Git and GitHub¶

1. What is Git — why it matters¶

Git is a version control system. It records the history of every change you make to your code, enabling:

Rolling back to a previous version when code breaks
Working on multiple features in parallel without them interfering (branches)
Team collaboration — multiple people working on the same project

GitHub is a cloud hosting service for Git repositories. Code on your computer is local, GitHub is remote — a backup copy on the internet.

Git ≠ GitHub

Git is a tool that runs on your computer. GitHub is a website that hosts code. You use Git to manage your code, and GitHub to share and back it up.

2. Git workflow¶

Git workflow

Area	Role
Working Directory	Where you write and edit code
Staging Area	A "shopping cart" — choose which changes go into the next commit
Local Repository	Commit history on your machine (the `.git` folder)
Remote (GitHub)	Cloud backup on the internet

3. Install Git¶

WindowsmacOSLinux

Download¶

Go to https://git-scm.com/download/win — the page auto-detects Windows and suggests the correct build.
Download the .exe file (about 60 MB).

Install¶

Run the .exe, click Yes to allow changes.
The components screen — leave defaults, click Next through each screen.
Choosing the default editor — select Notepad or Nano if you're not familiar with Vim, then Next.
Adjusting your PATH — select Git from the command line and also from 3rd-party software (the middle option) — this is important.
All remaining screens — keep defaults, click Next until Install, then Finish.

Verify¶

Open a new Command Prompt or PowerShell:

git --version

Example output: git version 2.47.0.windows.1

Option 1 — Xcode Command Line Tools (easiest)¶

Open Terminal and run:

git --version

If Git is not installed, macOS automatically asks if you want to install Xcode Command Line Tools — click Install. This takes 5–10 minutes.

Option 2 — Homebrew¶

brew install git

The Homebrew version is more up to date than the Apple-provided one.

# Ubuntu / Debian
sudo apt update && sudo apt install -y git

# Fedora
sudo dnf install -y git

# Arch
sudo pacman -S git

Verify: git --version

4. Configure Git for the first time¶

Before using Git, set your name and email. This information is attached to every commit you create.

git config --global user.name "Your Name"
git config --global user.email "email@example.com"

Replace "Your Name" and "email@example.com" with your actual name and email (ideally the same email as your GitHub account).

Verify the configuration:

git config --list

Only needs to be done once

The --global flag saves to ~/.gitconfig and applies to all repositories on this machine. You only need to run these two commands once.

5. Create a GitHub account¶

Go to https://github.com, click Sign up.
Enter your email address, create a password, choose a username.
Verify your email by clicking the link in GitHub's confirmation email.
Select the Free plan (sufficient for all learning purposes).

6. Create your first repository on GitHub¶

Log in to GitHub, click the + button in the top right → New repository.
Fill in the details:
- Repository name: java-practice (or any name you prefer)
- Description: optional
- Public / Private: Public if you want to share it, Private if only you should see it
- ✅ Check Add a README file
Click Create repository.

GitHub takes you to the new repository page, showing a URL like https://github.com/username/java-practice.

7. Connect your computer to GitHub — SSH Key¶

Git needs to verify your identity before allowing you to push code to GitHub. The most secure and convenient method is an SSH key.

Step 1 — Check for an existing SSH key¶

ls ~/.ssh

If you see files named id_ed25519 and id_ed25519.pub — you already have an SSH key; skip Step 2.

Step 2 — Generate a new SSH key¶

ssh-keygen -t ed25519 -C "email@example.com"

Replace email@example.com with your GitHub email.

When prompted: - Enter file in which to save the key — press Enter to use the default location. - Enter passphrase — leave empty (press Enter twice) or set a password for extra security.

Step 3 — Add the SSH key to GitHub¶

Copy the public key content:

macOSWindows (PowerShell)Linux

cat ~/.ssh/id_ed25519.pub

Get-Content "$env:USERPROFILE\.ssh\id_ed25519.pub"

cat ~/.ssh/id_ed25519.pub

Copy the entire line of text starting with ssh-ed25519 ....

On GitHub: 1. Click your profile picture → Settings → SSH and GPG keys. 2. Click New SSH key. 3. Title: a name to identify this machine (e.g. Personal laptop). 4. Key: paste the content you copied. 5. Click Add SSH key.

Step 4 — Test the connection¶

ssh -T git@github.com

Successful output:

Hi username! You've successfully authenticated, but GitHub does not provide shell access.

8. Clone the repository to your machine¶

git clone git@github.com:username/java-practice.git

Replace username with your GitHub username. This creates a java-practice directory containing the full repository.

cd java-practice

9. The daily Git workflow¶

This is the process you will repeat every day:

Step 1 — Check the current status¶

git status

This shows: - Which files have been changed (modified) - Which files are in staging (staged) - Which files are not yet tracked (untracked)

Step 2 — Add changes to staging¶

git add FileName.java         # add a specific file
git add .                     # add all changes in the current directory

Prefer specifying files explicitly

git add . is convenient but can accidentally include unwanted files. When starting out, name the files explicitly.

Step 3 — Create a commit¶

git commit -m "Short description of the change"

Example:

git commit -m "Add BankAccount class with deposit and withdraw methods"

Write good commit messages

Start with a verb: Add, Fix, Update, Remove
Keep it short and clear — someone reading the commit message should understand what you did
English is the international standard, but your own language is fine when working alone

Step 4 — Push code to GitHub¶

git push origin main

Step 5 — Pull the latest code from GitHub (when working with others or on multiple machines)¶

git pull origin main

10. Complete example — from start to finish¶

# Clone the repository
git clone git@github.com:username/java-practice.git
cd java-practice

# Create a new Java file
# ... (write code in your IDE or text editor) ...

# Check status
git status

# Add the file to staging
git add HelloWorld.java

# Create a commit
git commit -m "Add HelloWorld program"

# Push to GitHub
git push origin main

11. .gitignore — exclude files Git should not track¶

The .gitignore file lists files and directories that Git should completely ignore.

Create a .gitignore file in the root of your project:

# Bytecode
*.class

# Build output
target/
out/

# IDE files
.idea/
*.iml
.vscode/
.classpath
.project
.settings/

# macOS
.DS_Store

# Windows
Thumbs.db

Generate .gitignore automatically

The site gitignore.io lets you select your language and IDE and auto-generates the appropriate .gitignore content. Example: select Java, IntelliJ, macOS.

12. Git command reference¶

Command	Action
`git init`	Initialize a new Git repository in the current directory
`git clone <url>`	Copy a repository from remote to your machine
`git status`	View the state of working directory and staging
`git add <file>`	Add a file to staging
`git add .`	Add all changes to staging
`git commit -m "msg"`	Create a commit with a message
`git push origin main`	Push commits to the remote `main` branch
`git pull origin main`	Pull the latest code from remote
`git log`	View commit history
`git log --oneline`	Compact commit history
`git diff`	View unstaged changes

13. Troubleshooting¶

`Permission denied (publickey)`¶

Cause: The SSH key has not been added to GitHub, or has not been loaded into the SSH agent.

Fix:

eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519
ssh -T git@github.com   # verify again

`error: failed to push some refs`¶

Cause: The remote has commits your local repository doesn't have yet (someone else pushed first).