Commits are one of the key parts of a Git repository, and more so, the commit message is a life log for the repository. As the project/repository evolves over time (new features getting added, bugs being fixed, architecture being refactored), commit messages are the place where one can see what was changed and how. So it's important that these messages reflect the underlying change in a short, precise manner.

Why meaningful commit history is important

Git commit messages are the fingerprints that you leave on the code you touch. Any code that you commit today, a year from now when you look at the same change; you would be thankful for a clear, meaningful commit message that you wrote, and it will also make the lives of your fellow developers easier. When commits are isolated based on context, a bug which was introduced by a certain commit becomes quicker to find, and the easier it is to revert the commit which caused the bug in the first place.

While working on a large project, we often deal with a lot of moving parts that are updated, added or removed. Ensuring that commit messages are maintained in such cases could be tricky, especially when development spans across days, weeks, or even months. So to simplify the effort of maintaining concise commit history, this article will use some of the common situations that a developer might face while working on a Git repository.

But before we dive in, let's quickly go through how a typical development workflow looks like in a our hypothetical Ruby application.

Note: This article assumes that you are aware about basics of Git, how branches work, how to add uncommitted changes of a branch to stage and how to commit the changes. If you're unsure of these flows, our documentation is a great starting point.

A day in the life

Here, we are working on a small Ruby on Rails project where we need to add a navigation view on the homepage and that involves updating and adding several files. Following is a step by step breakdown of the entire flow:

You start working on a feature with updating a file; let's call it application_controller.rb

This feature requires you to also update a view: index.html.haml

You added a partial which is used in index page: _navigation.html.haml

Styles for the page also need to be updated to reflect the partial we added: styles.css.scss

Feature is now ready with the desired changes, time to also update tests; files to be updated are as follows: application_controller_spec.rb navigation_spec.rb

Tests are updated and passing as expected, now time to commit the changes!

Since all the files belong to different territories of the architecture, we commit the changes isolated of each other to ensure that each commit represents a certain context and is made in a certain order. I usually prefer backend -> frontend order where most backend-centric change is committed first, followed by the middle layer and then by frontend-centric changes in the commits list.

application_controller.rb & application_controller_spec.rb ; Add routes for navigation. _navigation.html.haml & navigation_spec.rb ; Page Navigation View. index.html.haml ; Render navigation partial. styles.css.scss ; Add styles for navigation.

Now that we have our changes committed, we create a merge request with the branch. Once you have merge request open, it typically gets reviewed by your peer before the changes are merged into repo's master branch. Now let's learn what different situations we may end up with during code review.

Situation 1: I need to change the most recent commit

Imagine a case where the reviewer looked at styles.css.scss and suggested a change. In such a case, it is very simple to do the change as the stylesheet changes are part of last commit on your branch. Here's how we can handle this;

You directly do the necessary changes to styles.css.scss in your branch.

in your branch. Once you're done with the changes, add these changes to stage; run git add styles.css.scss .

. Once changes are staged, we need to add these changes to our last commit; run git commit --amend . Command breakdown : Here, we're asking the git commit command to amend whatever changes are present in stage to the most recent commit.

. This will open your last commit in your Git-defined text editor which has the commit message Add styles for navigation .

. Since we only updated the CSS declaration, we don't need to alter the commit message. At this point, you can just save and exit the text editor that Git opened for you and your changes will be reflected in the commit.

Since you modified an existing commit, these changes are required to be force pushed to your remote repo using git push --force-with-lease <remote_name> <branch_name> . This command will override the commit Add styles for navigation on remote repo with updated commit that we just made in our local repo.

One thing to keep in mind while force pushing branches is that if you are working on the same branch with multiple people, force pushing may cause trouble for other users when they try to normally push their changes on a remote branch that has new commits force pushed. Hence, use this feature wisely. You can learn more about Git force push options here.

