A Detailed Guide to Git Merging and Conflict Resolution
A merge is the process of combining changes from different branches. Through merging, teams can develop different features in parallel and then integrate them into a single version. Understanding the principles of merging and conflict resolution is crucial for team collaboration.
1. Basic Concepts of Mergers
(1) What Is a Merger?
A merge is the process of applying changes from one branch to another. Git attempts to merge the changes automatically, but conflicts may arise when changes on the two branches affect the same location.
graph TB
A[mainBranch] --> B[MergefeatureBranch]
B --> C{Is there a conflict??}
C -->|No conflict| D[Automatic merge successful]
C -->|There is a conflict| E[Resolve conflicts manually]
E --> F[Mark the conflict as resolved]
F --> G[Complete the merger]
style D fill:#d4edda
style E fill:#fff3cd
(2) Types of Mergers
Git supports two types of merges:
- Fast-forward merge: If there are no new commits on the target branch, the pointer is moved directly
- Three-way merge: Both branches have new commits; create a merge commit
(3) Direction of the Merge
TEXT
git merge feature # Merge feature into the current branch
A merge always merges the specified branch into the current branch, so be sure to check the current branch before merging.
2. Fast-forward merge
(1) The Principle of Fast-Forward Merging
When the target branch (such as main) has no new commits, but the branch to be merged (such as feature) has new commits, Git can simply move the main pointer forward; this is called a fast-forward merge.
graph LR
subgraph Before the merger
C1[SubmitC1] --> C2[SubmitC2]
C2 --> C3[SubmitC3]
main1[main] --> C2
feature1[feature] --> C3
end
subgraph After the merger
C4[SubmitC1] --> C5[SubmitC2]
C5 --> C6[SubmitC3]
main2[main] --> C6
feature2[feature] --> C6
end
style main1 fill:#fff3cd
style main2 fill:#d4edda
(2) Perform a fast-forward merge
▶ Example: Fast-forward merge
BASH
# View Current Status
git log --oneline --graph --all
# Output：
# * d4e5f6g (feature) Add feature
# * a1b2c3d (HEAD -> main) Initial commit
# Switch tomainBranch
git switch main
# MergefeatureBranch
git merge feature
# Output：
# Updating a1b2c3d..d4e5f6g
# Fast-forward
# feature.js | 1 +
# 1 file changed, 1 insertion(+)
# create mode 100644 feature.js
# View the merge results
git log --oneline --graph --all
# Output：
# * d4e5f6g (HEAD -> main, feature) Add feature
# * a1b2c3d Initial commit
(3) Disable fast-forward merge
Sometimes, even when fast-forward merges are possible, I want to create a merge commit to preserve the branch history.
▶ Example: Disabling Fast-Forward Merge
BASH
# Usage--no-ffDisable Fast-Forward Merge
git merge --no-ff feature
# Output：
# Merge made by the 'ort' recursive strategy.
# feature.js | 1 +
# 1 file changed, 1 insertion(+)
# create mode 100644 feature.js
# View History
git log --oneline --graph --all
# Output：
# * a1b2c3d (HEAD -> main) Merge branch 'feature'
# |\
# | * d4e5f6g (feature) Add feature
# |/
# * h7i8j9k Initial commit
3. Three-Party Merger
(1) The Principle of Three-Way Merging
When both branches have new commits, Git needs to find a common ancestor (the merge base) and then perform a three-way merge.
graph TB
A[Common Ancestor<br/>Merged Base] --> B[mainBranch Commit]
A --> C[featureBranch Commit]
B --> D[Three-Party Merger]
C --> D
D --> E[Merge Commit]
