and perfected as possible.