Merging and conflicts

At this stage we have two branches, master and risky_idea. Let’s imagine that we have continued to work away on the risky_idea branch, committing our changes as we go...

Merging

At some stage we will want fold our changes in the risky_idea branch back into the master branch. We do this by “merging” the risky_idea branch into master.

First we checkout the master branch:

% git checkout master

If you run git lg you should see that none of your commits to the risky_idea branch are present. You can further confirm this by looking at the contents of paper.tex; The section “The Empire Strikes Back” shouldn’t be present.

Now merge risky_idea into our current branch using the following command:

% git merge risky_idea

If everything runs smoothly, running git lg should show your commits from the risky_idea branch.

At this stage you could either checkout the risky_idea branch again and continue working, or if your finished with it you can delete it.

We no longer need the risky_idea branch, so delete it using:

% git branch -d risky_idea

Dealing with conflicts

Typically, as above, a git merge will progress smoothly with Git automatically working out how to merge the two branches. Occasionally however, this is not the case. In particular if we have made changes to two different branches which directly conflict with each other, then a merge will require us to tell Git which change is the correct one. We will now engineer such a situation...

First create a new branch called episode5 and check it out:

% git branch episode5
% git checkout episode5

Tip

In situations where you want to create a new branch and immediately check it out (as above) you can use the following shortcut:

% git checkout -b <branch_name>

Then add another section to paper.tex with the following:

\section{Revenge of the Jedi}
That blast came from the Death Star! That thing's operational!

Commit your changes:

% git add paper.tex
% git commit

Now we have a new branch with a new commit that adds a section to our paper. However, imagine the situation where we decide we want to leave this section for the moment and go back to working on our second section. To do this, we return to our master branch. During the course of our edits we come up with another name for our newest section though, and pen this in so that we don’t forget. This will lead to a conflict when we later merge our episode5 branch back into master. Let’s replicate this conflict now to see what happens...

First checkout master:

% git checkout master

Then edit paper.tex, this time with the text:

\section{Return of the Jedi}
That blast came from the Death Star! That thing's operational!

Again, stage and commit your changes:

% git commit -a

Note

Note that we used git commit -a here to stage and commit our changes in one go. This is a very useful shortcut. However, it will only stage changes in files which are already being tracked by the repository. i.e. if you add a new file to your project, you will still need to run git add for Git to start tracking it. Additionally, it will stage all changes, so you have less control over what changes go into each commit.

Now our two branches master and episode5 have commits in them which directly conflict. Running the merge command from the master branch will flag this conflict and Git will ask us for help. Try it now:

% git merge episode5

and you should be presented with the following message:

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

This tells us that a conflict has occurred in paper.tex.

To resolve the conflict open up paper.tex in your favorite editor. The offending section will look something like this:

<<<<<<< HEAD
\section{Return of the Jedi}
=======
\section{Revenge of the Jedi}
>>>>>>> episode5

Everything between the lines <<<<<<< HEAD and ======= are what exists in the HEAD commit (the tip of the master branch in this case). Between the lines ======= and >>>>>>> episode5 is what exists in our episode5 branch.

In order to resolve the conflict, pick which of the section headings we want to use and remove the other lines (including the ======= line and those lines starting with > or < symbols. In our case we want to keep the section title from the master branch, and so we need to leave only that line:

\section{Return of the Jedi}

After you have edited and saved paper.tex, finish the merge by staging and committing your results:

% git commit -a

The commit message will be auto-populated for you, and so there is no need to edit it.

Command summary

Command	Description
git merge	Merge branches and commits.
git branch -d	Delete a branch.
git commit -a	Stage all changes in tracked files and commit them.