How to add text to disk image?

Adding Text and Files to a macOS Disk Image (DMG)

Learn how to customize macOS Disk Images (DMG files) by embedding text, READMEs, and other essential files directly into the image, enhancing user experience and distribution.

Disk Images (DMG files) are a common way to distribute software and data on macOS. While they typically contain application bundles, you might also want to include supplementary information like a README.txt file, licensing agreements, or custom instructions directly within the mounted image. This article will guide you through the process of adding text and other files to an existing or new DMG, ensuring your users have all the necessary information at their fingertips.

Understanding DMG Structure and Customization

A DMG file is essentially a container that, when mounted, appears as a volume on your macOS desktop. This volume behaves much like a regular folder, allowing you to add, remove, and modify its contents. The key to customizing a DMG with text or other files lies in manipulating its contents before the image is finalized. This typically involves creating a temporary read/write disk image, adding your desired content, and then converting it to a read-only, compressed format for distribution.

flowchart TD
    A[Start: Create/Open DMG] --> B{Mount DMG as Read/Write}
    B --> C[Add Text/Files to Mounted Volume]
    C --> D[Unmount Volume]
    D --> E[Convert to Read-Only/Compressed DMG]
    E --> F[End: Distributable DMG]

Workflow for adding content to a macOS Disk Image.

Method 1: Using Disk Utility and Finder (GUI Approach)

For those who prefer a graphical interface, macOS's built-in Disk Utility and Finder provide a straightforward way to add content to a DMG. This method is ideal for simple additions like a single text file or a small set of documents.

1. Create a New Blank Disk Image

Open Disk Utility (Applications > Utilities > Disk Utility). Go to File > New Image > Blank Image. Configure the image with a suitable name, size (ensure it's large enough for your content), and format (e.g., APFS or Mac OS Extended (Journaled)). Choose 'read/write disk image' for the Image Format. Save it to your desired location.

2. Mount the Disk Image

Double-click the newly created .dmg file. It will mount as a volume on your desktop and in Finder's sidebar.

3. Add Your Text/Files

Drag and drop your README.txt file, other documents, or even application bundles directly into the mounted volume's window in Finder. Arrange them as you wish.

4. Unmount the Disk Image

Eject the mounted volume by dragging it to the Trash, clicking the Eject icon in Finder, or right-clicking and selecting 'Eject'.

5. Convert to Read-Only (Optional but Recommended)

Go back to Disk Utility. Select the .dmg file from the sidebar. Click 'Image' in the menu bar, then 'Convert...'. Choose a new name and location for the converted image. For 'Image Format', select 'compressed' to make it read-only and reduce its size. Click 'Convert'.

Method 2: Using the Command Line (hdiutil)

For automation, scripting, or more granular control, the hdiutil command-line tool is invaluable. This method is particularly useful for developers or system administrators who frequently create and modify DMGs.

1. Create a Read/Write Disk Image

Open Terminal (Applications > Utilities > Terminal). Use the hdiutil create command to create a new read/write disk image. For example, to create a 100MB image named MySoftware.dmg:

2. Mount the Disk Image

Mount the newly created image using hdiutil attach. The output will show the mount point (e.g., /Volumes/MySoftware).

3. Add Your Text/Files

Copy your files into the mounted volume using the cp command. For example, to add a README.txt file:

4. Unmount the Disk Image

Eject the volume using hdiutil detach.

5. Convert to Read-Only and Compress

Convert the read/write image to a compressed, read-only format for distribution. This also reduces the file size.

hdiutil create -size 100m -fs APFS -volname "MySoftware" -type UDRW MySoftware_rw.dmg

hdiutil attach MySoftware_rw.dmg

cp README.txt /Volumes/MySoftware/
cp -R MyApplication.app /Volumes/MySoftware/

hdiutil detach /Volumes/MySoftware

hdiutil convert MySoftware_rw.dmg -format UDBZ -o MySoftware.dmg

Command-line sequence for creating, populating, and converting a DMG.

Adding Background Images and Custom Icons (Advanced)

Beyond just adding text files, you can further customize your DMG's appearance by adding a background image to the mounted volume's Finder window and setting custom icons for the volume itself. This often involves using AppleScript or specialized tools like create-dmg (a popular open-source script) to automate the process and ensure proper layout.

While hdiutil handles the core disk image operations, tools like create-dmg build upon it to add visual flair. These tools typically mount the DMG, set Finder view options (like background image, icon size, window position), and then unmount and convert the image. This level of customization is beyond simply adding text but is a common next step for professional software distribution.