Overview

Teaching: 15 min
Exercises: 0 min
Questions
  • What do I do when my changes conflict with someone else’s?

Objectives
  • Explain what conflicts are and when they can occur.

  • Resolve conflicts resulting from a merge.

As soon as people can work in parallel, it’s likely someone’s going to step on someone else’s toes. This will even happen with a single person: if we are working on a piece of software on both our laptop and a server in the lab, we could make different changes to each copy. Version control helps us manage these conflicts by giving us tools to resolve overlapping changes.

To see how we can resolve conflicts, we must first create one. The file mars.txt currently looks like this in both partners’ copies of our planets repository:

$ cat mars.txt
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity

Let’s add a line to one partner’s copy only:

$ nano mars.txt
$ cat mars.txt
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
This line added to Wolfman's copy

and then push the change to GitHub:

$ git add mars.txt
$ git commit -m "Add a line in our home copy"
[master 5ae9631] Add a line in our home copy
 1 file changed, 1 insertion(+)
$ git push origin master
Counting objects: 5, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 352 bytes, done.
Total 3 (delta 1), reused 0 (delta 0)
To https://github.com/vlad/planets
   29aba7c..dabb4c8  master -> master

Now let’s have the other partner make a different change to their copy without updating from GitHub:

$ nano mars.txt
$ cat mars.txt
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
We added a different line in the other copy

We can commit the change locally:

$ git add mars.txt
$ git commit -m "Add a line in my copy"
[master 07ebc69] Add a line in my copy
 1 file changed, 1 insertion(+)

but Git won’t let us push it to GitHub:

$ git push origin master
To https://github.com/vlad/planets.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to 'https://github.com/vlad/planets.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Merge the remote changes (e.g. 'git pull')
hint: before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

The Conflicting Changes

Git detects that the changes made in one copy overlap with those made in the other and stops us from trampling on our previous work. What we have to do is pull the changes from GitHub, merge them into the copy we’re currently working in, and then push that. Let’s start by pulling:

$ git pull origin master
remote: Counting objects: 5, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 1), reused 3 (delta 1)
Unpacking objects: 100% (3/3), done.
From https://github.com/vlad/planets
 * branch            master     -> FETCH_HEAD
Auto-merging mars.txt
CONFLICT (content): Merge conflict in mars.txt
Automatic merge failed; fix conflicts and then commit the result.

git pull tells us there’s a conflict, and marks that conflict in the affected file:

$ cat mars.txt
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
<<<<<<< HEAD
We added a different line in the other copy
=======
This line added to Wolfman's copy
>>>>>>> dabb4c8c450e8475aee9b14b4383acc99f42af1d

Our change—the one in HEAD—is preceded by <<<<<<<. Git has then inserted ======= as a separator between the conflicting changes and marked the end of the content downloaded from GitHub with >>>>>>>. (The string of letters and digits after that marker identifies the commit we’ve just downloaded.)

It is now up to us to edit this file to remove these markers and reconcile the changes. We can do anything we want: keep the change made in the local repository, keep the change made in the remote repository, write something new to replace both, or get rid of the change entirely. Let’s replace both so that the file looks like this:

$ cat mars.txt
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
We removed the conflict on this line

To finish merging, we add mars.txt to the changes being made by the merge and then commit:

$ git add mars.txt
$ git status
On branch master
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

	modified:   mars.txt

$ git commit -m "Merge changes from GitHub"
[master 2abf2b1] Merge changes from GitHub

Now we can push our changes to GitHub:

$ git push origin master
Counting objects: 10, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 697 bytes, done.
Total 6 (delta 2), reused 0 (delta 0)
To https://github.com/vlad/planets.git
   dabb4c8..2abf2b1  master -> master

Git keeps track of what we’ve merged with what, so we don’t have to fix things by hand again when the collaborator who made the first change pulls again:

$ git pull origin master
remote: Counting objects: 10, done.
remote: Compressing objects: 100% (4/4), done.
remote: Total 6 (delta 2), reused 6 (delta 2)
Unpacking objects: 100% (6/6), done.
From https://github.com/vlad/planets
 * branch            master     -> FETCH_HEAD
Updating dabb4c8..2abf2b1
Fast-forward
 mars.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

We get the merged file:

$ cat mars.txt
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
We removed the conflict on this line

We don’t need to merge again because Git knows someone has already done that.

Git’s ability to resolve conflicts is very useful, but conflict resolution costs time and effort, and can introduce errors if conflicts are not resolved correctly. If you find yourself resolving a lot of conflicts in a project, consider one of these approaches to reducing them:

Solving Conflicts that You Create

Clone the repository created by your instructor. Add a new file to it, and modify an existing file (your instructor will tell you which one). When asked by your instructor, pull her changes from the repository to create a conflict, then resolve it.

