GitLab - Rebase Operation

GitLab - Understanding and Performing a Rebase

What is Rebasing?

In Git, rebasing is the process of moving or combining a sequence of commits to a new base commit. When you’re working on a feature branch that has diverged from the main branch (commonly named ‘main’ or ‘master’), rebasing allows you to incorporate the latest changes from ‘main’ into your feature branch. Instead of creating a merge commit, rebase rewrites your feature branch’s history as if it started from the latest commit on ‘main’.

This results in a cleaner, more linear project history. It’s particularly useful for feature branches before merging them into the main branch.

Rebase vs. Merge

Both rebase and merge are used to integrate changes from one branch into another.

• git merge: Creates a new ‘merge commit’ in your current branch, tying together the histories of both branches. The project history will show the divergence and merging of branches. This is non-destructive; existing branches are not changed.
• git rebase: Re-applies commits from your current branch onto the tip of another branch. It rewrites the project history by creating brand new commits for each commit in the original branch. This can make the history look cleaner and more linear.

The Golden Rule of Rebasing: Never rebase commits that have been pushed to a public/shared repository. Rebasing rewrites history, which can cause significant issues for collaborators who have based their work on the original commits.

Steps for a Rebase Operation

Let’s walk through an example. Imagine you have a feature branch called my-feature and you want to incorporate the latest changes from the main branch.

Step 1: Ensure your local main branch is up-to-date.

$ git checkout main
$ git pull origin main

Step 2: Switch back to your feature branch.

$ git checkout my-feature

Step 3: Rebase your feature branch onto main.

$ git rebase main

This command tells Git to take all commits on my-feature that are not on main, move my-feature to point to the latest commit of main, and then re-apply those unique commits one by one onto the new tip of my-feature.

Example Scenario Walkthrough:

1. Create and switch to a new branch, say feature-x, from main:

$ git checkout -b feature-x main
Switched to a new branch 'feature-x'

2. Make some changes, add a new file, and commit it on feature-x:

$ echo "Initial feature work" > feature_file.txt
$ git add feature_file.txt
$ git commit -m "Add initial feature file"

3. Meanwhile, other work has been done on the main branch. To simulate this, switch to main, make a change, and commit:

$ git checkout main
Switched to branch 'main'
Your branch is up to date with 'origin/main'.

$ echo "Update on main" > main_update.txt
$ git add main_update.txt
$ git commit -m "Add update on main branch"

4. Now, switch back to feature-x to rebase it onto the updated main:

$ git checkout feature-x
Switched to branch 'feature-x'

5. Perform the rebase:

$ git rebase main
Successfully rebased and updated refs/heads/feature-x.

At this point, your feature-x branch includes its original commits plus all the commits from main that occurred after feature-x was branched. The history of feature-x will appear linear as if it was developed sequentially after the latest main branch changes.

Handling Conflicts During Rebase

If Git encounters conflicts while re-applying your commits, the rebase process will pause. You’ll need to resolve the conflicts manually:

1. Open the conflicting files and edit them to resolve the differences.
2. Stage the resolved files: git add <conflicted-file>.
3. Continue the rebase: git rebase --continue.
4. Alternatively, if you want to abort the rebase: git rebase --abort.

Pushing a Rebased Branch

If you have previously pushed your feature branch to a remote repository (e.g., GitLab) and then rebased it locally, Git will prevent you from doing a simple git push. This is because the rebase has rewritten history. You will need to force push:

$ git push origin feature-x --force-with-lease

--force-with-lease is generally safer than --force as it checks if the remote branch has been updated by someone else since your last pull.

Warning: Only force push if you are certain no one else is working on that specific branch or if you have coordinated with your team.

When to Use Rebase

• Updating your local feature branch with the latest changes from the main branch before continuing development.
• Cleaning up local commits (using interactive rebase git rebase -i) before sharing or merging them.
• Before submitting a merge request (pull request) to ensure a clean, linear history that is easier to review.

Understanding rebase is crucial for maintaining a clean and understandable Git history, especially in collaborative projects hosted on platforms like GitLab.