Git History

Parent Skill: /git Path: /git/history

Purpose

Explore commit history, compare changes, perform code archaeology, and find when bugs were introduced. Master log viewing, diffs, blame, bisect, and history filtering.

When to Use

Trigger automatically when:

• User mentions: log, diff, show, blame, bisect, history, who changed, when was
• Investigating bugs
• Code review preparation
• Finding when changes were made
• Understanding code evolution

Chat commands:

/git/history show me recent commits
/git/history find when this bug was introduced
/git/history who changed this line
/git/history compare branches
/git/history search commit messages for "auth"

Requirements

Verification

# Check repository has commits
git log --oneline -5

# Check file history exists
git log -- path/to/file.js

Critical Rules

Patterns

Pattern 1: View Commit History

Problem: Understand what changes were made and when

# ❌ BAD: Default log is overwhelming
git log  # Shows full commit messages, can be thousands of lines

# ✅ GOOD: Concise, readable format
git log --oneline -10

# ✅ BETTER: Custom format with author and date
git log --oneline --graph --decorate --all -20

# ✅ BEST: Alias for common format
git config --global alias.lg "log --oneline --graph --decorate --all"
git lg -20

Pretty formatting:

# One line per commit
git log --oneline
# abc1234 feat: add user authentication
# def5678 fix: resolve login bug

# With graph (show branches)
git log --oneline --graph
# * abc1234 feat: add user auth
# * def5678 fix: resolve login bug
# |\
# | * ghi9012 feat: add dashboard
# |/
# * jkl3456 Initial commit

# Custom format
git log --pretty=format:"%h - %an, %ar : %s"
# abc1234 - John Doe, 2 hours ago : feat: add user auth

# With stats
git log --stat -3
# Shows files changed and line counts

Time-based filtering:

# Commits from last week
git log --since="1 week ago"

# Commits from specific date range
git log --since="2024-01-01" --until="2024-02-01"

# Commits from last 3 days
git log --since="3 days ago"

# Commits from specific time
git log --since="2024-02-01 10:00" --until="2024-02-01 18:00"

Impact: Quickly understand project evolution

When to use:

• Code review preparation
• Understanding recent changes
• Writing changelogs
• Investigating project timeline

Pattern 2: Compare Changes (Diff)

Problem: See what changed between commits, branches, or states

# ❌ BAD: Unclear what's being compared
git diff  # Compares working directory to staging?

# ✅ GOOD: Explicit comparisons
# Unstaged changes (working directory vs staging)
git diff

# Staged changes (staging vs last commit)
git diff --staged
# or
git diff --cached

# All changes (working directory vs last commit)
git diff HEAD

# Specific file
git diff HEAD -- src/app.js

Compare commits:

# Compare two commits
git diff abc1234 def5678

# Compare commit to HEAD
git diff abc1234 HEAD

# Compare with parent
git diff HEAD^ HEAD
git diff HEAD~1 HEAD

# Show what commit changed
git show abc1234

Compare branches:

# Changes from main to feature branch
git diff main..feature-branch

# Changes that will be merged
git diff main...feature-branch  # Three dots!

# Files changed between branches
git diff --name-only main..feature-branch

# Stat summary
git diff --stat main..feature-branch

Diff output options:

# Side-by-side diff (if terminal supports)
git diff --color-words

# Show only file names
git diff --name-only

# Show with stats
git diff --stat

# Ignore whitespace
git diff -w

# Specific function/class (Git is smart!)
git diff -L :functionName:file.js

Impact: Understand exactly what changed

When to use:

• Before committing (review changes)
• Code review
• Comparing branches before merge
• Understanding specific changes

Pattern 3: Show Commit Details

Problem: See complete details of specific commit

# ❌ BAD: Just hash or log
git log abc1234
# Too much output

# ✅ GOOD: Show specific commit
git show abc1234

# ✅ BETTER: Show with stat summary
git show --stat abc1234

# Show only changed files
git show --name-only abc1234

# Show specific file from commit
git show abc1234:src/app.js

View file at specific commit:

# See file content from commit
git show abc1234:path/to/file.js

# Save file version to disk
git show abc1234:src/app.js > app-old.js

# Compare file between commits
git diff abc1234:src/app.js def5678:src/app.js

Show multiple commits:

# Show last 3 commits with details
git show HEAD~3 HEAD~2 HEAD~1

# Show range
git log -p abc1234..def5678  # -p shows patches (diffs)

Impact: Deep dive into specific changes

When to use:

• Investigating bug fix
• Understanding old code
• Code archaeology
• Reverting specific changes

Pattern 4: Find Who Changed Line (Blame)