Situation 2: I need to change a specific commit

In the previous situation, the fix was rather simple as we had to modify only our last commit, but imagine if reviewer suggested to change something in _navigation.html.haml . In this case, it is second commit from the top, so changing it won't be as direct as it was in the first situation. Let's see how we can handle this:

Whenever a commit is made in a branch, it is identified by a unique SHA1 hash string. Think of it as a unique ID that separates one commit from another. You can view all the commits, along with their SHA1 hashes in a branch by running git log . With this, you would see an output that looks somewhat as follows, where the most recent commits are at the top;

commit aa0a35a867ed2094da60042062e8f3d6000e3952 (HEAD -> add-page-navigation) Author: Kushal Pandya <kushal@gitlab.com> Date: Wed May 2 15:24:02 2018 +0530 Add styles for navigation commit c22a3fa0c5cdc175f2b8232b9704079d27c619d0 Author: Kushal Pandya <kushal@gitlab.com> Date: Wed May 2 08:42:52 2018 +0000 Render navigation partial commit 4155df1cdc7be01c98b0773497ff65c22ba1549f Author: Kushal Pandya <kushal@gitlab.com> Date: Wed May 2 08:42:51 2018 +0000 Page Navigation View commit 8d74af102941aa0b51e1a35b8ad731284e4b5a20 Author: Kushal Pandya <kushal@gitlab.com> Date: Wed May 2 08:12:20 2018 +0000 Add routes for navigation

This is where git rebase command comes into play. Whenever we wish to edit a specific commit with git rebase , we need to first rebase our branch by moving back HEAD to the point right before the commit we wish to edit. In our case, we need to change the commit that reads Page Navigation View .

Here, notice the hash of commit which is right before the commit we want to modify; copy the hash and perform the following steps:

Rebase the branch to move to commit before our target commit; run git rebase -i 8d74af102941aa0b51e1a35b8ad731284e4b5a20 Command breakdown : Here we're running Git's rebase command with interactive mode with provided SHA1 hash as commit to rebase to.

This will run rebase command for Git in interactive mode and will open your text editor showing all of your commits that came after the commit you rebased to. It will look somewhat like this:

pick 4155df1cdc7 Page Navigation View pick c22a3fa0c5c Render navigation partial pick aa0a35a867e Add styles for navigation # Rebase 8d74af10294..aa0a35a867e onto 8d74af10294 (3 commands) # # 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

Notice how each commit has a word pick in front of it, and in the contents below, there are all possible keywords we can use. Since we want to edit a commit, we need to change pick 4155df1cdc7 Page Navigation View to edit 4155df1cdc7 Page Navigation View . Save the changes and exit editor.

Now your branch is rebased to the point in time right before the commit you made which included _navigation.html.haml . Open the file and perform desired changes as per the review feedback. Once you're done with the changes, stage them by running git add _navigation.html.haml .

Since we have staged the changes, it is time to move branch HEAD back to the commit we originally had (while also including the new changes we added), run git rebase --continue , this will open your default editor in the terminal and show you the commit message that we edited during rebase; Page Navigation View . You can change this message if you wish, but we would leave it as it is for now, so save and exit the editor. At this point, Git will replay all the commits that followed after the commit you just edited and now branch HEAD is back to the top commit we originally had, and it also includes the new changes you made to one of the commits.

Since we again modified a commit that's already present in remote repo, we need force push this branch again using git push --force-with-lease <remote_name> <branch_name> .

Situation 3: I need to add, remove, or combine commits

A common situation is when you've made several commits just to fix something previously committed. Now let's reduce them as much as we can, combining them with the original commits.

All you need to do is start the interactive rebase as you would in the other scenarios.

pick 4155df1cdc7 Page Navigation View pick c22a3fa0c5c Render navigation partial pick aa0a35a867e Add styles for navigation pick 62e858a322 Fix a typo pick 5c25eb48c8 Ops another fix pick 7f0718efe9 Fix 2 pick f0ffc19ef7 Argh Another fix!

Now imagine you want to combine all those fixes into c22a3fa0c5c Render navigation partial . You just need to:

Move the fixes up so that they are right below the commit you want to keep in the end. Change pick to squash or fixup for each of the fixes.

Note: squash keeps the commits messages in the description. fixup will forget the commit messages of the fixes and keep the original.

You'll end up with something like this:

pick 4155df1cdc7 Page Navigation View pick c22a3fa0c5c Render navigation partial fixup 62e858a322 Fix a typo fixup 5c25eb48c8 Ops another fix fixup 7f0718efe9 Fix 2 fixup f0ffc19ef7 Argh Another fix! pick aa0a35a867e Add styles for navigation

Save the changes, exit the editor, and you're done! This is the resulting history:

pick 4155df1cdc7 Page Navigation View pick 96373c0bcf Render navigation partial pick aa0a35a867e Add styles for navigation

As before, all you need to do now is git push --force-with-lease <remote_name> <branch_name> and the changes are up.

