🌿 Working with Branches From Scratch (Part B)

🌳 Create & Switch Branches Manually!

Without git branch, git checkout, or git switch

📺 Video Reference

Resource	Link
🎬 Video	Creating Repo From Scratch
📄 Part A	Creating a Repository From Scratch

📋 Prerequisites

Make sure you've completed Part A and have:

scratch-repo/
├── hello.txt
└── .git/
    ├── HEAD                 ← ref: refs/heads/master
    ├── index
    ├── objects/
    │   ├── 5d/...          ← Tree
    │   ├── 93/...          ← Blob
    │   └── a5/...          ← Commit
    └── refs/
        └── heads/
            └── master       ← Points to commit

🌿 Understanding Branches Internally

Remember: A branch is just a file in .git/refs/heads/ containing a commit SHA!

🔧 Creating a Branch Manually

Method 1: Using echo

bash

# See current commit SHA
cat .git/refs/heads/master
# Output: a52f8f31b84c2e5c0ea76bf21c9f57f30476af91

# Create a new branch pointing to same commit
echo "a52f8f31b84c2e5c0ea76bf21c9f57f30476af91" > .git/refs/heads/feature

# Verify
git branch

Output:

  feature
* master

🎉 We just created a branch without git branch!

Method 2: Using git update-ref (safer)

bash

# Create another branch
git update-ref refs/heads/dev a52f8f31b84c2e5c0ea76bf21c9f57f30476af91

git branch

  dev
  feature
* master

🔄 Switching Branches Manually

How git checkout Works Internally

When you run git checkout feature:

Git updates .git/HEAD to point to the new branch
Git updates the working directory to match that branch's commit

Switch Manually with echo

bash

# Check current HEAD
cat .git/HEAD
# Output: ref: refs/heads/master

# Switch to feature branch
echo "ref: refs/heads/feature" > .git/HEAD

# Verify
git branch

* feature
  master
  dev

⚠️ Warning: Unlike git checkout, manually changing HEAD doesn't update the working directory! If the branches point to different commits with different files, you'll have mismatched state.

Switch Back to Master

bash

echo "ref: refs/heads/master" > .git/HEAD

git branch

  feature
* master
  dev

📸 Creating a Second Commit on a Branch

Let's make a commit on the feature branch to see branches diverge.

Step 1: Switch to Feature Branch

bash

echo "ref: refs/heads/feature" > .git/HEAD

Step 2: Create a New File's Blob

bash

# Create new content
echo "Feature content!" | git hash-object --stdin -w

Output:

2c4d8f6e31b2a7c9f4e5d6a7b8c9d0e1f2a3b4c5

Step 3: Update the Index

bash

# Add existing file (still tracked)
# ... it's already in index from previous commit

# Add new file
git update-index --add --cacheinfo 100644 \
    2c4d8f6e31b2a7c9f4e5d6a7b8c9d0e1f2a3b4c5 \
    feature.txt

Step 4: Create Tree

bash

git write-tree

Output:

8a7b9c4d5e6f1a2b3c4d5e6f7a8b9c0d1e2f3a4b

Step 5: Create Commit with Parent

🔑 Key Point: Use -p flag to specify the parent commit!

bash

# Get current branch's commit (our parent)
cat .git/refs/heads/feature
# Output: a52f8f31b84c2e5c0ea76bf21c9f57f30476af91

# Create commit with parent
git commit-tree 8a7b9c4d5e6f1a2b3c4d5e6f7a8b9c0d1e2f3a4b \
    -m "Add feature file" \
    -p a52f8f31b84c2e5c0ea76bf21c9f57f30476af91

Output:

f5e6d7c8b9a0f1e2d3c4b5a6d7e8f9a0b1c2d3e4

Step 6: Update Branch Reference

bash

echo "f5e6d7c8b9a0f1e2d3c4b5a6d7e8f9a0b1c2d3e4" > .git/refs/heads/feature

Verify the New Commit

bash

git cat-file -p f5e6d7c8b9a0f1e2d3c4b5a6d7e8f9a0b1c2d3e4

tree 8a7b9c4d5e6f1a2b3c4d5e6f7a8b9c0d1e2f3a4b
parent a52f8f31b84c2e5c0ea76bf21c9f57f30476af91    ← Links to first commit!
author Your Name <you@example.com> 1769447200 +0000
committer Your Name <you@example.com> 1769447200 +0000

Add feature file

📊 Branch Divergence Visualization

Current State:

🔍 Viewing the Log

bash

# Check log from feature branch
git log --oneline

f5e6d7c Add feature file
a52f8f3 Initial commit

bash

# Switch to master and check log
echo "ref: refs/heads/master" > .git/HEAD
git log --oneline

a52f8f3 Initial commit

The feature branch has 2 commits, master still has 1!

🎭 Detached HEAD State

What if we point HEAD directly to a commit instead of a branch?

bash

# Point HEAD directly to a commit SHA
echo "a52f8f31b84c2e5c0ea76bf21c9f57f30476af91" > .git/HEAD

git status

HEAD detached at a52f8f3
nothing to commit, working tree clean

⚠️ Detached HEAD Warning:

In detached HEAD state:

HEAD points directly to a commit, not a branch
Any new commits won't belong to any branch
If you switch branches, those commits may become "orphaned"
Use git checkout -b new-branch to create a branch and save your work!

Fix Detached HEAD

bash

# Go back to master branch
echo "ref: refs/heads/master" > .git/HEAD

🗑️ Deleting a Branch Manually

Since branches are just files:

bash

# Delete the dev branch
rm .git/refs/heads/dev

git branch

  feature
* master

💡 Tip: Using git branch -d is safer because it checks if the branch is fully merged first. Deleting the file directly skips this safety check!

