Here’s a quick howto for people that need to distribute files to Mac users. I used to create .tgz files when I needed to transfer a large amount of data to another mac, but I recently found an easier way. With a little bit of scripting (or a lot of mouseclicks in Disk Utility) you can create a compressed disk image (.dmg file); these are easier to use if you need to send files to people who aren’t familiar with command-line utilities, and the resulting files are only slightly larger than gzipped ones.

Here are three ways to create compressed .dmg files on MacOS X:

Create a disk image with Disk Utility and compress it later

This is the way I originally did this:

  • Start Disk Utility
  • Click the “New Image” button
  • Choose how large you want the image
  • The image will automatically be mounted and visible in the Finder
  • Drag files to it
  • Go back to Disk Utility and click “Convert”
  • Select the image you created earlier; the default settings will compress this image
  • Enter a new name for the compressed image
  • Eject the uncompressed image in the Finder and remove it


This works, but it’s a bit more complicated than I like. So here’s an easier way:

Create a compressed disk image without intermediate steps

This is a lot easier than the first method, but most people are unaware of this option.

  • Create a folder containing all the files you want to add to the disk image
  • Start Disk Utility
  • Choose File -> New -> Disk Image from Folder from the menu
  • Select the folder containing your files
  • Enter the name of your disk image

Disk Utility will now create a compressed image containing the files in the folder you created in the first step.

Using the CLI

And of course there is an even easier way: use the command line. The hdiutil command can perform all the tasks above; here’s a simple script that gives an example of what you can do with it:

echo "Creating uncompressed disk image:"
hdiutil create -size $SIZE -fs HFS+J -volname $IMAGENAME \
echo "Mounting uncompressed disk image:"
hdiutil attach $IMAGENAME-temp.dmg -readwrite -mount required
echo "Image mounted. Drag files to it and press enter when ready. "
echo "Unmounting uncompressed image"
hdiutil detach /Volumes/$IMAGENAME/
echo "Creating compressed image"
hdiutil convert $IMAGENAME-temp.dmg -format UDZO \
    -imagekey zlib-level=9 -o $IMAGENAME.dmg
echo "Removing temporary image"
rm $IMAGENAME-temp.dmg
echo "Done"