Problem: Find who last modified each line of file

# ❌ BAD: Blame without context
git blame file.js
# Overwhelming output

# ✅ GOOD: Blame with line numbers and author
git blame -L 10,20 src/app.js
# Shows lines 10-20 only

# ✅ BETTER: Ignore whitespace changes
git blame -w src/app.js

# Show email and date
git blame -e --date=short src/app.js

Advanced blame:

# Ignore specific commit (like reformatting)
git blame --ignore-rev abc1234 src/app.js

# Ignore all commits in file
git blame --ignore-revs-file .git-blame-ignore-revs src/app.js

# Show original line numbers (track through renames)
git blame -C -C src/app.js

# Interactive blame (show commit details)
git blame -L 50,60 src/app.js
# Copy commit hash
git show abc1234

Find deleted code:

# Find when line was deleted
git log -S "function oldFunction" -- src/app.js

# Show commits that added or removed string
git log -G "regex pattern" -- src/app.js

Impact: Understand code history and authorship

When to use:

• Bug investigation
• Understanding design decisions
• Code review context
• Finding subject matter expert

Pattern 5: Binary Search for Bugs (Bisect)

Problem: Find commit that introduced a bug

# ❌ BAD: Manually check each commit
git log --oneline
git checkout abc1234
# Test...
git checkout def5678
# Test...
# This takes forever!

# ✅ GOOD: Binary search with bisect
git bisect start
git bisect bad                 # Current commit is bad
git bisect good abc1234        # This old commit was good

# Git checks out middle commit
# Test it
npm test

# If test passes:
git bisect good

# If test fails:
git bisect bad

# Repeat until Git finds the culprit commit
# Git will tell you: "abc1234 is the first bad commit"

# End bisect
git bisect reset

Automated bisect:

# Let Git automatically test
git bisect start HEAD abc1234
git bisect run npm test

# Git will automatically:
# 1. Check out commit
# 2. Run npm test
# 3. Mark good (exit 0) or bad (exit 1)
# 4. Continue until found

# Works with any command that returns exit code
git bisect run ./test.sh
git bisect run python -m pytest

Bisect with build script:

# Create test script
cat > test.sh << 'EOF'
#!/bin/bash
npm install
npm run build
npm test
exit $?
EOF

chmod +x test.sh

git bisect start HEAD v1.0.0
git bisect run ./test.sh

Skip commits (if build broken):

git bisect start
git bisect bad
git bisect good abc1234

# Current commit can't be tested
git bisect skip

# Skip range
git bisect skip abc1234..def5678

Impact: Find bug introduction 10x faster than manual search

When to use:

• Regression testing
• Bug appeared between releases
• Code worked before, broken now
• Large commit history to search

Pattern 6: Search Commit Messages

Problem: Find commits related to specific feature or bug

# ❌ BAD: Search all commits with grep
git log | grep "auth"
# Misses context

# ✅ GOOD: Search commit messages
git log --grep="auth"

# Case insensitive
git log --grep="auth" -i

# Multiple patterns (OR)
git log --grep="auth" --grep="login"

# Multiple patterns (AND)
git log --grep="auth" --grep="login" --all-match

# Regular expressions
git log --grep="fix.*auth" -E

Search commit content:

# Find commits that added/removed string
git log -S "functionName"

# With patches showing context
git log -S "functionName" -p

# Regular expression in content
git log -G "function.*Auth"

# Find commits affecting specific file
git log -- src/auth.js

# Combination: message and file
git log --grep="fix" -- src/auth.js

Impact: Quickly find relevant commits

When to use:

• Finding when feature was added
• Locating bug fix
• Code review
• Understanding feature history

Pattern 7: Filter History by File/Author/Date

Problem: View commits for specific context

Filter by file:

# ❌ BAD: All commits, then search
git log > all.txt
# Search in editor

# ✅ GOOD: Filter by file path
git log -- src/auth.js

# Follow file through renames
git log --follow -- src/auth.js

# Multiple files
git log -- src/auth.js src/user.js

# Directory
git log -- src/components/

Filter by author:

# Specific author
git log --author="John Doe"

# Partial match
git log --author="John"

# Multiple authors
git log --author="John\|Jane"

# Exclude author
git log --author="^(?!John).*$"

# With stats
git log --author="John" --stat

Filter by date:

# Since date
git log --since="2024-01-01"

# Until date
git log --until="2024-02-01"

# Date range
git log --since="2024-01-01" --until="2024-02-01"

# Relative dates
git log --since="2 weeks ago"
git log --since="yesterday"
git log --since="1 month ago"

Combine filters:

# Author + date + file
git log --author="John" --since="1 week ago" -- src/auth.js

# Author + message
git log --author="John" --grep="fix"

# Multiple filters
git log --author="John" --since="2024-01-01" --grep="auth" -- src/

Impact: Narrow down to exact commits needed

When to use:

