Diagnosing corruption

Repositories corrupt through power loss, disk failure, partial network sync, or buggy non-Git tools touching .git. Symptoms range from "fatal: bad object" to wrong content silently delivered. Diagnosis starts with fsck:

git fsck --full
git fsck --full --strict --no-dangling
git fsck --connectivity-only
git fsck --lost-found

--lost-found writes orphaned objects to .git/lost-found/ for manual inspection.

Common failures and fixes

Loose object corrupted

If fsck reports broken link from ... to blob <sha>, see if a colleague's clone has the object:

# on healthy clone:
git cat-file -p <sha> > /tmp/recovered
# on corrupt clone:
cd .git/objects/<xx>
mv <rest-of-sha> <rest-of-sha>.bad
git hash-object -w /tmp/recovered

Pack file corrupted

Find the bad pack:

for p in .git/objects/pack/pack-*.pack; do
  git verify-pack -v "$p" >/dev/null 2>&1 || echo "BAD: $p"
done

Then either remove the bad pack and refetch:

rm .git/objects/pack/pack-<sha>.{pack,idx,bitmap,rev}
git fetch --all
git repack -ad

Reflog corrupted or missing

The reflog is per-clone; cannot recover from upstream. If lost, missing branches must be reconstructed from FETCH_HEAD or by asking colleagues for their tip SHAs.

Index corrupted

rm .git/index
git reset                    # rebuilds index from HEAD

Refs corrupted

git pack-refs --all
cat .git/packed-refs                  # readable text
git update-ref refs/heads/main <sha>  # recreate from known SHA

Repository-wide recovery from a clone

Often the fastest path is to clone fresh from a healthy remote, then copy local-only branches:

git clone https://example.com/repo.git fresh
cd corrupt
git push ../fresh 'refs/heads/local-only:refs/heads/local-only'

Prevention

• Run git fsck regularly; git maintenance can include it.
• Store bare backups: git clone --mirror on a separate disk.
• Avoid editing inside .git/ by hand.
• Use ECC RAM and reliable storage on long-lived servers.

Common mistakes

Running git gc on a corrupt repo — pruning may delete the only copies of recoverable objects. Always backup .git before recovery. Mixing recovered objects from different SHA-1 algorithms; SHA-256 repos cannot accept SHA-1 objects.

SHA-1 collision protection

Modern Git includes SHAttered hardening, refusing to write objects whose hashes match the known collision pair. Verify your build:

git --version
echo "test" | git hash-object --stdin

Related

See "Git garbage collection: gc, prune, and pack-refs", "Git internals: the object database", and "Repository size analysis: git count-objects and git sizer".