git-recovery（git 恢复）

Git Safety and Recovery

Purpose: This skill teaches AI agents to work safely with Git and recover from common mistakes without losing work.

Core Principles

1. Git Rarely Deletes - Almost everything is recoverable via reflog
2. Understand Before Destroying - Know what --hard, --force do
3. Backup Before Risky Operations - Use stash or branches
4. Revert, Don't Reset - For shared branches

Understanding Git's Safety Net

The Reflog (Your Safety Net)

What is reflog? Git's reflog records every HEAD movement. It's your "undo history" for Git operations.

# ✓ View reflog
git reflog

# Output shows every commit, checkout, reset, rebase, etc:
abc123 HEAD@{0}: commit: feat(auth): add login
def456 HEAD@{1}: checkout: moving from main to feature
ghi789 HEAD@{2}: pull: fast-forward
jkl012 HEAD@{3}: reset: moving to HEAD~1
mno345 HEAD@{4}: commit: fix(bug): temporary fix

How long does reflog keep data?

• Default: 90 days for reachable commits
• 30 days for unreachable commits
• Configurable via gc.reflogExpire

What Can Be Recovered?

# ✓ CAN RECOVER:
- Deleted branches (if deleted recently)
- Reset commits (within 90 days)
- Amended commits (original still in reflog)
- Rebased commits (original still in reflog)
- Dropped stashes (if recent)

# ✗ CANNOT RECOVER:
- Unstaged changes that were never committed
- Untracked files that were never added
- Commits older than gc.reflogExpire
- Files explicitly removed with git clean -f

Common Mistakes and Fixes

Mistake 1: Accidentally Deleted Branch

# ✗ MISTAKE: Deleted wrong branch
git branch -D feature/important-work
# Deleted branch feature/important-work (was abc123).

# ✓ FIX: Recover from reflog
git reflog | grep "important-work"
# abc123 HEAD@{5}: commit: Last commit on important-work

# Recreate branch at that commit
git switch -c feature/important-work abc123

# Or use branch -C to force create
git branch feature/important-work abc123
git switch feature/important-work

Mistake 2: Reset Too Far

# ✗ MISTAKE: Reset and lost commits
git reset --hard HEAD~3
# Now 3 commits are "gone"

# ✓ FIX: Find commits in reflog
git reflog
# abc123 HEAD@{0}: reset: moving to HEAD~3
# def456 HEAD@{1}: commit: feat(auth): important feature  ← Want this!

# Reset back to before the mistake
git reset --hard def456
# Or
git reset --hard HEAD@{1}

Mistake 3: Committed to Wrong Branch

# ✗ MISTAKE: Committed to main instead of feature branch
git switch main
git add src/feature.js
git commit -m "feat(feature): add new feature"
# Oops! This should be on feature branch

# ✓ FIX: Move commit to correct branch

# Create feature branch (includes the commit)
git switch -c feature/new-feature

# Go back to main and remove the commit
git switch main
git reset --hard HEAD~1

# Now:
# - feature/new-feature has the commit
# - main is back to previous state

Mistake 4: Amended Wrong Commit

# ✗ MISTAKE: Amended commit that shouldn't be changed
git commit --amend -m "New message"
# Oops! Original commit was important

# ✓ FIX: Find original commit in reflog
git reflog
# abc123 HEAD@{0}: commit (amend): New message
# def456 HEAD@{1}: commit: Original message  ← Want this!

# Reset to original commit
git reset --hard def456

Mistake 5: Rebased and Lost Commits

# ✗ MISTAKE: Rebase went wrong, some commits disappeared
git rebase -i HEAD~5
# Accidentally dropped important commits

# ✓ FIX: Abort or recover from reflog

# If still in rebase:
git rebase --abort

# If rebase already finished:
git reflog
# Find "rebase (start)" entry
# abc123 HEAD@{10}: rebase (start): checkout main
# def456 HEAD@{11}: commit: Important commit  ← Before rebase started

# Reset to before rebase
git reset --hard def456

Mistake 6: Force Pushed Wrong Branch

# ✗ MISTAKE: Force pushed and overwrote remote commits
git push --force origin main
# Overwrote teammate's commits!

# ✓ FIX: Depends on situation

# If you still have local copy of correct state:
git reflog
git reset --hard <correct-commit>
git push --force-with-lease origin main

# If you don't have it, ask teammate to push their version:
# Teammate runs:
git push --force origin main

# Or restore from remote reflog (if server keeps it):
git fetch origin
git log origin/main@{1}  # One state ago

Mistake 7: Accidentally Removed Staged Changes

# ✗ MISTAKE: Used git restore --staged and lost work
git restore --staged src/important.js
git restore src/important.js  # Oops! Lost changes

# ✓ PREVENTION: Check before restoring
git diff src/important.js  # See what will be lost
git stash push src/important.js  # Safer alternative

