How to resolve merge conflicts?

A merge conflict can occur when we try to merge one branch into another, if the two branches introduce any conflicting changes.

For example, consider trying to merge two branches that make the following changes to the same line of the file test.txt:

1. On the branch my-new-branch:

    First line
   -Second line
   +My new second line
    Third line
   

2. On the main branch:

    First line
   -Second line
   +A different second line
    Third line
   

When we attempt to merge my-new-branch into the main branch, git merge my-new-branch will tell us:

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

The test.txt file will now include the conflicting changes, which we can inspect with git diff:

diff --cc test.txt
index 18712c4,bc576a6..0000000
--- a/test.txt
+++ b/test.txt
@@@ -1,3 -1,3 +1,7 @@@
  First line
++<<<<<<< ours
 +A different second line
++=======
+ My new second line
++>>>>>>> theirs
  Third line

Note that this two-day diff shows:

1. "our" changes: from the commits on the branch that we are merging into; and
2. "their" changes: from the commits on the branch that we are merging from.

Each conflict is surrounded by <<<<<<< and >>>>>>> markers, and the conflicting changes are separated by a ======= marker.

If we instruct Git to use a three-way diff (see first-time Git setup), the conflict will be reported slightly differently:

diff --cc test.txt
index 18712c4,bc576a6..0000000
--- a/test.txt
+++ b/test.txt
@@@ -1,3 -1,3 +1,7 @@@
  First line
++<<<<<<< ours
 +A different second line
++||||||| base
++Second line
++=======
+ My new second line
++>>>>>>> theirs
  Third line

In addition to showing "our" changes and "their changes", this three-way diff also shows the original lines, between the ||||||| and ======= markers. This extra information can help you decide how to best resolve the conflict.

Resolving the conflicts

We can edit test.txt to reconcile these changes, and the commit our fix. For example, we might decide that test.txt should have the following contents:

First line
The corrected second line
Third line

We can then commit these changes to resolve the merge conflict:

git add test.txt
git commit -m "Resolved the merge conflict"

Cancelling the merge

Alternatively, you may decide you don't want to merge these two branches, in which case you cancel the merge by running:

git merge --abort