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git rebase is an extremely useful tool that is all about rewriting git history, though most commonly used to squash multiple commits into one. While useful in this scenario, this is not the only function that the git rebase command can perform. It in fact proves to be far more useful when used for the intended function that its name suggests: to essentially rebase a branch. Let me explain what I mean by that.

Let us say that you have a repository such as this:

                                                  --- Commit 5 ----------- auth branch
                                                /               
                                               --- Commit 4 -------------- dev branch 
                                             /
 --- Commit 1 ---- Commit 2 ---- Commit 3 -------------------------------- master branch

If you wanted to merge the auth branch with the dev branch, git will throw an error at you because your auth branch is out-of-date: it doesn't account for Commit 4. You'll have to bring your branch up-to-date.

Git provides you with two methods to do this: the merge command and the rebase command. For an exploration of the merge command, visit the relevant wiki article: https://github.com/FreeCodeCamp/FreeCodeCamp/wiki/git-merge

Let's run rebase now:

git checkout auth
git rebase dev

The repo will now look like this:

                                                                 --- Commit 5 --- auth branch
                                                               /
                                               --- Commit 4 --------------------- dev branch 
                                             /
 --- Commit 1 ---- Commit 2 ---- Commit 3 --------------------------------------- master branch

Do you see what happened? Git essentially saved the commits in the auth branch, 'removed' it, and then created it again with the same commits after the commits in the dev branch. This means that Commit 4 only exists in the dev branch and not the auth branch! And that is really all there is to it! This might seem a bit confusing at first, but try to understand the diagram. This is an extremely useful tool.

git-rebase at FCC

Preventing merge conflicts

If you contribute to the FCC codebase, or are planning to do it, always run this command before you make any changes in your local files and push them:

git pull --rebase upstream staging (thanks, @SaintPeter)

If you don't have upstream set up, then run this command before you run the above command (git will throw an error because it doesn't know what upstream is): git remote add upstream https://github.com/freecodecamp/freecodecamp.git

This will pull the latest changes from the FCC staging branch and rebase them with your fork's staging branch so that you will not have any conflicts when opening the PR :)

Squashing multiple commits into one

This is an awesome feature of rebase that can be used in the interactive mode. To squash the last n commits into one, run the following command:

git rebase -i HEAD~n

That will open up a text-editor with something similar to the following:

pick commit_1
pick commit_2
pick commit_3
...
pick commit_n
# Bunch of comments

Leave the first commit alone, and change the rest of the picks to squash. Save and exit the editor.

If you've already pushed to a remote before squashing your commits, you'll have to push to the remote again, with the -f flag, otherwise git will throw an error at you. Happy coding!