• Reviewing teammate's work
• File history investigation
• Preparing changelog
• Audit and compliance

Pattern 8: Visualize Branch History

Problem: Understand complex branch structure

# ❌ BAD: Linear log
git log --oneline
# Can't see branches

# ✅ GOOD: Graph view
git log --graph --oneline --all

# ✅ BETTER: With decorations and colors
git log --graph --oneline --decorate --all --color

# ✅ BEST: Custom format with details
git log --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit --all

Create alias:

git config --global alias.tree "log --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit --all"

# Now use:
git tree
git tree -20  # Last 20 commits

Branch-specific views:

# Show only current branch
git log --graph --oneline

# Show all branches
git log --graph --oneline --all

# Show specific branches
git log --graph --oneline main develop feature-branch

# Show branch divergence
git log --graph --oneline main..feature-branch

Visual tools:

# Use gitk (GUI tool)
gitk --all

# Or tig (terminal tool)
tig

# Or GitHub/GitLab web interface

Impact: Understand branch relationships visually

When to use:

• Complex branch structures
• Before merging
• Understanding team workflow
• Debugging merge issues

Common Workflows

Bug Investigation Workflow

# 1. Find when bug appeared
git bisect start HEAD v1.0.0
git bisect run npm test

# 2. Found commit abc1234
git show abc1234

# 3. Who wrote it?
git blame src/buggy-file.js

# 4. What was the context?
git log --oneline abc1234~5..abc1234

# 5. Related changes?
git log --grep="feature-name" --since="1 month ago"

Code Review Preparation

# 1. See all commits in PR
git log --oneline main..feature-branch

# 2. See detailed changes
git diff main...feature-branch

# 3. Review each commit
git log -p main..feature-branch

# 4. Check specific file changes
git log --follow -- src/important-file.js

Changelog Generation

# Get commits since last release
git log --oneline v1.0.0..HEAD

# Group by type
git log --oneline --grep="feat:" v1.0.0..HEAD
git log --oneline --grep="fix:" v1.0.0..HEAD

# With authors
git log --pretty=format:"- %s (%an)" v1.0.0..HEAD

# Export to file
git log --pretty=format:"- %s" v1.0.0..HEAD > CHANGELOG.md

Troubleshooting

Log is too long

# Limit number of commits
git log -10

# Use pager
git log | less

# Export to file
git log > log.txt

Can't find commit

# Search all branches
git log --all --grep="search term"

# Search reflog (even deleted commits)
git reflog | grep "search term"

# Search by date
git log --all --since="when I remember working on it"

Blame shows wrong author

# Blame shows reformatting commit
# Create .git-blame-ignore-revs
echo "abc1234" >> .git-blame-ignore-revs
git blame --ignore-revs-file .git-blame-ignore-revs file.js

# Or configure globally
git config blame.ignoreRevsFile .git-blame-ignore-revs

Bisect stuck

# Skip problematic commits
git bisect skip

# Abort and start over
git bisect reset
git bisect start

# Resume from saved state
git bisect log > bisect-log.txt
# Git automatically resumes from saved state

Best Practices

1. Use limits - Always limit git log output (-10, --since, etc.)
2. Specific comparisons - Be explicit with diff comparisons
3. Follow renames - Use --follow for file history
4. Automate bisect - Write test scripts for bisect run
5. Create aliases - Save complex log formats as aliases
6. Use graphical view - Visual tools for complex branches
7. Document bisect - Save bisect results in commit/issue

Related Sub-Skills

• Need to undo changes? → /git/undo
• Need to compare branches? → /git/branching
• Investigating merge conflicts? → /git/conflicts
• Understanding team workflow? → /git/workflows

Quick Command Reference

# View history
git log --oneline -10                    # Recent commits
git log --graph --all                    # Branch graph
git log --since="1 week ago"             # Time filter

# Compare
git diff                                 # Unstaged changes
git diff --staged                        # Staged changes
git diff main..feature                   # Branch comparison

# Show commit
git show abc1234                         # Commit details
git show abc1234:file.js                 # File at commit

# Blame
git blame file.js                        # Who changed what
git blame -L 10,20 file.js              # Specific lines

# Bisect
git bisect start HEAD v1.0.0            # Start bisect
git bisect good/bad                      # Mark commits
git bisect reset                         # End bisect

# Search
git log --grep="search"                  # Search messages
git log -S "code"                        # Search content
git log -- file.js                       # File history

# Filter
git log --author="John"                  # By author
git log --since="date"                   # By date

Last Updated: 2025-02-11 Version: 1.0.0