Merge and handling conflicts

Utilize common version control operations with Git

📘Cisco DevNet Associate (200-901 DEVASC)

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1. What is a Git Merge?

In Git, a merge is the process of combining changes from one branch into another.

• Think of branches as parallel lines of development in your code repository.
• When you finish work on one branch, you often want to integrate it back into the main branch (commonly called main or master).

Command to merge:

git checkout main   # Switch to the branch you want to merge into
git merge feature-branch   # Merge changes from feature-branch into main

After this, Git tries to automatically combine the changes.

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2. Types of Merges

1. Fast-forward merge
   • Happens when the branch you are merging has all commits ahead of your current branch.
   • Git simply moves the pointer forward; no new commit is created.
   • Example: git merge feature-branch
   • Exam tip: Fast-forward merge occurs only when there’s a straight line of commits with no divergence.
2. Three-way merge
   • Happens when both branches have new commits after they split.
   • Git uses the common ancestor of both branches to combine changes.
   • If there are conflicting changes, you may need to resolve conflicts manually.
   • Exam tip: This is the more common type of merge in team projects.

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3. Merge Conflicts

A merge conflict happens when Git cannot automatically combine changes.

• Common reasons:
  1. Two branches modified the same line of a file.
  2. One branch deleted a file that the other branch edited.

Example scenario:

• main branch: Line 10 says API_KEY = "123"
• feature branch: Line 10 says API_KEY = "456"
• When merged, Git doesn’t know which version to keep, causing a conflict.

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4. How to Identify Conflicts

After a merge, if there is a conflict, Git will:

• Stop the merge process.
• Mark the conflicting files with conflict markers:

<<<<<<< HEAD
API_KEY = "123"
=======
API_KEY = "456"
>>>>>>> feature-branch

• HEAD shows the current branch (main), while the other part shows the branch you are merging (feature-branch).

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5. How to Resolve Conflicts

Steps to resolve a conflict:

1. Open the conflicting file in a code editor.
2. Decide which changes to keep:
   • Keep changes from main.
   • Keep changes from feature-branch.
   • Or combine both.
3. Remove conflict markers (<<<<<<<, =======, >>>>>>>) after editing.
4. Mark the file as resolved:

git add <file>

5. Complete the merge:

git commit

• Git will auto-generate a merge commit message, or you can edit it.

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6. Useful Git Commands for Merges and Conflicts

Command	Purpose
git merge <branch>	Merge another branch into the current branch
git status	Shows which files have conflicts
git log --graph --oneline	Visualizes the branch structure and merges
git diff	Shows changes between branches or commits
git checkout --ours <file>	Keep current branch version during conflict
git checkout --theirs <file>	Keep incoming branch version during conflict
git merge --abort	Abort the merge and go back to previous state

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7. Tips for Avoiding Conflicts

While conflicts are normal in team environments, you can reduce them:

1. Pull updates often: git pull origin main
2. Work on small, focused branches (smaller changes = fewer conflicts).
3. Communicate with team members about changes in shared files.
4. Use code review tools (like GitHub Pull Requests) to catch conflicts before merging.

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8. Summary for Exam

• Merge = Combining changes from one branch into another.
• Fast-forward merge = Simple pointer move when no divergence.
• Three-way merge = Combines changes from branches with different commits.
• Merge conflict = Happens when Git cannot auto-merge changes.
• Resolution steps: Edit conflicting files → remove markers → git add → git commit.
• Key commands: git merge, git status, git diff, git checkout --ours/--theirs, git merge --abort.

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💡 Exam Tip: You might get questions like:

• What happens if two branches modify the same line of code? → Merge conflict.
• Which Git command resolves a conflict? → git add <file> after editing, then git commit.
• When does a fast-forward merge occur? → When the current branch has no new commits since branching.