Conflicts on Non-textual files

What does Git do when there is a conflict in an image or some other non-textual file that is stored in version control?

Solution

Let’s try it. Suppose Dracula takes a picture of Martian surface and calls it mars.jpg.

If you do not have an image file of Mars available, you can create a dummy binary file like this:

$ head --bytes 1024 /dev/urandom > mars.jpg
$ ls -lh mars.jpg
-rw-r--r-- 1 vlad 57095 1.0K Mar  8 20:24 mars.jpg

ls shows us that this created a 1-kilobyte file. It is full of random bytes read from the special file, /dev/urandom.

Now, suppose Dracula adds mars.jpg to his repository:

$ git add mars.jpg
$ git commit -m "Add picture of Martian surface"
[master 8e4115c] Add picture of Martian surface
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 mars.jpg

Suppose that Wolfman has added a similar picture in the meantime. His is a picture of the Martian sky, but it is also called mars.jpg. When Dracula tries to push, he gets a familiar message:

$ git push origin master
To https://github.com/vlad/planets.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'https://github.com/vlad/planets.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

We’ve learned that we must pull first and resolve any conflicts:

$ git pull origin master

When there is a conflict on an image or other binary file, git prints a message like this:

$ git pull origin master
remote: Counting objects: 3, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://github.com/vlad/planets.git
 * branch            master     -> FETCH_HEAD
   6a67967..439dc8c  master     -> origin/master
warning: Cannot merge binary files: mars.jpg (HEAD vs. 439dc8c08869c342438f6dc4a2b615b05b93c76e)
Auto-merging mars.jpg
CONFLICT (add/add): Merge conflict in mars.jpg
Automatic merge failed; fix conflicts and then commit the result.

The conflict message here is mostly the same as it was for mars.txt, but there is one key additional line:

warning: Cannot merge binary files: mars.jpg (HEAD vs. 439dc8c08869c342438f6dc4a2b615b05b93c76e)

Git cannot automatically insert conflict markers into an image as it does for text files. So, instead of editing the image file, we must check out the version we want to keep. Then we can add and commit this version.

On the key line above, Git has conveniently given us commit identifiers for the two versions of mars.jpg. Our version is HEAD, and Wolfman’s version is 439dc8c0.... If we want to use our version, we can use git checkout:

$ git checkout HEAD mars.jpg
$ git add mars.jpg
$ git commit -m "Use image of surface instead of sky"
[master 21032c3] Use image of surface instead of sky

If instead we want to use Wolfman’s version, we can use git checkout with Wolfman’s commit identifier, 439dc8c0:

$ git checkout 439dc8c0 mars.jpg
$ git add mars.jpg
$ git commit -m "Use image of sky instead of surface"
[master da21b34] Use image of sky instead of surface

We can also keep both images. The catch is that we cannot keep them under the same name. But, we can check out each version in succession and rename it, then add the renamed versions. First, check out each image and rename it:

$ git checkout HEAD mars.jpg
$ git mv mars.jpg mars-surface.jpg
$ git checkout 439dc8c0 mars.jpg
$ mv mars.jpg mars-sky.jpg

Then, remove the old mars.jpg and add the two new files:

$ git rm mars.jpg
$ git add mars-surface.jpg
$ git add mars-sky.jpg
$ git commit -m "Use two images: surface and sky"
[master 94ae08c] Use two images: surface and sky
 2 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 mars-sky.jpg
 rename mars.jpg => mars-surface.jpg (100%)

Now both images of Mars are checked into the repository, and mars.jpg no longer exists.

A Typical Work Session

You sit down at your computer to work on a shared project that is tracked in a remote Git repository. During your work session, you take the following actions, but not in this order:

In what order should you perform these actions to minimize the chances of conflicts? Put the commands above in order in the action column of the table below. When you have the order right, see if you can write the corresponding commands in the command column. A few steps are populated to get you started.

order action . . . . . . . . . . command . . . . . . . . . .
1    
2   echo 100 >> numbers.txt
3    
4    
5    
6 Celebrate! AFK

Solution

order action . . . . . . command . . . . . . . . . . . . . . . . . . .
1 Update local git pull origin master
2 Make changes echo 100 >> numbers.txt
3 Stage changes git add numbers.txt
4 Commit changes git commit -m "Add 100 to numbers.txt"
5 Update remote git push origin master
6 Celebrate! AFK

Key Points