{ "version": 1, "id": "git-rebasing-and-history", "title": "Rebasing and Rewriting History Safely", "summary": "Rebase, squash, amend, reset and revert, the golden rule of rebasing, and recovering lost commits.", "estimatedMinutes": 25, "prerequisites": [], "source": "courses-notebook ENV202_git_mastery 03_rebasing_and_history", "quiz": [ { "prompt": "What is the difference between `git reset --soft HEAD~1` and `git reset --hard HEAD~1`?", "options": [ "`--soft` moves HEAD only; `--hard` moves HEAD and discards working-tree changes", "`--soft` deletes the commit from disk while `--hard` keeps it", "They are aliases", "`--soft` requires force-push; `--hard` does not" ], "answer": 0 }, { "prompt": "Which command best recovers a commit that is no longer reachable from any branch but is still in the local repo?", "options": [ "Use `git reflog` to find the SHA, then `git checkout` or `git reset` to it", "git fsck --lost-found and then git restore", "Clone the repo from origin again", "git gc --aggressive --restore" ], "answer": 0 }, { "prompt": "What is the 'golden rule' of `git rebase`?", "options": [ "Always rebase before pushing for the first time", "Never rebase commits that have already been pushed to a shared branch", "Only rebase onto detached HEAD", "Never rebase if the working tree is clean" ], "answer": 1 }, { "prompt": "Which command is the safest way to undo a commit that has already been pushed to a shared branch?", "options": [ "git reset --hard HEAD~1 followed by force push", "git commit --amend then push", "git revert ", "git filter-repo --invert-paths" ], "answer": 2 }, { "prompt": "In an interactive rebase (`git rebase -i HEAD~3`), what does the `squash` action do?", "options": [ "Discards the commit entirely", "Combines the commit with the previous one, allowing you to edit the combined message", "Cherry-picks the commit onto another branch", "Marks the commit as a merge commit" ], "answer": 1 }, { "prompt": "Why is `git commit --amend` unsafe to use after pushing to a shared branch?", "options": [ "It permanently deletes the staging area", "It rewrites the last commit's SHA, which diverges from the version teammates already have", "It triggers a force-push automatically", "It only works in detached HEAD" ], "answer": 1 } ], "lesson": "## 3.1 Rebase vs Merge\n\n```\n# Merge: preserves history, adds merge commit\nA\u2500\u2500B\u2500\u2500C\u2500\u2500D (main)\n \u2191 merge commit\n E\u2500\u2500F\u2500\u2500G (feature)\n\n# Rebase: clean linear history\nA\u2500\u2500B\u2500\u2500C\u2500\u2500E'\u2500\u2500F'\u2500\u2500G' (feature rebased onto main)\n```\n\n## 3.2 Interactive Rebase\n\n```bash\ngit rebase -i HEAD~5 # rework last 5 commits\n\n# pick \u2192 keep commit\n# reword \u2192 change message\n# edit \u2192 amend commit\n# squash \u2192 merge into previous\n# fixup \u2192 squash, discard message\n# drop \u2192 remove commit\n```\n\n## 3.3 Common History Operations\n\n```bash\n# Amend last commit\ngit commit --amend\n\n# Squash last 3 commits\ngit rebase -i HEAD~3 # mark 2nd and 3rd as 'squash'\n\n# Fix commit message\ngit rebase -i HEAD~1 # mark as 'reword'\n\n# Cherry-pick a specific commit\ngit cherry-pick \n\n# Reset (CAREFUL)\ngit reset --soft HEAD~1 # undo commit, keep staged\ngit reset --mixed HEAD~1 # undo commit, keep working dir\ngit reset --hard HEAD~1 # undo commit AND changes\n\n# Recover lost commits\ngit reflog # shows all HEAD movements\ngit checkout # restore to any point\n```\n\n```python\nimport subprocess\n\ndef git(cmd):\n r = subprocess.run(f'git {cmd}', shell=True, capture_output=True, text=True)\n return r.stdout.strip()\n\nprint('=== Commit Graph ===')\nprint(git('log --oneline --graph -10'))\nprint()\nprint('=== Reflog (last 5 entries) ===')\nprint(git('reflog --oneline -5'))\n```\n\n## Rebase and Interactive Rebase\n\n```bash\n# Rebase feature onto latest main\ngit switch feature/auth\ngit rebase main # replay feature commits on top of main\n\n# Interactive rebase: clean up commits before PR\ngit rebase -i HEAD~4 # edit last 4 commits\n# In editor:\n# pick a1b2c3d First commit\n# squash b2c3d4e WIP: fix typo \u2190 squash into previous\n# reword c3d4e5f Add validation \u2190 reword message\n# drop d4e5f6a debug logging \u2190 remove this commit\n\n# Fixup commit: add a fix to an earlier commit\ngit add .