File Type Defaults

Set default applications for file types on macOS using the utiluti command-line tool.

Prerequisites

Ensure utiluti is installed:

brew install utiluti

Instructions

Recommended Approach: File-based Method

The most reliable method is to use utiluti file set with a sample file. This works even for extensions with dynamic UTIs.

Step 1: Get application bundle ID

osascript -e 'id of app "Application Name"'

Step 2: Set default using an existing file

# Use an existing file of that type (preferred)
utiluti file set /path/to/existing/file.<extension> <bundle-id>

# OR create a sample file if needed
touch /tmp/sample.<extension>
utiluti file set /tmp/sample.<extension> <bundle-id>

Example:

osascript -e 'id of app "Cursor"'
# Returns: com.todesktop.230313mzl4w4u92

# Using an existing .conf file
utiluti file set ~/.dotfiles/tmux/tmux.conf com.todesktop.230313mzl4w4u92

# Or using a temporary sample file
touch /tmp/sample.conf
utiluti file set /tmp/sample.conf com.todesktop.230313mzl4w4u92

Alternative: UTI-based Method (for well-known types)

This method works for file types with standard, non-dynamic UTIs.

Step 1: Get UTI for file extension

utiluti get-uti <extension>

Example:

utiluti get-uti txt
# Returns: public.plain-text

utiluti get-uti md
# Returns: net.daringfireball.markdown

utiluti get-uti pdf
# Returns: com.adobe.pdf

Step 2: Set default application

utiluti type set <uti> <bundle-id>

Example:

utiluti type set public.plain-text com.barebones.bbedit

Note: Some file extensions (like .conf) have dynamic UTIs (e.g., dyn.ah62d4rv4ge80g55sq2) that cannot be set using utiluti type set. Use the file-based method instead.

Finding application bundle IDs

Use osascript to find an application's bundle ID:

osascript -e 'id of app "Application Name"'

Examples:

osascript -e 'id of app "Visual Studio Code"'
# Returns: com.microsoft.VSCode

osascript -e 'id of app "BBEdit"'
# Returns: com.barebones.bbedit

osascript -e 'id of app "Preview"'
# Returns: com.apple.Preview

Common Use Cases

Configuration files (.conf) to Cursor

# Using an existing .conf file (recommended)
utiluti file set ~/.dotfiles/tmux/tmux.conf com.todesktop.230313mzl4w4u92

# Or using a temporary file
touch /tmp/sample.conf
utiluti file set /tmp/sample.conf com.todesktop.230313mzl4w4u92

Text files to BBEdit (UTI method)

utiluti get-uti txt
utiluti type set public.plain-text com.barebones.bbedit

Markdown files to Visual Studio Code (UTI method)

utiluti get-uti md
utiluti type set net.daringfireball.markdown com.microsoft.VSCode

JSON files to Cursor (UTI method)

utiluti get-uti json
utiluti type set public.json com.todesktop.230313mzl4w4u92

Python files to Cursor (file method for reliability)

touch /tmp/sample.py
utiluti file set /tmp/sample.py com.todesktop.230313mzl4w4u92

Verification

After setting a default, verify by:

1. Right-clicking a file of that type in Finder
2. Checking "Open With" → default should show your selected app
3. Or use command-line verification:

   # For UTI-based settings
   utiluti type get <uti>
   
   # For file-based settings
   utiluti file app /path/to/sample.file
   

Reference

• utiluti GitHub: https://github.com/scriptingosx/utiluti
• List of common UTIs: https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html

Notes

• Changes take effect immediately for new file operations
• Prefer using existing files over temporary files - this ensures the exact file type is matched
• Some file types may require logging out and back in for changes to fully propagate
• System applications (like TextEdit) might override custom settings for certain types
• When in doubt, use the file-based method - it works universally for all file extensions, including those with dynamic UTIs
• The file-based method (utiluti file set) sets the association system-wide for that extension
• If you still get "No application knows how to open" errors after setting the association:
  • Try right-clicking the file in Finder → "Get Info" → Change "Open with:" → Click "Change All..."
  • Or try opening via right-click → "Open With" → select the app
  • The Launch Services database may need time to update