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Version Control with Git

Conflicts

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Overview

Teaching: 15 min
Exercises: 15 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 ingredients.txt currently looks like this in both partners’ copies of our ingredients.txt repository:

$ cat ingredients.txt

2 eggs
salt
2 sausages
butter
pepper
½ tomato

Let’s add a new ingredient to one partner’s copy only:

$ nano ingredients.txt
$ cat ingredients.txt

2 eggs
salt
2 sausages
butter
pepper
½ tomato
Marmite

and then push the change to GitHub:

$ git add ingredients.txt
$ git commit -m "Recommendation from dinner discussion"

[master e3814e4] Recommendation from dinner discussion
 1 file changed, 1 insertion(+)

$ git push

Counting objects: 5, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 342 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To https://github.com/breakfastmaster/breakfast.git
   7fb6b7e..e3814e4  master -> master

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

$ nano ingredients.txt
$ cat ingredients.txt

2 eggs
salt
2 sausages
butter
pepper
½ tomato
plenty of bacon

We can commit the change locally:

$ git add ingredients.txt
$ git commit -m "Add bacon"

[master 07ebc69] Add bacon
 1 file changed, 5 insertion(+)

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

$ git push

To https://github.com/breakfastmaster/breakfast.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'https://github.com/breakfastmaster/breakfast.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.

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

remote: Counting objects: 3, done.
remote: Compressing objects: 100% (1/1), done.
remote: Total 3 (delta 2), reused 3 (delta 2), pack-reused 0
Unpacking objects: 100% (3/3), done.
From https://github.com/breakfastmaster/breakfast
   12d2b5e..5fa9f4b  master     -> origin/master
Auto-merging ingredients.txt
CONFLICT (content): Merge conflict in ingredients.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 ingredients.txt

2 eggs
salt
2 sausages
butter
pepper
½ tomato
<<<<<<< HEAD
Marmite
=======
plenty of bacon
>>>>>>> 5fa9f4bbc64cfea4231f462afb1147b8e4703aaa

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.

$ nano ingredients.txt
$ cat ingredients.txt

2 eggs
salt
2 sausages
butter
pepper
½ tomato
plenty of bacon

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

$ git add ingredients.txt
$ git status

On branch master
Your branch and 'origin/master' have diverged,
and have 1 and 1 different commits each, respectively.
  (use "git pull" to merge the remote branch into yours)
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

	modified:   ingredients.txt

$ git commit -m "Marmite has no place in breakfast"

[master 2abf2b1] Merging changes from GitHub

Now we can push our changes to GitHub:

$ git push

Counting objects: 6, done.
Delta compression using up to 24 threads.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 591 bytes | 0 bytes/s, done.
Total 6 (delta 4), reused 0 (delta 0)
remote: Resolving deltas: 100% (4/4), completed with 2 local objects.
To https://github.com/breakfastmaster/breakfast.git
   5fa9f4b..3fec3a2  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

remote: Counting objects: 6, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 6 (delta 4), reused 6 (delta 4), pack-reused 0
Unpacking objects: 100% (6/6), done.
From https://github.com/breakfastmaster/breakfast
   5fa9f4b..3fec3a2  master     -> origin/master
Updating 5fa9f4b..3fec3a2
Fast-forward
 ingredients.txt | 6 ++++++
 1 file changed, 6 insertions(+)

We get the edited file:

$ cat ingredients

2 eggs
salt
2 sausages
butter
pepper
½ tomato
plenty of bacon

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

Version control’s ability to merge conflicting changes is another reason users tend to divide their programs and papers into multiple files instead of storing everything in one large file. There’s another benefit too: whenever there are repeated conflicts in a particular file, the version control system is essentially trying to tell its users that they ought to clarify who’s responsible for what, or find a way to divide the work up differently.

Solving Conflicts that You Create

Each partner edit methods.txt to create another conflict. Use the same steps as above to resolve the conflict

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?

Hint

Try it! Commit a small binary image file (jpg or png) and create a conflict

Solution

You must choose one version or the other, because it’s impractical to edit the actual binary differences

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
2 Make changes echo 100 >> numbers.txt
3 Stage changes git add numbers.txt
4 Commit changes git commit -m "add 100"
5 Update remote git push
6 Celebrate! AFK

Key Points

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