What is dmg?
A DMG (short for Disk Image) is a single file that acts like a virtual hard drive on macOS. It stores the exact copy of a folder, program, or entire disk, and can be opened (mounted) to view or run its contents just like a physical disk.
Let's break it down
- File extension: .dmg
- Container: Holds files and folder structure inside one package.
- Compression: Often shrinks the data to make downloads smaller.
- Encryption: Can be password‑protected to keep the data safe.
- Mounting: When you double‑click it, macOS creates a temporary drive on your desktop where you can access the files.
Why does it matter?
DMG files make it easy to share software and large collections of files without losing the original folder layout. They also let developers protect their apps with encryption and ensure the download isn’t corrupted, because the whole image can be verified with a checksum.
Where is it used?
- Distributing macOS applications and updates.
- Providing installers for games, utilities, and developer tools.
- Creating backups of folders or entire drives.
- Sending large sets of files (e.g., media assets) over the internet.
Good things about it
- Convenient: One file contains everything you need.
- Secure: Optional password protection and encryption.
- Compressed: Saves bandwidth and storage space.
- Integrity‑checked: macOS can verify the image isn’t damaged before mounting.
- Native: Works seamlessly on any Mac without extra software.
Not-so-good things
- Mac‑only: Windows and Linux can’t open DMGs without third‑party tools.
- Mount time: Large, compressed images may take a few seconds to mount.
- Size: If not compressed, a DMG can be as big as the original data.
- Limited editing: Modifying a DMG after it’s created often requires special utilities.