style A fill:#fff3cd
style E fill:#d4edda
(2) Execute a three-party merger
▶ Example: Three-Party Merger
BASH
# View Branch Status
git log --oneline --graph --all
# Output：
# * d4e5f6g (feature) Feature work
# | * a1b2c3d (HEAD -> main) Main work
# |/
# * h7i8j9k Initial commit
# MergefeatureBranch
git merge feature
# Output：
# Merge made by the 'ort' recursive strategy.
# feature.js | 1 +
# 1 file changed, 1 insertion(+)
# View the merge results
git log --oneline --graph --all
# Output：
# * k9l0m1n (HEAD -> main) Merge branch 'feature'
# |\
# | * d4e5f6g (feature) Feature work
# * | a1b2c3d Main work
# |/
# * h7i8j9k Initial commit
(3) View the merge base
▶ Example: Finding a Common Ancestor
BASH
# Find the common ancestor of two branches
git merge-base main feature
# Output：
# h7i8j9k
# View the content to be merged
git diff $(git merge-base main feature) feature
# Or use the three-point syntax
git diff main...feature
4. Resolving Merge Conflicts
(1) The Origins of Conflict
When two branches modify the same location in the same file, Git cannot automatically merge them, resulting in a conflict.
▶ Example: Conflict
BASH
# Modifying Files on a Branch
echo "main content" > file.js
git add file.js
git commit -m "Main modification"
# Switch tofeatureBranch
git switch feature
# Modify the same location in the same file
echo "feature content" > file.js
git add file.js
git commit -m "Feature modification"
# Switch back tomainCombine and Merge
git switch main
git merge feature
# Output：
# Auto-merging file.js
# CONFLICT (content): Merge conflict in file.js
# Automatic merge failed; fix conflicts and then commit the result.
(2) View Conflict Status
▶ Example: Analyzing Conflicts
BASH
# View Status
git status
# Output：
# On branch main
# You have unmerged paths.
# (fix conflicts and run "git commit")
#
# Unmerged paths:
# (use "git add <file>..." to mark resolution)
# both modified: file.js
# View Conflicting Files
cat file.js
# Output：
# <<<<<<< HEAD
# main content
# =======
# feature content
# >>>>>>> feature
# View Conflict Details
git diff
# Output：
# <<<<<<< HEAD
# -main content
# =======
# -feature content
# >>>>>>> feature
(3) Resolving Conflicts
▶ Example: Resolving Conflicts Manually
BASH
# Editing Files with Conflicts，Select the content to keep
cat file.js
# Method1：RetainmainChanges to
# main content
# Method2：RetainfeatureChanges to
# feature content
# Method3：Merge the changes from both
# main content
# feature content
# Mark the conflict as resolved
git add file.js
# Complete the merger
git commit
# Output：
# [main d4e5f6g] Merge branch 'feature'
(4) Using Tools to Resolve Conflicts
▶ Example: Using the Merge Tool
BASH
# Using a Graphical Merge Tool
git mergetool
# Using Specific Tools
git mergetool --tool=vimdiff
# View available merge tools
git mergetool --tool-help
# UsagecheckoutSelect a Version
git checkout --ours file.js # Select the current branch version
git checkout --theirs file.js # Select a branch version to merge
(5) Detailed Explanation of Conflict Markers
TEXT
<<<<<<< HEAD
Content of the current branch（HEAD）
=======
To merge the contents of the branches
>>>>>>> feature
<<<<<<< HEAD: Start of the current branch's content=======: Separator; the current branch is above it, and the branch to be merged is below it>>>>>>> feature: End of content to be merged from the branch
5. Merger Strategy
(1) Merge Strategy Options
Git supports a variety of merge strategies:
| Strategy | Description | Use Case |
|---|---|---|
| recursive (default) | Recursive three-way merge | Most cases |
| octopus | Merging multiple branches | Automatically used when merging multiple branches |
| ours | Ignore other branches | Keep the current branch |
| theirs | Ignore the current branch | Keep the branch to be merged |
| resolve | Simple three-way merge | Special cases |
▶ Example: Using a Merge Strategy
BASH
# UsagerecursiveStrategy（Default）
git merge feature
# UsageoursStrategy（IgnorefeatureChanges to）
git merge -s ours feature
# UsagerecursiveStrategy Options
git merge -X ours feature # In case of a conflict, prioritize the current branch
git merge -X theirs feature # In the event of a conflict, prioritize the branch to be merged
git merge -X ignore-space-change feature # Ignore changes in whitespace
(2) Compression and Merging
Compress multiple commits into a single commit and then merge them.
