Git Merge Conflicts

Learning Goals

• Demonstrate steps for merge conflict resolution
• Execute resolving a merge conflict

Important Update

In order to complete this lab, please be sure to clone it from the following source:

https://github.com/learn-co-curriculum/git-workflow-merge-conflicts-lab

This is required because our teaching system works to make sure we don't deploy "broken git repositories" to our learners. But we're trying to teach you to work with... broken git repositories 😂. Cloning from the URL above, into a new directory is the best way to complete this lab.

Introduction

We've discussed how merge conflicts happen and how we can solve them. Let's work through a scenario using characters from Back to the Future to resolve some issues with conflicting Git histories!

Marty McFly and Doc Brown just finished their student profiles for Flatiron School. Now they need to merge their profiles into the master branch so that they'll have a completed profile page.

This is what we will be working towards:

Demonstrate Steps for Merge Conflict Resolution

On the master branch, there is a placeholder profile in place. The index.html page on that branch looks like this:

Doc's finished profile is in the doc-brown branch. The index page on his branch looks like this:

Meanwhile, Marty's finished profile is stored in the marty-mcfly branch. His index page on his branch looks like this:

You are going to merge both branches onto the master branch and resolve the merge conflicts.

To accomplish this, you're going to be following six steps, listed below:

1. Make sure you have all three branches
2. Switch to the master branch
3. Merge Doc's branch into the master branch. This will merge "cleanly"
4. Merge Marty's branch in. This will not merge cleanly due to a "merge conflict"
5. Fix the merge conflict
6. Delete Doc and Marty's branches on your computer
7. Verify that Doc and Marty's branches have been integrated to your local master branch.

Step 1: Confirm You Have Both Branches

Remember to fork then clone down this repo. Then change directories into it using:

• cd git-merge-conflicts-< your cohort identifier >

The first step is to see how many branches you have locally. Run git branch from your terminal to see all of the branches. The output should look like this:

$ git branch
* master

To fetch the doc-brown or marty-mcfly remote branches, run the following commands in order:

• git checkout -t origin/doc-brown
• git checkout -t origin/marty-mcfly

This creates a local tracking-branch on your computer that matches the doc-brown and marty-mcfly branches on GitHub. Let's verify this by re-running the git branch command. The output should look like this:

$ git branch
  doc-brown
* marty-mcfly
  master

If you don't have all three branches, get help.

As you can see, the marty-mcfly branch is starred and highlighted. This is Git's way of telling you which branch you're on. Git "put" us "on" this branch when we issued that last checkout command. Therefore, you're on the marty-mcfly branch. Since we want to merge into master we need to "get" back "on" it.

Step 2: Navigate Into The master Branch

Remember, checkout allows you to switch between branches that are on your local machine. It's time to check out the master branch:

• git checkout master

You should now be in the master branch. Remember, you can confirm you're on the master branch if it's starred and highlighted when you run git branch:

$ git branch
  doc-brown
  marty-mcfly
* master

From the master branch, if you are using the in-browser IDE, you can open the index page and take a look by running httpserver (or, if you are using a local environment, open the file by running open index.html). You should see a web page with just a placeholder avatar. Marty and Doc should not be there.

Step 3: Merge!

You're going to add both the doc-brown branch and the marty-mcfly branch to the master branch using git merge. Merge the doc-brown branch first by running:

git merge doc-brown -m "merge doc brown"

Here, we're saying: "Integrate the differences between master and doc-brown back into master."

When you merge doc-brown into your master branch, your terminal should print a readout that looks something like this:

Updating 7d220f6..bb73c64
Fast-forward
 img/students/doc_brown_index_profile.jpg    | Bin 0 -> 32589 bytes
 img/students/student_name_background.jpg    | Bin 72485 -> 0 bytes
 img/students/student_name_index_profile.jpg | Bin 17565 -> 0 bytes
 img/students/student_name_profile.jpg       | Bin 12632 -> 0 bytes
 index.html                                  |  23 ++++++++++++++++++++++-
 5 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 img/students/doc_brown_index_profile.jpg
 delete mode 100644 img/students/student_name_background.jpg
 delete mode 100644 img/students/student_name_index_profile.jpg
 delete mode 100644 img/students/student_name_profile.jpg

This readout confirms that you've merged all of Doc Brown's profile information into the master branch. Take a look at the index page by again running open index.html in the terminal. It's important to keep looking at index.html to make sure that it looks exactly how you want it to look.

The index.html page should look like this:

Now try merging in Marty McFly's profile information into the master branch. You probably already are, but ensure that you are currently on your master branch (type git branch). Then run:

git merge marty-mcfly -m "Merge marty mcfly"

Here, we're saying: "Integrate the differences between master and marty-mcfly back into master."

Auto-merging index.html
CONFLICT (content): Merge conflict in index.html
Automatic merge failed; fix conflicts and then commit the result.