# ✗ CANNOT FIX: Unstaged changes cannot be recovered
# Prevention is key!

Reset vs Revert vs Restore

Decision Tree

What do you want to undo?
├─ Local changes not committed yet
│  └─> git restore <file>
│
├─ Staging area (unstage)
│  └─> git restore --staged <file>
│
├─ Last commit (not pushed)
│  ├─ Keep changes in working directory
│  │  └─> git reset --soft HEAD~1
│  │
│  └─ Discard changes completely
│     └─> git reset --hard HEAD~1
│
└─ Commit already pushed
   └─> git revert <commit>  (creates new commit)

Git Reset (For Local Changes)

# Three modes of reset:

# 1. --soft: Move HEAD, keep staged and working directory
git reset --soft HEAD~1
# Use case: Undo commit, keep all changes staged
# Safe: Yes (changes preserved)

# 2. --mixed (default): Move HEAD, unstage, keep working directory
git reset HEAD~1
# OR
git reset --mixed HEAD~1
# Use case: Undo commit, keep changes but unstaged
# Safe: Yes (changes preserved in working directory)

# 3. --hard: Move HEAD, discard staged and working directory
git reset --hard HEAD~1
# Use case: Completely undo commit and discard changes
# Safe: NO (changes lost, but recoverable from reflog)

Reset Examples

# ✓ SAFE: Undo last commit, keep changes
git reset --soft HEAD~1
# Now changes are staged, ready to re-commit differently

# ✓ SAFE: Undo last commit, unstage changes
git reset HEAD~1
# Now changes are in working directory, unstaged

# ✗ DESTRUCTIVE: Undo commit and discard all changes
git reset --hard HEAD~1
# Changes are "gone" (but recoverable from reflog)

# ✓ SAFE: Reset to specific commit
git reset --soft abc123
# All commits after abc123 are undone, changes staged

# ✓ VISUAL: See what would be reset
git log HEAD~3..HEAD  # Shows commits that would be undone
git diff HEAD~3  # Shows changes that would be affected

Git Revert (For Shared Branches)

# ✓ SAFE: Undo commit on shared branch
git revert abc123

# Creates NEW commit that undoes changes:
#   * Revert "feat(auth): add OAuth"  ← New commit
#   * feat(auth): add OAuth           ← Original commit
#   * Previous commit

# Why use revert instead of reset?
# - Doesn't rewrite history
# - Safe for shared branches
# - Preserves audit trail

Revert Examples

# ✓ Revert single commit
git revert abc123
# Opens editor for revert commit message

# ✓ Revert without opening editor
git revert abc123 --no-edit

# ✓ Revert multiple commits
git revert abc123 def456 ghi789

# ✓ Revert range of commits
git revert HEAD~3..HEAD

# ✓ Revert merge commit
git revert -m 1 abc123
# -m 1 means keep first parent (usually main)

Comparison Table

Recovery Workflows

Workflow 1: Find and Recover Lost Commit

# 1. Search reflog for lost commit
git reflog | grep -i "search term"

# OR view all reflog
git reflog

# 2. Examine commit content
git show abc123

# 3. Recover options:

# Option A: Create new branch at that commit
git switch -c recovered-work abc123

# Option B: Cherry-pick the commit
git cherry-pick abc123

# Option C: Reset current branch to that commit
git reset --hard abc123

Workflow 2: Recover Deleted Branch

# 1. Find branch in reflog
git reflog | grep "branch-name"
# OR
git reflog | grep "checkout"

# 2. Find last commit on that branch
# abc123 HEAD@{5}: commit: Last commit on branch-name

# 3. Recreate branch
git switch -c branch-name abc123

# 4. Verify
git log

Workflow 3: Undo Bad Merge

# Just merged, but it was wrong

# ✓ OPTION 1: Reset (if not pushed)
git reset --hard HEAD~1

# ✓ OPTION 2: Revert (if already pushed)
git revert -m 1 HEAD
# -m 1 keeps the main branch parent

# ✓ OPTION 3: Abort (if still in merge)
git merge --abort

Workflow 4: Recover from Bad Rebase

# Rebase went wrong

# ✓ OPTION 1: Abort (if still in rebase)
git rebase --abort

# ✓ OPTION 2: Recover from reflog (if already finished)
git reflog
# Find entry before rebase started:
# abc123 HEAD@{10}: checkout: moving from feature to main
# def456 HEAD@{11}: commit: Last commit before rebase

git reset --hard def456

Workflow 5: Recover Dropped Stash

# Accidentally dropped stash

# 1. Find stash in reflog
git reflog | grep stash
# abc123 WIP on main: Previous stash
# def456 index on main: Another stash

# 2. Create branch from stash commit
git switch -c recovered-stash abc123

# 3. Apply the stash
git stash apply abc123

# OR directly apply from reflog
git stash apply abc123

Preventive Measures

Before Risky Operations

# ✓ BACKUP 1: Create safety branch
git switch -c backup-before-rebase
git switch feature-branch
# Do risky operation
# If successful, delete backup:
git branch -d backup-before-rebase

# ✓ BACKUP 2: Use stash
git stash push -m "Backup before risky operation"
# Do risky operation
# If successful:
git stash drop
# If failed:
git stash pop

