Ever run into this problem during a git pull?

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

This is a merge conflict; they can be messy to resolve.

What causes merge conflicts?

One common cause is concurrent editing of the same file.

  1. Alice and Bob clone the same repository
  2. Bob makes changes to a file in the repository; add, commit, push
  3. Alice makes a change to the same file at the same line as Bob
  4. Alice adds, commits, and then pushes… which gets an error because her clone is out of date.
$ vi README.md
$ git commit README.md -m "made some changes"
[main 3d2af64] made some changes
 1 file changed, 1 insertion(+)
$ git push origin main
To git@github.com:git@github.com:cs50-2022-fall/demo.git
 ! [rejected]        main -> main (fetch first)
error: failed to push some refs to 'git@github.com:cs50-2022-fall/demo.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.
$

To resolve that error, Alice tries a git pull… which reports the merge conflict:

$ git pull origin main
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 0), reused 3 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), 326 bytes | 163.00 KiB/s, done.
From git@github.com:cs50-2022-fall/demo
   e05f510..7a92782  main       -> origin/main
Auto-merging README.md
CONFLICT (content): Merge conflict in README.md
Automatic merge failed; fix conflicts and then commit the result.
$

Dang! a merge conflict. As a result, git modified the conflicting file to add some ‘markers’:

$ cat README.md 
# demo repo

This is a small repo for demonstrating merge conflicts.

<<<<<<< HEAD
This line was added by Alice, in the local clone.
=======
This line was added by Bob.
>>>>>>> 7a92782aa4bec41785407b0e71651396eb10e941
$

GitHub calls this a competing line change merge conflict.

Alice manually resolves the conflict, by editing the file; she decides to combine them:

$ vi README.md
$ cat README.md 
# demo repo

This is a small repo for demonstrating merge conflicts.

This line was added by Alice, in the local clone, and
This line was added by Bob.
$ 
$ git add README.md
$ git commit -m "resolved README.md merge conflict"
[main 665235f] resolved README.md merge conflict
$ git push origin main
Enumerating objects: 10, done.
Counting objects: 100% (10/10), done.
Delta compression using up to 16 threads
Compressing objects: 100% (4/4), done.
Writing objects: 100% (6/6), 629 bytes | 629.00 KiB/s, done.
Total 6 (delta 1), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (1/1), done.
To github.com:cs50spring2021/demo.git
   7a92782..665235f  main -> main
$

Another kind of merge conflict

What happens if one person removes a file and others on the team don’t know?

  1. Alice makes changes to a file, then git add, and git commit only.
  2. Bob removes that same file in his copy of the repository, and git add, git commit, and git push .
  3. Alice tries git push and gets an error reporting the missing file.

Alice then has to decide whether to keep the file (and her changes). If so, Alice adds it back to git.
For example, if the file is named “file.md” and Alice wanted to keep it, she should

git add file.md
git commit -m "Resolved merge conflict by keeping file.md"
git push origin master

If Alice decides to accept the file’s removal, she has to update her repository:

git rm file.md
git commit -m "Resolved merge conflict by removing file.md"
git push origin master

A far better solution

The best way to handle merge conflicts is to avoid them. We advise you to follow the Git Flow style of development, described in Git flow.