Q: What is `git fsck` and when would you use it?

git fsck (file system check) verifies the integrity of the Git object database — it walks every object and checks that nothing is corrupted, missing, or orphaned.

• Every object's SHA matches its content (detects corruption).
• Every commit's parent exists.
• Every tree's referenced blobs and sub-trees exist.
• No invalid object types or malformed headers.
• dangling commit — a commit not reachable from any branch or tag. Often from amended commits, rebases, or deleted branches. Harmless; git gc cleans them up.
• dangling blob — file content not referenced by any tree. Usually from staged-but-uncommitted files.

What it checks: What it reports: When to use it: Recovery from corruption: if fsck reports missing objects, try re-fetching from the remote (git fetch origin). If the remote has the objects, they'll fill the gaps. For truly lost objects, restore from a backup or re-clone.

git fsck --full

• missing object — an object referenced but not found. This is actual corruption.
• broken link — a tree or commit references an object that doesn't exist.
• After a disk failure or unclean shutdown — verify the repo isn't corrupted.
• Recovering lost commits — git fsck --unreachable lists orphaned commits you might want to rescue (alternative to reflog).
• Debugging weird Git errors — "fatal: bad object" errors often point to corruption that fsck can diagnose.