018-git-merge.md

Git Merge

Once the developers are ready with their changes, they can merge their feature branches into the main branch and make those features live on the website.

If you followed the steps from the previous chapter, then your newFeature branch is now ahead of the main branch with 1 commit. So in order to get that new changes over to the main branch, we need to merge the newFeature branch into our main branch.

Merging a branch

You can do that by following these steps:

• First switch to your main branch:

git checkout main

• After that, in order to merge your newFeature branch and the changes that we created in the last chapter, run the following git merge command:

git merge newFeature

Output:

Updating ab1007b..a281d25
Fast-forward
 feature1.html | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 feature1.html

As you were on the main branch when you ran the git merge command, Git will take all of the commits from that branch and merge them into the main branch.

Now, if you run the ls command, you will be able to see the new feature1.html file, and if you check the commit history with the git log command, you will see the commit from the newFeature branch present on your main branch.

Before doing the merge, you could again use the git diff command to check the differences between your current branch and the branch that you want to merge. For example, if you are currently on the main branch, you could use the following:

git diff newFeature

In this case, the merge went through smoothly as there were no merge conflicts. However, if you are working on a real project with multiple people making changes, there might be some merge conflicts. Essentially this happens when changes are made to the same line of a file, or when one developer edits a file on one branch and another developer deletes the same file.

Resolving conflicts

Let's simulate a conflict. To do so, create a new branch:

git checkout -b conflictDemo

Then edit the feature1.html file:

echo "<p>Conflict Demo</p>" >> feature1.html

The command above will echo out the <p>Conflict Demo</p> string, and thanks to the double grater sign >>, the string will be added to the bottom of the feature1.html file. You can check the content of the file with the cat command:

cat feature1.html

Output:

<h1>My First Feature Branch</h1>
<p>Conflict Demo</p>

You can again run git status and git diff to check what exactly has been modified before committing.

After that, go ahead and commit the change:

git commit -am "Conflict Demo 1"

Note that we did not run the git add command, but instead, we used the -a flag, which stands for add. You can do that for files that have been added to git and have just been modified. If you've added a new file, then you would have to stage it first with the git add command.

Now go switch back to your main branch:

git checkout main

And now, if you check the feature1.html file, it will only have the <h1>My First Feature Branch</h1> line as the change that we made is still only present on the conflictDemo branch.

Now let's go ahead and make a change to the same file:

echo "<p>Conflict: change on main branch</p>" >> feature1.html

Now we are adding again a line to the bottom of the feature1.html file with different content.

Go ahead and stage this and commit the change:

git commit -am "Conflict on main"

Now your main branch and the conflictDemo branch have changes to the same file, on the same line. So let's run the git merge command and see what happens:

git merge conflictDemo

Output:

Auto-merging feature1.html
CONFLICT (content): Merge conflict in feature1.html
Automatic merge failed; fix conflicts and then commit the result.

As we can see from the output, the merge is failing as there were changes to the same file on the same line, so Git is unsure which is the correct change.

As always, there are multiple ways to fix conflicts. Here we will go through one.

Now if you were to check the content of the feature1.html file you will see the following output:

<h1>My First Feature Branch</h1>
<<<<<<< HEAD
<p>Conflict: change on main branch</p>
=======
<p>Conflict Demo</p>
>>>>>>> conflictDemo

Initially, it could be a little bit overwhelming, but let's quickly review it:

• <<<<<<< HEAD: this part here indicates the start of the changes on your current branch. In our case, the <p>Conflict: change on main branch</p> line is present on the main branch, which is also the branch that we've currently switched to.
• =======: this line indicates where the changes from the current branch end and where the changes from the new branch are coming from. In our case, the change from the new branch is the <p>Conflict Demo</p> line.
• >>>>>>> conflictDemo: this indicates the name of the branch that the changes are coming from.

You can resolve the conflict by manually removing the lines that are not needed, so at the end, the file will look like this:

<h1>My First Feature Branch</h1>
<p>Conflict: change on main branch</p>

In case that you are using an IDE like VS Code, for example, it will allow you to choose which changes to keep with a click of a button.

After resolving the conflict, you will need to make another commit as the conflict is now resolved:

git commit -am "Resolve merge conflict"

Conclusion

Git branches and merges allow you to work on a project together with other people. One important thing to keep in mind is to make sure that you pull the changes to your local main branch on a regular basis so that it does not get behind the remote one.

A few more commands which you might find useful once you feel comfortable with what we've covered so far are the git rebase command and the git cherry-pick command, which lets you pick which commits from a specific branch you would like to carry over to your current branch.