Advanced Git: Interactive Rebase, Cherry-pick, Reflog, and History Rewriting

# Start interactive rebase for last 5 commits
git rebase -i HEAD~5

# An editor opens showing:
# pick f7f3f6d Add user authentication
# pick 310154e Fix typo in login form
# pick a5f4a0d Add password validation
# pick c6f9e8e Fix minor bug in validation
# pick 0d1a2b3 Improve error messages

# Commands you can use:
# pick (p) - Use commit as-is
# reword (r) - Change commit message
# edit (e) - Stop to amend commit
# squash (s) - Combine with previous commit
# fixup (f) - Combine, discard message
# drop (d) - Remove commit
# reorder - Just move lines to reorder

# Example: Squash Fix typo into Add user authentication
# pick f7f3f6d Add user authentication
# fixup 310154e Fix typo in login form
# squash a5f4a0d Add password validation
# fixup c6f9e8e Fix minor bug in validation
# reword 0d1a2b3 Improve error messages

# Save and close - Git executes the rebase

# Merge preserves history, creates merge commit
# Rebases rewrite history, creates linear history

# Merge (non-linear history)
git checkout feature
git merge main
#        A---B---C feature
#       /         \
# D---E---F---G---H main

# Rebase (linear history)
git checkout feature
git rebase main
#                 A'--B'--C' feature
#                /
# D---E---F---G main

# When to use each:
# - Merge: public branches, preserving exact history
# - Rebase: private branches before sharing

# Rebase feature branch onto updated main
git checkout feature
git rebase main

# Resolve any conflicts, then
git rebase --continue
# or
git rebase --abort

# Apply a single commit to current branch
git cherry-pick abc123def

# Apply a range of commits
git cherry-pick abc123..def456

# Apply but don't auto-commit
git cherry-pick -n abc123def

# Apply multiple commits
git cherry-pick abc123 def456 ghi789

# Real-world: Backport a bug fix
git checkout release/v1.0
git cherry-pick -x abc123def  # -x adds source info

# Resolve conflicts if needed
git cherry-pick --continue
git cherry-pick --abort

# Example: Extract specific commits from a feature branch
# BEFORE: feature branch has 10 commits
# AFTER: main branch has only the 3 useful commits

# View reflog (local history of HEAD movements)
git reflog

# Output example:
# abc1234 HEAD@{0}: commit: Add new feature
# def5678 HEAD@{1}: rebase -i (finish): Rebase completed
# ghi9012 HEAD@{2}: rebase -i (start): Checkout
# jkl3456 HEAD@{3}: commit: WIP

# Recover deleted branch
git checkout -b recovered-branch abc1234

# Undo a rebase
git reset --hard HEAD@{1}

# Go back in time (temporarily)
git checkout HEAD@{2}

# See reflog for specific branch
git reflog show feature-branch

# Recover after hard reset
git reflog
# Find commit before reset
git reset --hard HEAD@{5}

# Clean reflog (remove entries older than 90 days)
git reflog expire --expire=now --all
git gc --prune=now

# Start bisect
git bisect start

# Mark current commit as bad (has the bug)
git bisect bad

# Mark a known good commit (no bug)
git bisect good v1.0

# Git checks out a commit halfway between
# Test the commit, then mark:
git bisect good  # If commit is good
git bisect bad   # If commit is bad

# Repeat until Git finds the bad commit
# When done, show results
git bisect log

# End bisect session
git bisect reset

# Automatic bisect with script
git bisect start HEAD v1.0
git bisect run npm test

# Example: Find where the API broke
# Run API test automatically
cat > test-api.sh << 'EOF'
#!/bin/bash
curl -f http://localhost:3000/health || exit 1
EOF
chmod +x test-api.sh
git bisect start HEAD v1.0
git bisect run ./test-api.sh

# Scenario 1: Wrong commit message
git commit --amend -m "Correct message"
git push --force-with-lease origin main

# Scenario 2: Forgot to add a file
git add forgotten-file.txt
git commit --amend --no-edit
git push --force-with-lease

# Scenario 3: Need to remove a commit (but keep changes)
git reset --soft HEAD~1  # Keep changes in working directory
git reset --mixed HEAD~1 # Keep changes unstaged
git reset --hard HEAD~1  # Discard changes completely

# Scenario 4: Revert a pushed commit (safe way)
git revert abc123def  # Creates new revert commit
git push origin main

# Scenario 5: Remove last 3 commits (if not pushed)
git reset --hard HEAD~3

# Scenario 6: Remove last 3 commits (if pushed - use with caution!)
git reset --hard HEAD~3
git push --force-with-lease

#!/bin/bash
# cleanup_history.sh - Script to clean messy Git history

echo "=== Git History Cleanup Tool ==="

# Display current messy log
echo "Current messy history:"
git log --oneline -10

# Option 1: Squash recent commits
echo ""
echo "Squashing last 5 commits..."
git reset --soft HEAD~5
git commit -m "Feature: Complete implementation with all fixes"
echo "Done! Now have 1 commit instead of 5"

# Option 2: Interactive rebase for more control
echo ""
echo "For more control, use: git rebase -i HEAD~N"
echo "Commands you can use:"
echo "  - squash: combine with previous"
echo "  - fixup: combine, discard message"
echo "  - reword: change commit message"
echo "  - edit: stop to amend"
echo "  - drop: delete commit"

# Option 3: Clean up after rebase
echo ""
echo "After cleaning, force push (if branch is private):"
echo "git push --force-with-lease origin feature-branch"

# Option 4: Clean up old branches
echo ""
echo "Deleting merged branches:"
git branch --merged | grep -v "\*" | grep -v "main" | xargs -n 1 git branch -d

# Option 5: Remove large files from history
echo ""
echo "To remove large files from history:"
echo "git filter-branch --tree-filter 'rm -f largefile.zip' HEAD"
echo "# or use BFG Repo-Cleaner for better performance"