Git-Mastery: Lessons

T6L3. Resolving Merge Conflicts


When merging branches, you need to tell Git how to resolve conflicting changes in different branches.

This lesson covers that part.

A conflict occurs when Git cannot automatically reconcile different changes made to the same part of a file.

A merge conflict happens when Git can't automatically combine branches because both branches changed the same part of a file in different ways. When this happens, Git pauses the merge and marks the conflicting sections in the affected files so you can resolve them yourself. Once you've resolved the conflicts, you can tell Git to continue the merge.

HANDS-ON: Resolve a merge conflict

Scenario In the nouns repo (revision graph shown below), both main and fix1 modify the same location in the same file. main inserts black where fix1 inserts green.

gitGraph BT:
    %%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'main'}} }%%
    commit id: "Add colours.txt"
    branch fix1
    checkout fix1
    commit id: "[fix1] Add green, red, white"
    checkout main
    commit id: "[HEAD → main] Add black, red, white"
colours.txt
blue
black
red
white

[main branch]

colours.txt
blue
green
red
white

[fix1 branch]

Target Merge the two branches and reconcile their conflicting changes.

Preparation

Option 1: Create a fresh sandbox using the Git-Mastery app

  • Navigate inside the gitmastery-exercises folder.
  • Run the gitmastery download hp-merge-conflicts command.

The sandbox will be set up inside the gitmastery-exercises/hp-merge-conflicts folder.

Git-Mastery commands list | Report Git-Mastery bugs | How to update Git-Mastery

Option 2: Manually set up a sandbox

Create a repo with two branches containing conflicting changes, as follows:

  1. Create a repo named nouns with one commit.
  2. Start a branch named fix1 in the repo. Create a commit that adds a line of text to one of the files.
  3. Switch back to main. Create a commit with a conflicting change that adds different text in that exact location.

You can do this with the following commands:

mkdir nouns
cd nouns
git init -b main

echo "blue" > colours.txt
git stage colours.txt
git commit -m "Add colours.txt"

git switch -c fix1
echo -e "green\nred\nwhite" >> colours.txt
git commit -am "Add green, red, white"

git switch main
echo -e "black\nred\nwhite" >> colours.txt
git commit -am "Add black, red, white"

1 Try to merge the fix1 branch into the main branch. Git will pause the merge and report a merge conflict. If you open the conflicted file colours.txt, you will see something like this:

colours.txt
blue
<<<<<<< HEAD
black
=======
green
>>>>>>> fix1
red
white

2 Observe how the conflicted part is marked between a line starting with <<<<<<< and a line starting with >>>>>>>, separated by another line starting with =======.

The yellow highlight below shows the conflicting part from main. The HEAD label on line 2 means this conflicting change comes from the currently active branch, main:

blue
<<<<<<< HEAD
black
=======
green
>>>>>>> fix1
red

Similarly, this is the conflicting part that comes from the fix1 branch:

blue
<<<<<<< HEAD
black
=======
green
>>>>>>> fix1
red

3 Resolve the conflict by editing the file. Assume you want the merged version to keep both lines. Remove the conflict-marker lines and keep black and green:

blue
black
green
red
white

General steps for resolving a conflict:

  1. Remove conflict markers (e.g., <<<<<<< HEAD)
  2. Edit the remaining text as needed (e.g., keep, edit, or delete one or both lines; you can also insert new text).

If there are multiple conflicts (in multiple files or different locations within the same file), resolve them the same way.

4 Stage the changes.

5 Complete the merge by doing one of the following:

  • Option 1: Commit the staged changes (as you normally would).
  • Option 2: Ask Git to resume the merge using the command git merge --continue.

done!

EXERCISE: conflict-mediator