Git Diff and Log
Git diff and log are essential tools for understanding changes in your repository. They help you see what changed, when it changed, and who changed it.
git diff - Viewing Changes
Basic Diff Operations
bash
# Show unstaged changes (working directory vs staging area)
git diff
# Show staged changes (staging area vs last commit)
git diff --staged
# or
git diff --cached
# Show all changes (working directory vs last commit)
git diff HEAD
Practical Example: Portfolio Updates
Let's create some changes to demonstrate diff:
bash
# Make changes to existing files
cd portfolio-website
# Modify index.html
cat > index.html << EOF
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>John Doe - Portfolio</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
<header>
<nav>
<a href="index.html">Home</a>
<a href="contact.html">Contact</a>
<a href="gallery.html">Gallery</a>
</nav>
</header>
<main>
<h1>Welcome to my portfolio</h1>
<p>I'm a web developer passionate about creating amazing user experiences.</p>
<section id="skills">
<h2>Skills</h2>
<ul>
<li>HTML5 & CSS3</li>
<li>JavaScript (ES6+)</li>
<li>React & Node.js</li>
</ul>
</section>
</main>
<footer>© 2024 John Doe</footer>
<script src="script.js"></script>
</body>
</html>
EOF
# Add some changes to CSS
cat >> style.css << EOF
/* Header Styles */
header {
background-color: #333;
padding: 1rem 0;
}
nav a {
color: white;
text-decoration: none;
margin: 0 1rem;
padding: 0.5rem 1rem;
border-radius: 4px;
transition: background-color 0.3s;
}
nav a:hover {
background-color: #555;
}
/* Skills Section */
#skills {
margin: 2rem 0;
padding: 1rem;
background-color: #f8f9fa;
border-radius: 8px;
}
#skills ul {
list-style-type: none;
padding: 0;
}
#skills li {
padding: 0.5rem 0;
border-bottom: 1px solid #dee2e6;
}
#skills li:last-child {
border-bottom: none;
}
EOF
# View unstaged changes
git diff
Understanding Diff Output
diff
diff --git a/index.html b/index.html
index 1234567..abcdefg 100644
--- a/index.html
+++ b/index.html
@@ -1,8 +1,25 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
+ <meta charset="UTF-8">
+ <meta name="viewport" content="width=device-width, initial-scale=1.0">
- <title>Portfolio</title>
+ <title>John Doe - Portfolio</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
+ <header>
+ <nav>
+ <a href="index.html">Home</a>
+ <a href="contact.html">Contact</a>
+ <a href="gallery.html">Gallery</a>
+ </nav>
+ </header>
Diff symbols:
---and+++: Old and new file versions@@: Hunk header showing line numbers-: Lines removed (red in terminal)+: Lines added (green in terminal)
Diff Options and Formats
bash
# Word-level diff (more granular)
git diff --word-diff
# Character-level diff
git diff --word-diff=color --word-diff-regex=.
