How to fix broken link from tree abc1234 to tree def5678 in Git

Before attempting any repairs, make a complete backup of your repository in its current state:

# Create a tarball backup
tar -czvf repo-backup-$(date +%Y%m%d).tar.gz /path/to/your/repo

# Or use rsync for a copy
rsync -av /path/to/your/repo/ /path/to/backup/repo-copy/

This ensures you can recover if the repair process makes things worse.

Get detailed information about what's broken using git fsck with the --name-objects flag (Git 2.10+):

git fsck --full --name-objects

This will show you exactly which refs lead to the broken objects:

broken link from tree b5eb6ff... (refs/heads/main~37:src/) to tree ec5cf80...
missing tree ec5cf80...

The output tells you:
- Which tree contains the broken reference
- How to reach it (e.g., through refs/heads/main)
- The hash of the missing object

If the repository has a remote origin, try fetching all objects fresh:

# Fetch all branches and tags from all remotes
git fetch --all

# Git 2.36+: Force re-fetch all objects like a fresh clone
git fetch --refetch

If the remote has the missing objects, this may resolve the issue. Run git fsck again to verify.

If you have a backup or another clone of the repository:

# Copy all objects from the backup into your corrupted repo
cp -r /path/to/backup/.git/objects/* /path/to/corrupted/.git/objects/

# Alternatively, unpack objects from a fresh clone's pack files
cd /path/to/corrupted/
git unpack-objects < /path/to/fresh-clone/.git/objects/pack/pack-*.pack

After copying, run git fsck to verify the objects are restored.

If you can find a working reference to the parent tree, list its contents:

# Find what the parent tree should contain
git ls-tree <parent-tree-sha>

This shows you the expected contents, which can help you understand what was lost and potentially reconstruct it.

The git-repair tool can automatically attempt to recover corrupted repositories:

# Install git-repair (Ubuntu/Debian)
sudo apt install git-repair

# Or on macOS with Homebrew
brew install git-repair

# Run the repair tool
cd /path/to/corrupted-repo
git-repair

git-repair will:
1. Delete corrupt objects
2. Retrieve missing objects from remotes
3. Optionally reset branches if recovery isn't possible

After running, use git fsck to verify the repair was successful.

For simple cases where you know the tree's contents, you can recreate it:

# Example: Recreating a tree with a single .gitkeep file
echo -e "100644 blob e69de29bb2d1d6434b8b29ae775ad8c2e48c5391\t.gitkeep" | git mktree

This outputs the new tree's SHA. If it matches the missing tree's expected SHA, the object is restored.

Note: This only works if you know exactly what the tree should contain and all referenced blobs already exist.

If all else fails, create a fresh clone and preserve your working directory:

# Save your working directory files
cp -r /path/to/corrupted-repo /path/to/working-files-backup
rm -rf /path/to/working-files-backup/.git

# Fresh clone
git clone <remote-url> /path/to/fresh-repo

# Copy your working files into the fresh clone
cp -r /path/to/working-files-backup/* /path/to/fresh-repo/

# Check status and commit any changes
cd /path/to/fresh-repo
git status
git diff

This approach loses any unpushed commits and stashes, but preserves your current working directory changes.