Yikes! Now we have a merge conflict! Why did this happen? Well, both Doc Brown and Marty McFly took master at the same moment in the Git history. This idea has become commonplace in sci-fi films but they "branched" reality. They started "two" timelines from that point. In those separate "timelines," they changed the same thing. We merged in the changes from Doc Brown's timeline to master but now we're trying to merge in Marty McFly's same changes from his timeline. Git needs us to help it decide how to integrate these two timelines. That's what all merge conflicts are: Git telling us it's not sure how to move forward and it needs our help! Nobody said reweaving the fabric of time would be easy!

This is what index.html should look like with the merge conflict:

Step 4: Fix the Conflicts

Open up the index.html file. Scroll down to around line 114 and 137. You should see something that looks like this:

<<<<<<< HEAD
  <!-- Begin Profile -->
  <li class="home-blog-post">
    <div class="blog-thumb">
      <img width="304" height="304" class="prof-image" src="img/students/doc_brown_index_profile.jpg" class="attachment-blog-thumb wp-post-image" alt="doc brown">
    </div>

   <div class="blog-title">
      <div class="big-comment">
        <h3>Doc Brown</h3>
      </div>
      <p class="home-blog-post-meta">"Great Scott!"</p>
    </div>
    <div class="clear"></div>

    <div class="excerpt">
      <p>Doctor Emmett Lathrop "Doc" Brown was the inventor of the DeLorean time machine. Doc's role models were scientists, as evidenced by the names of his dogs and the portraits of Isaac Newton and Albert Einstein found inside his laboratory.</p>
    </div>
    <div class="clear"></div>
  </li>
  <!-- End Profile -->
=======
>>>>>>> marty-mcfly
... (MORE CODE) ...

Identify the Commits to Merge

Remember, Git does its best to merge the code, but sometimes it just doesn't work. You need to complete the merge yourself by manually adjusting the code. Git gives you a few hints to help us out:

• <<<<<<< HEAD - the beginning of the original branch (master)
• ======= - the end of the original branch/the beginning of the branch being merged in (marty-mcfly)
• >>>>>>> marty-mcfly - the end of the new branch ( marty-mcfly)

Take your time and shift the code around, separating the MARTY MCFLY and DOC BROWN code blocks. Use the markers from git as a guide.

Hint: You can also use the HTML tags as guides. If one section ends with an opening <a> tag, look for the closing </a> tag in the next section.

When you're done the code should look something like this:

<!-- Begin MARTY MCFLY -->
<li class="home-blog-post">
 <div class="blog-thumb">
   <a href="students/marty_mcfly.html">
     <img width="304" height="304" class="prof-image" src="img/students/marty_mcfly_index_profile.jpg" class="attachment-blog-thumb wp-post-image" alt="doc brown">
   </a>
 </div>

... (MORE CODE) ...

</li>
<!-- End MARTY MCFLY -->

<!-- Begin DOC BROWN -->
<li class="home-blog-post">
 <div class="blog-thumb">
   <a href="students/doc_brown.html">
     <img width="304" height="304" class="prof-image" src="img/students/doc_brown_index_profile.jpg" class="attachment-blog-thumb wp-post-image" alt="doc brown">
   </a>
 </div>

... (MORE CODE) ...

</li>
<!-- End DOC BROWN -->

Open the page with httpserver (or open index.html). The page should look like the picture at the very top of this readme.

If everything is looking good, we're ready to commit the changes before moving on.

• run git add . to stage all changes made in index.html
• run git commit -am "merge marty and doc index pages" to commit, or finalize, these changes

Step 5: Confirm the Changes Are Correct

Almost done! The next and last step is to confirm that the master branch has everything we need.

Confirm that index.html in the master branch has the following for both Doc Brown and Marty McFly:

• profile images
• profile names
• descriptions

Once you have that, make sure you're still on the master branch. Now delete the doc-brown and marty-mcfly branches:

• run git branch -D doc-brown to delete the doc-brown branch
• run git branch -D marty-mcfly to delete, you guessed it, the marty-mcfly branch

That's it! Open up index.html in your browser to see your beautiful work!

Step 6: Wrap Up

Remember, while your computer has these updates, GitHub has no idea that you made them. These are all local.

Typically, the next step would be to create a local branch, push that branch to the remote, and then create a pull request to merge your branch into the remote master branch.

In this lab, you do not need to create a pull request.

Conclusion

Congrats on fixing your first merge conflict! This is a topic that takes a while to get a hang of. As you edit more text you'll learn to be better at understanding why Git is asking you for help. It's an intermediate-level Git skill, so if it's not easy right now, that's OK. The important thing to understand is that developers can split timelines and decide to edit the same material so that Git needs our help to integrate things during merges.

Resources

• Git Branching - Basic Branching and Merging
• Stack Overflow - Best (and safest) way to merge a git branch into master