# Ignore whitespace changes
git diff -w
# or
git diff --ignore-all-space
# Show only file names that changed
git diff --name-only
# Show file names with status
git diff --name-status
# Show statistics
git diff --stat
# Compact summary
git diff --shortstat
Comparing Specific Commits
bash
# Stage some changes first
git add index.html
git commit -m "Improve HTML structure and add navigation"
# Add more changes
echo "/* Additional mobile styles */" >> style.css
git add style.css
git commit -m "Add header and skills styling"
# Compare commits
git diff HEAD~2 HEAD~1 # Compare two commits ago with one commit ago
git diff HEAD~1 HEAD # Compare last commit with current
git diff a1b2c3d..e4f5g6h # Compare specific commits
# Compare branches
git diff main..feature/branch
git diff main...feature/branch # Three dots: common ancestor to branch tip
Diff for Specific Files
bash
# Diff specific file
git diff index.html
git diff HEAD~1 index.html
# Diff multiple files
git diff index.html style.css
# Diff files in directory
git diff src/
# Diff with path filtering
git diff '*.css'
git diff '*.js'
git log - Viewing History
Basic Log Operations
bash
# Basic log
git log
# One line per commit
git log --oneline
# Show last N commits
git log -5
git log -n 3
# Show commits since date
git log --since="2024-01-01"
git log --since="2 weeks ago"
git log --since="yesterday"
# Show commits until date
git log --until="2024-01-31"
git log --before="1 week ago"
Pretty Formats
bash
# Built-in formats
git log --pretty=oneline
git log --pretty=short
git log --pretty=medium
git log --pretty=full
git log --pretty=fuller
# Custom format
git log --pretty=format:"%h - %an, %ar : %s"
git log --pretty=format:"%C(yellow)%h%C(reset) - %C(blue)%an%C(reset), %C(green)%ar%C(reset) : %s"
Custom Format Placeholders
| Placeholder | Description |
|---|---|
%H | Full commit hash |
%h | Abbreviated commit hash |
%an | Author name |
%ae | Author email |
%ad | Author date |
%ar | Author date, relative |
%cn | Committer name |
%cd | Committer date |
%cr | Committer date, relative |
%s | Subject (commit message) |
%b | Body (commit message) |
%d | Ref names (branches, tags) |
Advanced Log Examples
bash
# Beautiful one-line format
git log --pretty=format:"%C(auto)%h%d %s %C(black)%C(bold)%cr" --graph
# Detailed format with colors
git log --pretty=format:"%C(yellow)%h %C(red)%d %C(reset)%s %C(green)(%cr) %C(blue)<%an>%C(reset)" --graph
# Show file changes
git log --stat
git log --name-only
git log --name-status
# Show actual changes
git log -p
git log --patch
# Limit patch output
git log -p -2 # Show patches for last 2 commits
Filtering Commits
By Author
bash
# Commits by specific author
git log --author="John Doe"
git log --author="john@example.com"
# Multiple authors (regex)
git log --author="John\|Jane"
# Exclude author
git log --author="^(?!John Doe).*$" --perl-regexp
By Message
bash
# Commits containing specific text
git log --grep="fix"
git log --grep="feature"
# Case insensitive
git log --grep="FIX" -i
# Multiple patterns
git log --grep="fix" --grep="bug" --all-match
# Invert match
git log --grep="WIP" --invert-grep
By File Changes
bash
# Commits that changed specific file
git log -- index.html
git log --follow -- index.html # Follow renames
# Commits that changed files matching pattern
git log -- '*.css'
git log -- src/
# Commits that added or removed specific text
git log -S "function validateForm" # Pickaxe search
git log -G "function.*validate" # Regex search
By Date Range
bash
# Specific date range
git log --since="2024-01-01" --until="2024-01-31"
# Relative dates
git log --since="2 weeks ago"
git log --since="yesterday" --until="today"
# Specific format
git log --since="Jan 1 2024" --until="Jan 31 2024"
Graphical Log
bash
# Simple graph
git log --graph --oneline
# Detailed graph
git log --graph --pretty=format:"%h -%d %s (%cr) <%an>" --abbrev-commit
# All branches
git log --graph --oneline --all
# Specific branches
git log --graph --oneline main feature/branch
Example output:
* b2c3d4e (HEAD -> feature/portfolio) Add header and skills styling
* a1b2c3d Improve HTML structure and add navigation
| * f9e8d7c (main) Fix contact form validation
|/
* 5h4i3j2 Add contact form feature
* 9k8l7m6 Initial commit
Practical Log Workflows
Daily Standup Report
bash
# What did I work on yesterday?
git log --author="$(git config user.name)" --since="yesterday" --pretty=format:"%h %s"
