git-history-detective

Git History Detective

Find when bugs were introduced and understand code evolution.

When to Use

• Finding when a bug was introduced
• Understanding why code was changed
• Finding who last modified a line
• Tracking down a regression
• Searching for commits by content

Quick Reference

Task	Command
Who changed this line?	git blame <file>
Find commit by message	git log --grep="keyword"
Find commit by code change	git log -S "code"
Binary search for bug	git bisect
See file at old commit	git show <commit>:<file>

Investigation Techniques

1. Git Blame - Find Line Authors

# Basic blame
git blame <file>

# Blame specific lines
git blame -L 10,20 <file>

# Ignore whitespace changes
git blame -w <file>

# Show original author (ignore moves/copies)
git blame -M -C <file>

# Blame with email
git blame -e <file>

Reading blame output:

abc1234 (John Doe 2024-01-15 10:30:45 +0000 42) function processData() {
│        │         │                       │    │
│        │         │                       │    └─ Line content
│        │         │                       └─ Line number
│        │         └─ Timestamp
│        └─ Author
└─ Commit SHA (first 7 chars)

2. Git Log - Search History

# Search commit messages
git log --grep="fix login"

# Search code that was added/removed
git log -S "functionName"  # "pickaxe" search

# Search with regex
git log -G "function.*deprecated"

# Find commits touching a file
git log --follow -- <file>

# Find commits by author
git log --author="John"

# Find commits in date range
git log --since="2024-01-01" --until="2024-02-01"

# Combine searches
git log --author="John" --grep="refactor" --since="2024-01-01"

3. Git Bisect - Binary Search for Bugs

When you know a bug exists now but didn't before:

# Start bisect
git bisect start

# Mark current (broken) commit as bad
git bisect bad

# Mark known good commit
git bisect good v1.2.0  # or a commit SHA

# Git checks out middle commit
# Test it, then:
git bisect good  # if bug NOT present
git bisect bad   # if bug IS present

# Repeat until Git finds the culprit
# "abc1234 is the first bad commit"

# End bisect
git bisect reset