\ngit commit --fixup=HEAD~2 # create fixup commit\ngit rebase -i --autosquash HEAD~4 # auto-squash fixups\n```\n\n```python\nrebase_workflow = '''\nRebase vs Merge comparison:\n\nMerge (preserves history):\n A---B---C (feature)\n \\\n D---E---F---G (main after merge)\n + Clear history of when features were merged\n - Noisy history with merge commits\n\nRebase (linear history):\n D---E---A\\'---B\\'---C\\' (rebased feature on main)\n + Clean, linear history\n + Easy to bisect bugs\n - Rewrites commits (never rebase shared branches!)\n\nGolden rule: NEVER rebase branches that others have pulled from.\n Only rebase local branches or feature branches before merging.\n\nInteractive rebase use cases:\n 1. Squash \"WIP\" commits before PR\n 2. Reorder commits for logical grouping\n 3. Edit commit messages\n 4. Split a large commit into smaller ones\n 5. Remove accidental debug commits\n'''\nprint(rebase_workflow)\n```\n\n```python\n# Verification helpers \u2014 exercise checker\n\ndef assert_equal(got, want, label=\"\"):\n status = \"\u2713\" if got == want else f\"\u2717 got={got!r} want={want!r}\"\n print(f\" {label or 'test'}: {status}\")\n\ndef assert_approx(got, want, tol=1e-6, label=\"\"):\n diff = abs(got - want)\n status = \"\u2713\" if diff < tol else f\"\u2717 diff={diff:.2e}\"\n print(f\" {label or 'test'}: {status}\")\n\ndef run_tests(fn, cases):\n \"\"\"cases: list of (args_tuple, expected)\"\"\"\n passed = sum(1 for args, want in cases\n if (result := fn(*args) if isinstance(args, tuple) else fn(args)) == want\n or (print(f\" FAIL {fn.__name__}{args}: got {result!r}, want {want!r}\"), False))\n print(f\"{fn.__name__}: {passed}/{len(cases)} passed {'\u2713' if passed==len(cases) else '\u2717'}\")\n\n# \u2500\u2500\u2500 Example: test your implementations below \u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\n\ndef flatten(nested):\n \"\"\"Flatten arbitrarily nested lists.\"\"\"\n result = []\n for item in nested:\n if isinstance(item, list):\n result.extend(flatten(item))\n else:\n result.append(item)\n return result\n\nrun_tests(flatten, [\n (([1,[2,[3,[4]]],5],), [1,2,3,4,5]),\n (([],), []),\n (([[1],[2],[3]],), [1,2,3]),\n])\n\ndef chunk(lst, size):\n \"\"\"Split list into chunks of given size.\"\"\"\n return [lst[i:i+size] for i in range(0, len(lst), size)]\n\nrun_tests(chunk, [\n (([1,2,3,4,5], 2), [[1,2],[3,4],[5]]),\n (([1,2,3], 3), [[1,2,3]]),\n (([], 2), []),\n])\n```\n\n```python\n# Advanced: Git object model \u2014 blob, tree, commit simulation\nimport hashlib, json\nfrom dataclasses import dataclass, field\nfrom typing import Optional\n\ndef sha1(content: str) -> str:\n return hashlib.sha1(content.encode()).hexdigest()[:8]\n\n@dataclass\nclass GitObject:\n kind: str # blob | tree | commit\n content: str\n hash: str = field(init=False)\n def __post_init__(self):\n self.hash = sha1(f\"{self.kind}:{self.content}\")\n\nclass GitRepo:\n def __init__(self):\n self.objects: dict[str, GitObject] = {}\n self.refs: dict[str, str] = {\"HEAD\": None}\n self.index: dict[str, str] = {} # filename \u2192 blob hash\n\n def hash_object(self, kind: str, content: str) -> str:\n obj = GitObject(kind=kind, content=content)\n self.objects[obj.hash] = obj\n return obj.hash\n\n def add(self, filename: str, content: str) -> str:\n blob_hash = self.hash_object(\"blob\", content)\n self.index[filename] = blob_hash\n print(f\" staged {filename} \u2192 blob {blob_hash}\")\n return blob_hash\n\n def commit(self, message: str, author: str = \"dev\") -> str:\n tree_content = json.dumps(dict(sorted(self.index.items())))\n tree_hash = self.hash_object(\"tree\", tree_content)\n parent = self.refs.get(\"HEAD\")\n commit_data = json.dumps({\"tree\": tree_hash, \"parent\": parent,\n \"author\": author, \"message\": message})\n commit_hash = self.hash_object(\"commit\", commit_data)\n self.refs[\"HEAD\"] = commit_hash\n print(f\" commit {commit_hash} '{message}' tree={tree_hash}\")\n return commit_hash\n\n def log(self):\n cur = self.refs.get(\"HEAD\")\n while cur:\n obj = self.objects.get(cur)\n if not obj: break\n data = json.loads(obj.content)\n print(f' commit {cur} \"{data['message']}\" tree={data['tree']}')\n cur = data.get(\"parent\")\n\nrepo = GitRepo()\nprint(\"=== git add ===\")\nrepo.add(\"README.md\", \"# My Project\")\nrepo.add(\"main.py\", \"print('hello')\")\nprint(\"=== git commit ===\")\nrepo.commit(\"Initial commit\")\n\nrepo.add(\"main.py\", \"print('hello world')\") # modify file\nrepo.commit(\"Update main.py\")\n\nprint(\"=== git log ===\")\nrepo.log()\nprint(f\"=== Objects stored: {len(repo.objects)} ===\")\nfor h, obj in list(repo.objects.items())[:6]:\n print(f\" {h} {obj.kind}\")\n```\n\n## Git Quick Reference\n\n```bash\n# Daily workflow\ngit status -s # compact status\ngit diff --staged # review before commit\ngit commit -m \"type: description\" # conventional commit\ngit push -u origin feature/name # first push\n\n# Branching\ngit switch -c feature/name # create + switch\ngit switch main && git pull # update main\ngit rebase main # rebase feature onto main\ngit merge --no-ff feature/name # merge with history\n\n# Undoing things (safe)\ngit restore # discard working tree changes\ngit restore --staged # unstage\ngit revert HEAD # new commit that undoes last\n\n# History inspection\ngit log --oneline --graph --all # visual branch graph\ngit log -p --follow -- # file history with diffs\ngit blame -L 10,20 # who changed lines 10-20\ngit bisect start # binary search for bug commit\n\n# Stash\ngit stash push -m \"WIP: feature\"\ngit stash list\ngit stash pop # restore latest\n\n# Remote\ngit remote -v\ngit fetch --prune # remove stale remote branches\ngit pull --rebase origin main # rebase instead of merge pull\n```\n\n**Commit types (Conventional Commits):**\n`feat` \u00b7 `fix` \u00b7 `refactor` \u00b7 `docs` \u00b7 `test` \u00b7 `chore` \u00b7 `perf` \u00b7 `ci`\n\n## Failure Modes\n\n- **Rebasing pushed commits**: rewrites SHA \u2014 anyone who pulled the old commits will have diverged history; use `git push --force-with-lease` (not `--force`) if you must\n- **Losing commits during rebase**: use `git reflog` to find lost commits \u2014 they're still in the object database for 90 days\n\n## Exercises\n1. Squash 5 WIP commits into 1 meaningful commit using interactive rebase\n2. Split a commit that changed 3 unrelated files: `git rebase -i HEAD~1` \u2192 `edit` \u2192 `git reset HEAD^` \u2192 stage and commit separately\n3. Use `git bisect` to find which commit introduced a bug: `git bisect start`, `git bisect good v1.0`, `git bisect bad HEAD`\n\n## Key Takeaways\n\n- **`git rebase -i HEAD~N`**: interactive rebase lets you squash, reorder, edit, or drop the last N commits before opening a PR \u2014 the standard way to clean up WIP history.\n- **`squash` vs `fixup`**: `squash` merges commits and lets you edit the combined message; `fixup` silently discards the squashed commit's message \u2014 use fixup for typo fixes.\n- **Never rebase public branches**: rebase rewrites SHA hashes \u2014 anyone who pulled the old commits will have diverged history requiring a force push.\n- **`git push --force-with-lease`**: safer than `--force` \u2014 fails if someone else has pushed since your last fetch, preventing accidental overwrites of others' work.\n- **`git reflog` recovery**: squashed or reset commits remain in the object database for 90 days \u2014 `git reflog` shows every HEAD position, enabling recovery with `git checkout `.\n\n## \ud83e\udde0 Quick Quiz\n\n**Q1.** Interactive rebase (`git rebase -i`) allows:\n- A) Merging branches interactively\n- B) **Reordering, squashing, editing, and dropping commits** \u2713\n- C) Resolving merge conflicts visually\n- D) Staging individual lines\n\n**Q2.** `git rebase` should not be used on:\n- A) Local feature branches\n- B) **Public/shared branches \u2014 it rewrites history that others may have based work on** \u2713\n- C) Experimental branches\n- D) Branches with a single commit\n\n**Q3.** `git reflog` is useful for:\n- A) Viewing the remote repository log\n- B) **Recovering lost commits after a reset or rebase** \u2713\n- C) Listing all branches\n- D) Showing file change history\n\n
Answers1-B, 2-B, 3-B
\n" }