# What did the team work on this week?
git log --since="1 week ago" --pretty=format:"%h - %an: %s" --graph
Release Notes
bash
# Changes since last tag
git log v1.0.0..HEAD --pretty=format:"- %s (%h)"
# Features and fixes since last release
git log v1.0.0..HEAD --grep="feat:" --grep="fix:" --pretty=format:"- %s"
Code Review Preparation
bash
# Changes in feature branch
git log main..feature/branch --oneline
# Detailed changes for review
git log main..feature/branch --stat
# All changes with patches
git log main..feature/branch -p
Advanced Diff and Log Techniques
Diff Tools Integration
bash
# Configure external diff tool
git config --global diff.tool vimdiff
# or
git config --global diff.tool vscode
# Use external tool
git difftool
git difftool HEAD~1
# Configure VS Code as diff tool
git config --global diff.tool vscode
git config --global difftool.vscode.cmd 'code --wait --diff $LOCAL $REMOTE'
Log Aliases
Create useful aliases for common log commands:
bash
# Add to ~/.gitconfig or use git config
git config --global alias.lg "log --graph --pretty=format:'%C(yellow)%h%C(reset) -%C(red)%d%C(reset) %s %C(green)(%cr) %C(blue)<%an>%C(reset)' --abbrev-commit"
git config --global alias.lga "log --graph --pretty=format:'%C(yellow)%h%C(reset) -%C(red)%d%C(reset) %s %C(green)(%cr) %C(blue)<%an>%C(reset)' --abbrev-commit --all"
git config --global alias.ls "log --pretty=format:'%C(yellow)%h %C(blue)%ad%C(reset) %C(white)%s%C(reset) %C(green)[%an]%C(reset)' --decorate --date=short"
# Usage
git lg
git lga
git ls
Searching Through History
bash
# Find when a line was added
git log -S "specific code line" --source --all
# Find when a function was modified
git log -G "function myFunction" --patch
# Find commits that touch specific lines
git log -L 10,20:filename.js
git log -L :functionName:filename.js
Performance and Large Repositories
bash
# Limit log output for performance
git log --oneline -100 # Last 100 commits
# Skip merge commits
git log --no-merges
# Only merge commits
git log --merges
# Simplify history
git log --simplify-by-decoration
Practical Examples
Example 1: Bug Investigation
bash
# Find when a bug was introduced
# Look for commits that changed the problematic file
git log --oneline -- problematic-file.js
# Check specific function changes
git log -G "buggyFunction" --patch -- problematic-file.js
# See what changed in suspicious commit
git show a1b2c3d
# Compare with previous version
git diff a1b2c3d~1 a1b2c3d -- problematic-file.js
Example 2: Code Review
bash
# Review all changes in feature branch
git log main..feature/new-feature --stat
# See detailed changes
git diff main...feature/new-feature
# Check individual commits
git log main..feature/new-feature --oneline
git show commit-hash
Example 3: Release Preparation
bash
# Generate changelog
git log v1.0.0..HEAD --pretty=format:"- %s" --reverse
# Check what files changed
git diff --name-only v1.0.0..HEAD
# Get statistics
git diff --stat v1.0.0..HEAD
Troubleshooting
Large Diff Output
bash
# Limit context lines
git diff -U1 # 1 line of context instead of 3
# Show only changed files
git diff --name-only
# Use pager
git diff | less
Log Performance Issues
bash
# Limit history depth
git log --max-count=50
# Skip expensive operations
git log --no-patch
# Use shallow clone for large repos
git clone --depth 50 <url>
Finding Lost Changes
bash
# Search in all commits (including deleted)
git log --all --full-history -- deleted-file.txt
# Search in reflog
git reflog | grep "search term"
# Find dangling commits
git fsck --lost-found
Best Practices
- Use Descriptive Commit Messages: Makes log more useful
- Regular Small Commits: Easier to track changes
- Use Aliases: Create shortcuts for common log formats
- Learn Format Strings: Customize output for your needs
- Combine with Other Tools: Use with grep, less, etc.
Quick Reference
bash
# Diff commands
git diff # Working directory vs staging
git diff --staged # Staging vs last commit
git diff HEAD # Working directory vs last commit
git diff commit1..commit2 # Between commits
git diff --stat # Show statistics
# Log commands
git log # Basic log
git log --oneline # Compact format
git log --graph # Show branch graph
git log --stat # Show file changes
git log -p # Show patches
git log --author="name" # Filter by author
git log --grep="text" # Filter by message
git log --since="date" # Filter by date
git log -- filename # Filter by file
# Useful aliases
git config --global alias.lg "log --graph --oneline --decorate"
git config --global alias.lga "log --graph --oneline --decorate --all"
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