If you want to remove a commit altogether, instead of squash or fixup , just write drop or simply delete that line.

Avoid conflicts

To avoid conflicts, make sure the commits you're moving up the timeline aren't touching the same files touched by the commits left after them.

pick 4155df1cdc7 Page Navigation View pick c22a3fa0c5c Render navigation partial fixup 62e858a322 Fix a typo # this changes styles.css fixup 5c25eb48c8 Ops another fix # this changes image/logo.svg fixup 7f0718efe9 Fix 2 # this changes styles.css fixup f0ffc19ef7 Argh Another fix! # this changes styles.css pick aa0a35a867e Add styles for navigation # this changes index.html (no conflict)

Pro-tip: Quick fixup s

If you know exactly which commit you want to fixup, when committing you don't have to waste brain cycles thinking of good temporary names for "Fix 1", "Fix 2", …, "Fix 42".

Step 1: Meet --fixup

After you've staged the changes fixing whatever it is that needs fixing, just commit the changes like this:

git commit --fixup c22a3fa0c5c

(Note that this is the hash for the commit c22a3fa0c5c Render navigation partial )

This will generate this commit message: fixup! Render navigation partial .

Step 2: And the sidekick --autosquash

Easy interactive rebase. You can have git place the fixup s automatically in the right place.

git rebase -i 4155df1cdc7 --autosquash

History will be shown like so:

pick 4155df1cdc7 Page Navigation View pick c22a3fa0c5c Render navigation partial fixup 62e858a322 Fix a typo fixup 5c25eb48c8 Ops another fix fixup 7f0718efe9 Fix 2 fixup f0ffc19ef7 Argh Another fix! pick aa0a35a867e Add styles for navigation

Ready for you to just review and proceed.

If you're feeling adventurous you can do a non-interactive rebase git rebase --autosquash , but only if you like living dangerously, as you'll have no opportunity to review the squashes being made before they're applied.

Situation 4: My commit history doesn't make sense, I need a fresh start!

If we're working on a large feature, it is common to have several fixup and review-feedback changes that are being committed frequently. Instead of constantly rebasing the branch, we can leave the cleaning up of commits until the end of development.

This is where creating patch files is extremely handy. In fact, patch files were the primary way of sharing code over email while collaborating on large open source projects before Git-based services like GitLab were available to developers. Imagine you have one such branch (eg; add-page-navigation ) where there are tons of commits that don't convey the underlying changes clearly. Here's how you can create a patch file for all the changes you made in this branch:

The first step to create the patch file is to make sure that your branch has all the changes present from master branch and has no conflicts with the same.

branch and has no conflicts with the same. You can run git rebase master or git merge master while you're checked out in add-page-navigation branch to get all the changes from master on to your branch.

or while you're checked out in branch to get all the changes from on to your branch. Now create the patch file; run git diff master add-page-navigation > ~/add_page_navigation.patch . Command breakdown : Here we're using Git's diff feature, and asking for a diff between master branch and add-page-navigation branch, and redirecting the output (via > symbol) to a file named add_page_navigation.patch in our user home directory (typically ~/ in *nix operating systems).

. You can specify any path you wish to keep this file in and the file name and extension could be anything you want.

Once the command is run and you don't see any errors, the patch file is generated.

Now checkout master branch; run git checkout master .

branch; run . Delete the branch add-page-navigation from local repo; run git branch -D add-page-navigation . Remember, we already have changes of this branch in a created patch file.

from local repo; run . Remember, we already have changes of this branch in a created patch file. Now create a new branch with the same name (while master is checked out); run git checkout -b add-page-navigation .

is checked out); run . At this point, this is a fresh branch and doesn't have any of your changes.

Finally, apply your changes from the patch file; git apply ~/add_page_navigation.patch .

. Here, all of your changes are applied in a branch and they will appear as uncommitted, as if all your modification where done, but none of the modifications were actually committed in the branch.

Now you can go ahead and commit individual files or files grouped by area of impact in the order you want with concise commit messages.

As with previous situations, we basically modified the whole branch, so it is time to force push!

Conclusion

While we have covered most common and basic situations that arise in a day-to-day workflow with Git, rewriting Git history is a vast topic and as you get familiar with above tips, you can learn more advanced concepts around the subject in the Git Official Documentation. Happy git'ing!

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