# ✓ BACKUP 3: Tag current state
git tag backup-$(date +%Y%m%d-%H%M%S)
# Do risky operation
# If successful:
git tag -d backup-20240115-143000

Configure Safety Settings

# ✓ Require confirmation for destructive operations
git config --global alias.yolo '!echo "Are you sure? Use git reset --hard if certain"'

# ✓ Increase reflog retention
git config --global gc.reflogExpire 180  # 180 days instead of 90
git config --global gc.reflogExpireUnreachable 60  # 60 days instead of 30

# ✓ Always use --force-with-lease
git config --global alias.pushf 'push --force-with-lease'

# ✓ Protect against accidental git clean
git config --global clean.requireForce true

Pre-operation Checklist

Before running destructive commands:

• Do I have a backup? (branch, tag, or stash)
• Is this branch pushed to remote? (additional backup)
• Have I verified what will be changed? (git diff, git log)
• Do I understand what this command does?
• Can I recover using reflog if something goes wrong?

Advanced Recovery

Recover Specific Files from History

# ✓ Restore file from specific commit
git restore --source=abc123 src/important.js

# ✓ Restore file from before it was deleted
git log --all --full-history -- src/deleted-file.js
# Find commit where it existed
git restore --source=abc123 src/deleted-file.js

# ✓ Restore file from stash
git show stash@{0}:src/file.js > src/file.js

Recover from git clean

# ✗ PROBLEM: Ran git clean and deleted untracked files
git clean -fd
# Deleted important-file.js (untracked)

# ✓ LIMITED RECOVERY OPTIONS:

# Option 1: Check editor auto-save
# Many editors keep unsaved file backups

# Option 2: Check OS trash/recycle bin
# Some Git GUIs move files to trash instead of deleting

# Option 3: File recovery tools
# Use OS-level file recovery (PhotoRec, TestDisk, etc.)

# ✗ PREVENTION: Always use -n first
git clean -nfd  # -n = dry run, shows what would be deleted
# Review output
git clean -fd   # Actually delete

Recover from Rewritten History

# Someone force-pushed and rewrote history

# ✓ Find old commits in local reflog
git reflog
git log --all --oneline

# ✓ Recovery options:

# Option 1: Push your version (if yours is correct)
git push --force-with-lease origin main

# Option 2: Merge both versions
git fetch origin
git merge origin/main
# Resolve conflicts
git push

# Option 3: Cherry-pick missing commits
git cherry-pick abc123 def456
git push

Common Dangerous Commands

Commands to Use Carefully

# ✗ DANGEROUS: Can lose work

git reset --hard <commit>
# Discards all changes in working directory
# Prevention: git stash first

git clean -fd
# Deletes untracked files permanently
# Prevention: git clean -nfd first (dry run)

git push --force
# Overwrites remote, can lose others' work
# Prevention: Use --force-with-lease instead

git rebase -i
# Can drop/edit commits, complex conflicts
# Prevention: Create backup branch first

git branch -D <branch>
# Deletes branch even if not merged
# Prevention: Use -d first (safer), recover from reflog if needed

Safe Alternatives

# ✓ SAFER ALTERNATIVES:

# Instead of: git reset --hard
git stash  # Or git stash push -u to include untracked

# Instead of: git push --force
git push --force-with-lease

# Instead of: git clean -fd
git clean -nfd  # Dry run first
git stash push -u  # Stash untracked files

# Instead of: git branch -D
git branch -d  # Refuses if not merged
git merge --no-ff <branch>  # Merge first, then delete

# Instead of: git reset --hard HEAD~3
git reset --soft HEAD~3  # Keep changes
git restore --staged .   # Unstage if needed

Quick Reference

Recovery Commands

# View reflog
git reflog
git reflog show <branch>

# Recover deleted branch
git switch -c <branch> <commit-hash>

# Undo last commit (keep changes)
git reset --soft HEAD~1

# Undo last commit (discard changes)
git reset --hard HEAD~1
git reflog  # Can still recover

# Revert commit (safe for shared branches)
git revert <commit>

# Recover from bad merge
git reset --hard HEAD~1  # If not pushed
git revert -m 1 HEAD     # If pushed

# Find deleted file
git log --all --full-history -- <file>
git restore --source=<commit> <file>

# Recover dropped stash
git reflog | grep stash
git stash apply <stash-hash>

Summary

Key Principles:

1. Git rarely deletes - Reflog keeps 90 days of history
2. Backup before risky operations - Branch, tag, or stash
3. Use revert for shared branches - Never reset public history
4. Check before destroying - Dry runs, diffs, logs
5. Force-with-lease, not force - Prevents accidental overwrites

Essential Commands:

git reflog                    # Your safety net
git reset --soft HEAD~1       # Undo commit, keep changes
git reset --hard <commit>     # Dangerous! Backup first
git revert <commit>           # Safe undo for shared branches
git switch -c name <hash>     # Recover deleted branch

Remember: Almost every mistake is recoverable. Stay calm, check reflog, and understand what each command does before running it.