How to Create a DMG File for .app on Mac

Creating a professional DMG file for your Mac application not only makes installation easier for your users but also adds a polished touch with custom branding, security, and convenience. In this guide, we’ll walk you through the step-by-step process to create a DMG file for .app on Mac using only the command line, without any third-party tools. This includes how to add a custom icon, set up your folders, and ensure your DMG file is secure for distribution.

What is a DMG file?

A DMG file is a disk image format commonly used on macOS to distribute software and files. It acts like a virtual disk, allowing users to mount and access its contents as if they were on a physical drive. DMG files are often used for application installers because they can be compressed, encrypted, and made read-only, providing both security and convenience for software distribution.

Steps to Create a DMG File

Step 1: Prepare Your Custom Icon

Adding an ICNS file is not strictly required, but it gives your DMG a custom, branded icon when mounted. This makes your installer look more professional, reinforces your brand, and creates a better first impression for users.

1. Get an Icon

Prepare your icon file in PNG format (1024*1024 pixels is recommended) with a transparent background.

2. Generate ICNS File

• Open Applications > TextEdit.
• In the menu bar, select Format > Make Plain Text (or use Shift + Command + T).
• Paste the following script:

#!/bin/bash

# Auto-generate icon.icns from icon.png (1024x1024)

ICON_NAME=icon

ICONSET_NAME=${ICON_NAME}.iconset

ICNS_NAME=${ICON_NAME}.icns

 

if [ ! -f ${ICON_NAME}.png ]; then

  echo "Please put ${ICON_NAME}.png in the current directory."

  exit 1

fi

 

mkdir -p $ICONSET_NAME

sips -z 16 16     ${ICON_NAME}.png --out ${ICONSET_NAME}/icon_16x16.png

sips -z 32 32     ${ICON_NAME}.png --out ${ICONSET_NAME}/[email protected]

sips -z 32 32     ${ICON_NAME}.png --out ${ICONSET_NAME}/icon_32x32.png

sips -z 64 64     ${ICON_NAME}.png --out ${ICONSET_NAME}/[email protected]

sips -z 128 128   ${ICON_NAME}.png --out ${ICONSET_NAME}/icon_128x128.png

sips -z 256 256   ${ICON_NAME}.png --out ${ICONSET_NAME}/[email protected]

sips -z 256 256   ${ICON_NAME}.png --out ${ICONSET_NAME}/icon_256x256.png

sips -z 512 512   ${ICON_NAME}.png --out ${ICONSET_NAME}/[email protected]

sips -z 512 512   ${ICON_NAME}.png --out ${ICONSET_NAME}/icon_512x512.png

sips -z 1024 1024 ${ICON_NAME}.png --out ${ICONSET_NAME}/[email protected]

iconutil -c icns $ICONSET_NAME

rm -rf $ICONSET_NAME

echo "ICNS file created: ${ICNS_NAME}"

• Save the file as make_icns.sh (make sure the option If no extension is provided, use “.txt” is deselected). Click here to download make_icns.sh.
• Place both make_icns.sh and your icon.png in the same folder.
• Open Terminal, navigate to that folder, and run:

chmod +x make_icns.sh
./make_icns.sh

Then the file Icon.icns will be generated in the same folder.

Step 2: Organize Your Folders

Organizing all necessary files into a dedicated folder ensures that your DMG package is tidy and user-friendly. This approach not only helps users find everything they need quickly but also minimizes installation errors and enhances the overall professionalism of your app distribution.

1. Create Folders

On your Desktop, right-click and create two folders: dmg-temp and dmg-final.

• dmg-temp is used to store all source files of the DMG file.
• dmg-final is used to store the generated DMG file.

2. Prepare the Applications Alias

• In Finder, click Go > Go to Folder (Command + Shift + G), enter /, and open the root directory.
  
• Right-click the Applications folder and select Make Alias.
  
• Drag the Applications alias (with the shortcut arrow) into the dmg-temp folder.

3. Copy .app

• Open the Applications folder.
• Right-click your .app and select Copy.
  
• Open the dmg-temp folder, right-click, and select Paste Item.

4. Add the Custom Icon

• Copy your VolumeIcon.icns file into the dmg-temp folder.
• To set it as the DMG volume icon, open Terminal and run:

cd ~/Desktop/dmg-temp
mv VolumeIcon.icns .VolumeIcon.icns
setfile -a CV .VolumeIcon.icns

You may need to install Xcode Command Line Tools to use setfile.

Step 3: Create the DMG File

First, create a writable DMG from your prepared folder so you can make final adjustments, such as customizing the layout or adding icons. Once everything looks right, convert it to a compressed, read-only DMG to ensure your installer is secure, professional, and ready for distribution.

1. Open Terminal to create a writable DMG:

hdiutil create -srcfolder ~/Desktop/dmg-temp -volname WiseX -fs HFS+ -format UDRW ~/Desktop/WiseX.dmg

• -srcfolder specifies the source folder.
• -volname sets the mounted volume name.
• -fs HFS+ specifies the file system (recommended for most uses).
• -format UDRW creates a writable DMG.
• WiseX.dmg is the output file.

2. Mount the DMG

Double-click WiseX.dmg to mount it.

3. Apply the Custom Icon

• Right-click the mounted WiseX drive icon on your Desktop and choose Get Info.
  
• Double-click the mounted WiseX drive, and press Command + Shift + . to show hidden file .VolumeIcon.icns.
• Drag .VolumeIcon.icns to the top-left icon in the Info window to set it as the volume icon.
  
• Close the Info window and eject the WiseX drive.

4. Finalize & Compress the DMG

Open Terminal, and convert the DMG to a compressed, read-only format for release.

hdiutil convert ~/Desktop/WiseX.dmg -format UDZO -o ~/Desktop/dmg-final/WiseX-final.dmg

• UDZO is the compressed, read-only format recommended for distribution.

Step 4: Add Security with SHA256 Checksum

Adding a security checksum to your DMG file is important because it verifies the integrity of your installer, reassuring users that the file hasn’t been tampered with and ensuring a safe installation process.

Open a terminal and run the following command to generate a SHA256 checksum to verify the integrity and authenticity of the DMG file.

shasum -a 256 ~/Desktop/dmg-final/WiseX-final.dmg

Then publish the checksum alongside your DMG so users can confirm it hasn’t been tampered with.

Bottomline

Creating a custom DMG file on Mac is a straightforward process that ensures your application looks professional and is secure for your users. By following these steps, preparing a custom icon, organizing your folders, creating and customizing the DMG, and adding a security checksum, you'll deliver a polished installation experience that builds trust and reinforces your brand. Whether you're a developer or a tech enthusiast, mastering DMG creation is a valuable skill for Mac software distribution.

Reference Documents

Apple's official HDIUtil command manual