How to fix error: unable to index file 'file.txt' in Git

The most common cause is another program holding a lock on the file. Close any applications that might have the file open:

# Check what processes might be using the file (Linux/macOS)
lsof <filename>

# On Windows, use Process Explorer or:
# Task Manager -> Details tab -> look for the application

Common culprits:
- IDEs: VS Code, Visual Studio, JetBrains products
- Office applications: Excel, Word (especially for .xlsx, .docx files)
- Git GUIs: SourceTree, GitKraken, GitHub Desktop

After closing the application, retry your git add command.

If a previous Git operation was interrupted (Ctrl+C, crash, power outage), it may have left behind a lock file:

# Check if lock file exists
ls -la .git/index.lock

# If it exists and no Git process is running, remove it
rm .git/index.lock

On Windows:

del .git\index.lock

Important: Only remove this file if you're certain no other Git operation is running. Check for running Git processes first:

# Linux/macOS
ps aux | grep git

# Windows (PowerShell)
Get-Process | Where-Object {$_.ProcessName -like "*git*"}

Antivirus software can lock files during scanning, causing Git operations to fail. Try temporarily disabling real-time protection:

Windows Defender:
1. Open Windows Security
2. Go to Virus & threat protection
3. Click "Manage settings"
4. Toggle off "Real-time protection"

Better long-term solution: Add exclusions for your development directories:

# Typical paths to exclude:
C:\Users\YourName\Projects\
C:\Users\YourName\Projects\**\.git\

Remember to re-enable protection after testing if this was the cause.

If your repository is in a folder synced by Dropbox, Google Drive, OneDrive, or similar services, they may interfere with Git operations:

1. Pause syncing in the cloud sync application
2. Retry your Git operation
3. Resume syncing after the operation completes

Better approach: Move your Git repositories outside of cloud-synced folders. Use remote repositories (GitHub, GitLab) for backup instead.

# Move repo out of synced folder
mv ~/Dropbox/my-project ~/Projects/my-project
cd ~/Projects/my-project
git add .

If the error mentions "Permission denied", you may need to fix ownership or permissions:

Linux/macOS:

# Check current ownership
ls -la .git/

# Fix ownership (replace 'yourusername' with your actual username)
sudo chown -R $(whoami):$(whoami) .git/

# Ensure write permissions
chmod -R u+w .git/

Windows (run as Administrator):

icacls .git /grant %USERNAME%:F /T

Note: Avoid using sudo with Git commands, as it can create files owned by root that cause permission issues later.

If the index file is corrupted, you can rebuild it from the last commit:

# Make a backup first
cp .git/index .git/index.backup

# Remove the corrupted index
rm .git/index

# Rebuild from HEAD (last commit)
git reset

This recreates the index based on your last commit. Your working directory files remain unchanged, but you'll need to re-stage any changes you had previously staged.

Alternative using git read-tree:

rm .git/index
git read-tree HEAD

If you need to add multiple files and only some are problematic, you can skip the errors:

git add --ignore-errors .

This will add all files that can be added and report errors for the ones that fail, without aborting the entire operation.

Note: This is a workaround, not a fix. You should still investigate why specific files cannot be indexed.

On Windows, certain filenames are reserved and cannot be used, even in Git:

Reserved names: CON, PRN, AUX, NUL, COM1-COM9, LPT1-LPT9

If your file uses one of these names (e.g., aux.js, con.txt), you'll need to rename it:

# Example: rename aux.js to auxiliary.js
git mv aux.js auxiliary.js

If the file was created on Linux/macOS and you're cloning on Windows, you may need to rename it on the original system or use WSL.