How to fix error: unable to read sha1 file of file.txt in Git

First, identify the extent of the corruption by running Git's file system check:

git fsck --full

This will report all corrupted, missing, or dangling objects. Common output includes:
- broken link from tree <sha1> to blob <sha1> - a tree references a missing file
- missing blob <sha1> - a file object is missing
- invalid sha1 pointer - a reference points to nothing

For more detail about which files are affected:

git fsck --full --name-objects

If you have a remote repository (GitHub, GitLab, etc.), Git may be able to recover the missing objects:

git fetch --all

Or fetch with full depth if you have a shallow clone:

git fetch --unshallow

After fetching, run git fsck --full again to check if the issue is resolved.

Sometimes Git creates 0-byte object files that block operations. Find and remove them:

# Find empty files in .git/objects (Linux/macOS)
find .git/objects -type f -size 0

# Remove empty files (verify the list first!)
find .git/objects -type f -size 0 -delete

After removing empty files, fetch from remote again:

git fetch origin

This allows Git to re-download the missing objects.

If you have a backup of the repository or another working clone, you can extract the missing objects:

From a packed backup:

# In the backup repository
git cat-file blob <missing-sha1> > recovered-file.dat

# In the damaged repository
git hash-object -w recovered-file.dat

Copy objects directly:

# Copy missing object from backup (adjust path based on SHA-1)
# For sha1 = abc123..., copy from .git/objects/ab/c123...
cp /path/to/backup/.git/objects/ab/c123... .git/objects/ab/

If the corruption is in the index (staging area) rather than the objects, reset it:

# Remove the index file
rm .git/index

# Rebuild it from HEAD
git reset HEAD

For cache-tree corruption specifically:

git update-index --refresh

If the corruption is severe, the safest approach is to clone fresh and copy your work:

# Clone the repository fresh
git clone <remote-url> fresh-repo

# Copy your uncommitted changes (excluding .git)
rsync -av --exclude='.git' ./damaged-repo/ ./fresh-repo/

# Or manually copy changed files
cp -r src/ fresh-repo/src/

Then use the fresh repository going forward and verify with git status.

The git-repair tool can automatically fix many types of repository corruption:

Install git-repair:

# Debian/Ubuntu
sudo apt install git-repair

# macOS with Homebrew
brew install git-repair

Run the repair:

git-repair

This tool will attempt to fetch missing objects from remotes and repair references automatically.

After recovery, confirm the repository is fully functional:

# Full integrity check
git fsck --full

# Test common operations
git status
git log --oneline -10
git branch -a

If git fsck reports no errors, the repository has been successfully repaired.