📋 Complete Plumbing Commands Reference

Creating & Inspecting Objects

Command	Purpose	Example
`git hash-object --stdin -w`	Create blob from stdin	`echo "hi" \| git hash-object --stdin -w`
`git hash-object -w FILE`	Create blob from file	`git hash-object -w hello.txt`
`git cat-file -t SHA`	Get object type	`git cat-file -t a52f8f31`
`git cat-file -p SHA`	Pretty print object	`git cat-file -p a52f8f31`
`git cat-file -s SHA`	Get object size	`git cat-file -s a52f8f31`

Index Operations

Command	Purpose
`git update-index --add --cacheinfo MODE SHA FILE`	Add blob to index
`git ls-files --stage`	Show staged files with SHAs
`git write-tree`	Create tree from index

Commit Operations

Command	Purpose
`git commit-tree TREE -m "msg"`	Create commit without parent
`git commit-tree TREE -m "msg" -p PARENT`	Create commit with parent
`git commit-tree TREE -m "msg" -p P1 -p P2`	Create merge commit

Reference Operations

Command	Purpose
`git update-ref refs/heads/BRANCH SHA`	Update branch pointer
`git symbolic-ref HEAD refs/heads/BRANCH`	Update HEAD (safe way)
`echo "ref: refs/heads/BRANCH" > .git/HEAD`	Update HEAD (manual)

🧠 Quiz: Test Your Understanding

1. How do you create a branch called "hotfix" pointing to commit abc123?

bash

echo "abc123" > .git/refs/heads/hotfix
# OR
git update-ref refs/heads/hotfix abc123

2. How do you switch to a branch without using git checkout?

bash

echo "ref: refs/heads/branchname" > .git/HEAD
# OR
git symbolic-ref HEAD refs/heads/branchname

⚠️ Note: This doesn't update the working directory!

3. What's the difference between these two HEAD values?

ref: refs/heads/master    (A)
a52f8f31b84c2e5c...       (B)

(A): HEAD points to a branch (normal state)
(B): HEAD points directly to a commit (detached HEAD state)

4. How do you create a commit with a parent?

bash

git commit-tree TREE_SHA -m "message" -p PARENT_SHA

The -p flag specifies the parent commit.

5. Why use git update-ref instead of echo?

git update-ref is safer because it:

Validates the SHA exists
Creates a reflog entry
Handles errors gracefully
Works with packed refs

🎯 Summary: Complete Manual Git Workflow

🏆 What You've Learned

✅ Create a Git repo manually
Without git init

✅ Create blobs and trees
With hash-object and write-tree

✅ Create commits with parents
Using commit-tree -p

✅ Create and switch branches
By manipulating .git/refs/heads/

🚀 Next Steps

🔄 Congratulations! You've mastered Git internals!

You now understand how Git really works under the hood. Use this knowledge to:

Debug Git issues more effectively
Write Git hooks and automation
Understand what happens when things go wrong
Recover from "impossible" Git situations

📚 Further Reading

Resource	Description
Git Book - Internals	Official Git documentation
Git Object Model	Deep dive into objects
Git References	Understanding refs

🎓 Course Complete!

You've finished the Git Internals Deep Dive series.

← Back to Course Index

🌿 Working with Branches From Scratch (Part B) ​

🌳 Create & Switch Branches Manually!

📺 Video Reference ​

📋 Prerequisites ​

🌿 Understanding Branches Internally ​

🔧 Creating a Branch Manually ​

Method 1: Using echo ​

Method 2: Using git update-ref (safer) ​

🔄 Switching Branches Manually ​

How git checkout Works Internally ​

Switch Manually with echo ​

Switch Back to Master ​

📸 Creating a Second Commit on a Branch ​

Step 1: Switch to Feature Branch ​

Step 2: Create a New File's Blob ​

Step 3: Update the Index ​

Step 4: Create Tree ​

Step 5: Create Commit with Parent ​

Step 6: Update Branch Reference ​

Verify the New Commit ​

📊 Branch Divergence Visualization ​

🔍 Viewing the Log ​

🎭 Detached HEAD State ​

Fix Detached HEAD ​

🗑️ Deleting a Branch Manually ​

📋 Complete Plumbing Commands Reference ​

Creating & Inspecting Objects ​

Index Operations ​

Commit Operations ​

Reference Operations ​

🧠 Quiz: Test Your Understanding ​

🎯 Summary: Complete Manual Git Workflow ​

🏆 What You've Learned ​

🚀 Next Steps ​

🔄 Congratulations! You've mastered Git internals!

📚 Further Reading ​

🎓 Course Complete!

🌿 Working with Branches From Scratch (Part B)

📺 Video Reference

📋 Prerequisites

🌿 Understanding Branches Internally

🔧 Creating a Branch Manually

Method 1: Using echo

Method 2: Using git update-ref (safer)

🔄 Switching Branches Manually

How git checkout Works Internally

Switch Manually with echo

Switch Back to Master

📸 Creating a Second Commit on a Branch

Step 1: Switch to Feature Branch

Step 2: Create a New File's Blob

Step 3: Update the Index

Step 4: Create Tree

Step 5: Create Commit with Parent

Step 6: Update Branch Reference

Verify the New Commit

📊 Branch Divergence Visualization

🔍 Viewing the Log

🎭 Detached HEAD State

Fix Detached HEAD

🗑️ Deleting a Branch Manually

📋 Complete Plumbing Commands Reference

Creating & Inspecting Objects

Index Operations

Commit Operations

Reference Operations

🧠 Quiz: Test Your Understanding

🎯 Summary: Complete Manual Git Workflow

🏆 What You've Learned

🚀 Next Steps

📚 Further Reading