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refactor: simplify 01_essentials/04-merging

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Bjarke Sporring
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01_essentials/04-merging/README.md
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In this module, you will: In this module, you will:
- Understand what merging means in Git - Understand what merging means in Git
- Merge divergent branches together - Merge a feature branch back into main
- Understand merge commits and their purpose
- Use `git merge` to combine branches - Use `git merge` to combine branches
- Visualize merge history with `git log --graph` - Visualize merged branches with `git log --graph`
## Challenge ## Challenge
@@ -19,121 +18,97 @@ Run the setup script to create your challenge environment:
.\setup.ps1 .\setup.ps1
``` ```
This will create a `challenge/` directory with a Git repository that has two divergent branches ready to merge. This will create a `challenge/` directory with a Git repository that has a main branch and a feature branch.
### Your Task ### Your Task
You have two branches that have diverged - both `main` and `feature-login` have new commits since they split. Your task is to merge them back together. You've been working on a new login feature in a separate branch. Now it's time to merge your work back into the main branch!
**Scenario:** **Scenario:**
- You're working on a team project
- You created a `feature-login` branch to add login functionality - You created a `feature-login` branch to add login functionality
- While you were working, your teammate added documentation to `main` - You made some commits on that branch
- Now you need to merge your login feature back into main - Meanwhile, your teammate updated the README on main
- Now you need to bring your login feature back into main
**Steps:** **Steps:**
1. Navigate to the challenge directory: `cd challenge` 1. Navigate to the challenge directory: `cd challenge`
2. Check current branch: `git branch` (should be on main) 2. Check which branch you're on: `git branch`
3. View the branch structure: `git log --oneline --graph --all` 3. View all branches: `git log --oneline --graph --all`
4. Merge the feature-login branch: `git merge feature-login` 4. Merge the feature-login branch into main: `git merge feature-login`
5. View the merge result: `git log --oneline --graph --all` 5. View the result: `git log --oneline --graph --all`
6. Examine the merge commit: `git show HEAD`
> **Important Notes:** > **That's it!** Merging is how you bring work from one branch into another.
> - When both branches have new commits, Git creates a **merge commit**
> - The merge commit has TWO parent commits (one from each branch)
> - Git will open an editor for you to write a merge commit message
> - You can use the default message or customize it
> - The `--graph` option helps visualize how branches merged
## Key Concepts ## What is Merging?
### What is Merging? **Merging** is the process of taking changes from one branch and bringing them into another branch.
**Merging** combines changes from different branches into one branch. When you merge, you're telling Git: "Take all the changes from branch X and incorporate them into my current branch." Think of it like combining two streams into one river - all the water (code) flows together.
### Divergent Branches ### Before Merging
When two branches have different commits since they split, they are **divergent**: You have two branches with different work:
``` ```
main: A---B---C main: A---B---C
\ \
feature: D---E feature-login: D---E
``` ```
- Both branches split from commit B - Main branch has commits A, B, and C
- `main` has commit C - Feature-login branch has commits D and E
- `feature` has commits D and E - They split apart at commit B
- They have **diverged** - each has unique work
### The Merge Commit ### After Merging
When you merge divergent branches, Git creates a special **merge commit**: You bring the feature branch into main:
``` ```
Before merge: main: A---B---C---M
main: A---B---C \ /
\ feature-login: D-E
feature: D---E
After merge (on main):
main: A---B---C-------M
\ /
feature: D---E---/
``` ```
**Merge commit (M)** is special because: - Commit M is a **merge commit** - it combines both branches
- It has **two parent commits**: C and E - Main now has all the work from both branches
- Your login feature is now part of main!
## How to Merge
Merging is simple - just two steps:
**1. Switch to the branch you want to merge INTO**
```bash
git switch main
```
This is the branch that will receive the changes.
**2. Merge the other branch**
```bash
git merge feature-login
```
This brings changes from feature-login into main.
**That's it!** Git does the work of combining the changes.
## Understanding the Merge Commit
When you merge, Git creates a special commit called a **merge commit**.
**What makes it special?**
- It has TWO parent commits (one from each branch)
- It represents the point where branches came back together - It represents the point where branches came back together
- It contains the combined changes from both branches - Git writes a message like "Merge branch 'feature-login'"
### How Git Merges
Git uses a **three-way merge** algorithm:
1. **Common ancestor** (B) - Where the branches split
2. **Your branch** (C) - Latest commit on main
3. **Their branch** (E) - Latest commit on feature
Git compares all three to figure out what changed on each branch and combines them intelligently.