▶ Example: Compress and Merge
BASH
# Compress and Merge
git merge --squash feature
# View Status
git status
# Output：
# Changes to be committed:
# new file: feature1.js
# new file: feature2.js
# Commit the compressed changes
git commit -m "Merge feature branch (squashed)"
# Advantages：Keep the main branch history clean
# Disadvantages：LostfeatureDetailed History of the Branch
(3) Merge only, do not commit
▶ Example: Preview Merge
BASH
# Merge but do not commit automatically
git merge --no-commit feature
# View the merge results
git status
git diff --staged
# If you're satisfied，Manual Submission
git commit -m "Merge feature branch"
# If you are not satisfied，Cancel Merge
git merge --abort
6. Merging Operations
(1) Suspension of the Merger
▶ Example: Canceling an Ongoing Merge
BASH
# Conflicts Arising from Mergers
git merge feature
# CONFLICT (content): Merge conflict in file.js
# Cancel Merge，Restore to the state prior to the merge
git merge --abort
# Or usereset
git reset --merge
(2) View Merge History
▶ Example: View a merged commit
BASH
# View All Merge Commits
git log --merges --oneline
# Output：
# d4e5f6g Merge branch 'feature'
# a1b2c3d Merge branch 'develop'
# View Unmerged Commits
git log --no-merges --oneline
# View details of the merge
git show --stat d4e5f6g
(3) Revoke the Merger
▶ Example: Undoing a Completed Merge
BASH
# Undo the Last Merge（Merge requests are still available）
git reset --hard HEAD~1
# UsagerevertRevoke Merger（Create a New Commit）
git revert -m 1 HEAD
# -m 1 Indicates that the first parent commit is being preserved（mainBranch）
# View Results
git log --oneline --graph
❓ FAQ
Q What is the difference between a fast-forward merge and a three-way merge?
A A fast-forward merge is performed when the target branch has no new commits; it simply moves the pointer without creating a merge commit. A three-way merge is performed when both branches have new commits; it finds a common ancestor to merge and creates a merge commit.
Q How do I resolve merge conflicts?
A Edit the file with conflicts, remove the conflict markers (<<<<<<<, =======, >>>>>>>), keep the content you need, then use
git add to mark the conflict as resolved, and finally use git commit to complete the merge.Q When should I use the --no-ff option?
A Use this when you want to preserve the branch history and clearly indicate that this is a merge operation. It is typically used when merging a feature branch into the main branch so that the merge point is visible in the history.
Q How do I cancel a merge that is in progress?
A Use
git merge --abort to cancel the merge and revert to the pre-merge state. If you have already resolved some of the conflicts, you can also use git reset --merge to reset.Q What is the difference between a squash merge and a regular merge?
A A squash merge compresses all commits on a branch into a single commit, without preserving the branch's detailed history. The advantage is that the main branch history remains concise; the disadvantage is that the branch's commit history and contextual information are lost.
📖 Summary
- A merge is the process of integrating changes from one branch into another.
- Fast-forward merge: If there are no new commits on the target branch, simply move the pointer
- Three-way merge: Both branches have new commits; create a merge commit
- Conflict Resolution: Edit the file with conflicts, remove the markers, and keep the content you need
- Merge strategies: recursive (default), ours, theirs, squash, etc.
- Merge operations: --abort to cancel, --no-commit to preview, revert to undo
📝 Exercises
-
Basic Exercise: Create two branches, modify different files on each branch, and then merge them to experience the difference between a fast-forward merge and a three-way merge.
-
Advanced Exercise: Simulate a conflict scenario: Make changes to the same location in the same file on two branches; when merging, a conflict occurs; manually resolve the conflict and complete the merge.
-
Challenge: Merge branches using different merge strategies (--no-ff, --squash, -X ours, etc.), and compare the results and appropriate use cases for each strategy.
