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

Automated bisect with test script:

git bisect start
git bisect bad HEAD
git bisect good v1.0.0
git bisect run npm test  # or any command that exits 0 for good, 1 for bad

4. Track File Changes Over Time

# See all commits that touched a file
git log --oneline -- <file>

# See actual changes in each commit
git log -p -- <file>

# Follow file through renames
git log --follow -p -- <file>

# Show file at specific commit
git show <commit>:<file>

# Compare file between commits
git diff <commit1> <commit2> -- <file>

5. Find Deleted Code

# Find when code was deleted
git log -S "deletedFunction" --diff-filter=D

# Find deleted file
git log --all --full-history -- "**/deleted-file.js"

# Restore deleted file
git checkout <commit-before-deletion>^ -- <file>

Investigation Workflow

Scenario: "This feature used to work"

Step 1: Identify the symptom

# Find when the feature last worked
# Check release tags, deployment dates
git tag -l --sort=-version:refname | head -10

Step 2: Narrow the time range

# Find commits between working and broken state
git log --oneline v1.2.0..HEAD -- src/feature/

Step 3: Use bisect to pinpoint

git bisect start
git bisect bad HEAD
git bisect good v1.2.0
# Test each checkout

Step 4: Analyze the culprit commit

git show <culprit-commit>
git log -1 --format="%B" <culprit-commit>  # Full message

Scenario: "Who broke this?"

# Find who last touched the broken code
git blame -L 45,50 src/broken-file.js

# See the full commit
git show <commit-from-blame>

# See what else changed in that commit
git show --stat <commit>

Scenario: "When was this function added?"

# Find first commit with the function
git log --reverse -S "functionName" --oneline

# See the full commit
git show <first-commit>

Scenario: "Why was this changed?"

# Get the commit that last changed this line
git blame -L 42,42 <file>

# See full commit message (hopefully explains why)
git show <commit>

# See related commits around that time
git log --oneline --since="2024-01-01" --until="2024-01-15" -- <file>

Advanced Techniques

Find Commit That Introduced Regex Match

git log -G "TODO:.*hack" --oneline

Compare Branches

# Commits in feature not in main
git log main..feature --oneline

# Commits in either but not both
git log main...feature --oneline

Visualize History

# ASCII graph
git log --graph --oneline --all

# Show merge history
git log --first-parent --oneline

Search Across All Branches

git log --all -S "searchTerm"
git branch --contains <commit>

Tools Integration

# Open blame in browser (GitHub)
gh browse --blame <file>

# Interactive blame in VS Code
# Install GitLens extension

# GUI tools
gitk --all  # Built-in
tig        # Terminal UI

Common Patterns

Find Security Vulnerabilities Introduced

git log -S "eval(" --oneline
git log -S "innerHTML" --oneline
git log -G "password.*=.*['\"]" --oneline

Find When Tests Started Failing

git bisect start
git bisect bad HEAD
git bisect good <last-known-good>
git bisect run npm test

Track Refactoring Impact

# Find the refactoring commit
git log --grep="refactor" --oneline -- src/

# See what files it touched
git show --stat <refactor-commit>

# Compare before/after
git diff <commit>^..<commit>