## Understanding Merge Output
When you run `git merge feature-login`, you'll see:
You can view the merge commit just like any other commit:
```bash
git show HEAD
``` ```
Merge made by the 'ort' strategy.
login.py | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
create mode 100644 login.py
```
**What this means:**
- **"Merge made by the 'ort' strategy"** - Git used its merge algorithm to combine changes
- The summary shows what files changed in the merge
- A new merge commit was created
### The Merge Commit Message
Git will open your editor with a default message:
```
Merge branch 'feature-login'
# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
```
**Tips for merge messages:**
- The default message is usually fine for simple merges
- For important merges, explain WHY you're merging
- Example: "Merge feature-login - adds user authentication system"
## Visualizing Merges ## Visualizing Merges
Use `git log --graph` to see the branch structure: Use `git log --graph` to see how branches merged:
```bash ```bash
git log --oneline --graph --all git log --oneline --graph --all
@@ -152,84 +127,82 @@ git log --oneline --graph --all
**Reading the graph:** **Reading the graph:**
- `*` = A commit - `*` = A commit
- `|` = Branch line continuing - `|` = Branch line
- `/` and `\` = Branch splitting or merging - `/` and `\` = Branches splitting or joining
- `(HEAD -> main)` = Current branch position - `(HEAD -> main)` = You are here
- The merge commit has two lines coming into it
The graph shows how the branches split and came back together!
## Useful Commands ## Useful Commands
### Merging
```bash ```bash
git merge <branch> # Merge branch into current branch # Merging
git merge --abort # Cancel a merge in progress git merge <branch-name> # Merge a branch into current branch
```
### Viewing Merge History # Viewing
git log --oneline --graph # See branch history visually
git log --oneline --graph --all # Include all branches
```bash # Branch management
git log --oneline --graph # Simple graph view git branch # List branches
git log --oneline --graph --all # Show all branches git switch <branch-name> # Switch to a branch
git log --merges # Show only merge commits git branch -d <branch-name> # Delete a branch (after merging)
git show <commit> # View details of a merge commit
```
### Branch Management
```bash
git branch # List local branches
git branch -a # List all branches (including remote)
git branch -d <branch> # Delete a merged branch
``` ```
## Common Questions ## Common Questions
### "What if the merge fails?" ### "What if I'm on the wrong branch when I merge?"
Sometimes Git can't automatically merge changes (this is called a **merge conflict**). Don't worry! We'll cover this in the next module (Module 05: Merge Conflicts). Don't worry! The branch you're currently on is the one that receives the changes.
If you see a conflict and want to cancel: **Example:**
```bash
git switch main # Go to main
git merge feature-login # Bring feature-login INTO main
```
Always switch to the destination branch first!
### "Can I undo a merge?"
Yes! Before you push to a remote:
```bash
git reset --hard HEAD~1
```
This removes the merge commit. (We'll cover this more in later modules)
### "What happens to the feature branch after merging?"
The feature branch still exists! The merge just copies its commits into main.
You can delete it if you're done:
```bash
git branch -d feature-login # Safe delete (only if merged)
```
The commits are still in history - you're just removing the branch label.
### "What if Git can't merge automatically?"
Sometimes Git needs your help when the same lines were changed in both branches. This is called a **merge conflict**.
Don't worry - we'll learn how to handle conflicts in the next module!
If you encounter a conflict now and want to cancel:
```bash ```bash
git merge --abort git merge --abort
``` ```
### "Can I undo a merge?"
Before you've pushed to remote:
```bash
git reset --hard HEAD~1 # Undo the merge commit
```
We'll cover more advanced undoing in later modules.
### "Should I delete the feature branch after merging?"
It's common practice to delete feature branches after merging:
```bash
git branch -d feature-login # Safe delete (only works if merged)
```
This cleans up your branch list. The commits are still in the history - the branch pointer is just removed.
### "What does 'ort strategy' mean?"
Git uses different merge algorithms:
- **ort** (default in Git 2.34+) - Newer, faster merge algorithm
- **recursive** (older) - Traditional merge algorithm
You don't need to worry about this - Git picks the best one automatically!
## Verification ## Verification
Once you've completed the merge, verify your solution: Once you've merged the feature-login branch, verify your solution:
```powershell ```powershell
.\verify.ps1 .\verify.ps1
``` ```
The verification script will check that you've successfully merged the feature branch. The verification script will check that you've successfully merged.
## Need to Start Over? ## Need to Start Over?
@@ -239,10 +212,18 @@ If you want to reset the challenge and start fresh:
.\reset.ps1 .\reset.ps1
``` ```
This will remove the challenge directory and run the setup script again, giving you a clean slate.
## What's Next? ## What's Next?
Now that you understand basic merging, the next module covers **Merge Conflicts** - what happens when Git can't automatically merge changes and how to resolve them manually. **Next module:** Merge Conflicts - Learn what to do when Git can't automatically merge changes.
**Advanced Topic:** In Module 06 (Advanced section), you'll learn about different merge strategies including fast-forward merges and when to use `--no-ff`. **Later:** In the advanced modules, you'll learn about different merging strategies and when to use them. For now, understanding basic merging is all you need!
## Quick Summary
✅ **Merging** combines work from one branch into another
✅ Switch to the destination branch, then run `git merge <source-branch>`
✅ Git creates a **merge commit** to record the merge
✅ Use `git log --graph` to visualize how branches merged
✅ The feature branch still exists after merging - you can delete it if you want
That's all there is to basic merging! 🎉