How to work with DMG files on Linux

So you’re running Linux on your computer, maybe Ubuntu, and you have some files with the .dmg extension. In this guide, we’re going to talk about how to open, mount, extract, and otherwise get your files from these pesky DMG images. You could always just extract the files on a Mac, then transfer them back to your Linux machine. But if you really want to do this on Linux, without having to rely on Mac, here’s how to do it.

What are DMG image files?

Simply put, it’s a kind of image file. But not an image like a jpeg is an image. DMG is Apple’s proprietary disk image format, native to Mac OS X. There are actually a whole bunch of different types, format and options within this format. There are options for encryption, compression, and different kinds of partition schemes, among others. Unfortunately, this can make things pretty confusing when we’re trying to gain access to the data contained in one of these images.

DMG images are typically a kind of Universal Disk Image Format (UDIF), although there are others, namely NDIF and SPARSE. Although the .dmg file extension is usually used, they can also sometimes have an .img extension, or in some cases no extension at all. Their MIME type is application/x-apple-diskimage.

The HFS/HFS+ (Mac OS Extended/Journaled) file system is typically used in DMGs. However, this isn’t always the case. You may also sometimes find FAT and ExFAT files systems, as well as variations on HFS.

Does my system support DMG?

Perhaps the biggest hurdle to overcome when trying to work with DMG files is working with the HFS file system (Mac OS Extended). Linux supports HFS through the “hfs” and “hfsplus” kernel modules.

There’s an easy way to test if your system has these kernel modules. Plug in a USB drive formatted with the Mac OS Extended file system. If your particular distribution doesn’t have the appropriate modules, you will likely get an error message. On Ubuntu, you’ll get a popup window declaring “Ubuntu: Unable to mount ".

Alternatively, we can see if the kernel module files are present with find:

~$ find /lib/modules -name "hfs*.ko"
/lib/modules/3.13.0-32-generic/kernel/fs/hfs/hfs.ko
/lib/modules/3.13.0-32-generic/kernel/fs/hfsplus.ko

We want to see two files: “hfs.ko” and “hfsplus.ko”. If find doesn’t return these files, your system probably doesn’t support HFS.

You could also try “modinfo”: modinfo hfs and modinfo hfsplus should return something like:

~$ modinfo hfs
filename:    /lib/modules/.../fs/hfs/hfs.ko
aias:        fs-hfs
license:     GPL
srcversion:  A122BE...
depends:
intree:      Y
vermagic:    3...-generic SMP mod_unload modversions
...

If you get "modinfo: ERROR: Module hfsplus not found" your system doesn’t have these modules.

Not all Linux kernels and distributions support HFS. This is especially the case for certain distributions that are a few years old. If you have kernel support for HFS, great! If not, don’t worry. There are still ways to extract data from your DMG files. While it’s nice to have the option to mount the images we’re working with, this is really the only functionality we’re losing without having the hfs and hfsplus modules. The two programs we’re going to use later on (P7ZIP and dmg2img) do not require kernel support to function.

What kinds of DMG images can be opened in Linux?

This guide is about how to open, mount, and extract files from read/write, read only, and compressed DMG image files. The following partition schemes have all been tested with the techniques discussed here.

Apple Partition Map
CD/DVD (partitioned)
CD/DVD (partitioned) with ISO data
Hard disk
Master Boot Record Partition Map
No partition map

This guide does not cover how to handle sparse disk images (.sparseimage), sparse bundle disk images (.sparsebundle), or CD/DVD masters. DMG images with partition scheme types of “CD/DVD” and “GUID Partition Map” do not appear to work with the techniques described here.

Option 1: Mount the DMG

If the Linux distribution you’re on has HFS support in the kernel (Ubuntu 14.04.1 LTS supports it), it’s pretty easy to just mount the DMG image:

sudo mount -t hfsplus image.dmg /mnt

We’re using “sudo” because we need root privileges to mount things. The HFS+ file system type is specified with “-t hfsplus”. The “/mnt” at the end of the command specifies where we’re mounting the image.

Unmount the image with sudo umount /mnt

If you get a wrong fs type message like the one below, it means the DMG file is either of an unsupported type, or it’s compressed. Unsupported images include sparse images, sparse disk bundles, CD/DVD masters, and images with partition schemes of the CD/DVD or GUID Partition Map types.

mount: wrong fs type, bad option, bad superblock on /dev/loop0,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so

Use “file” to learn a little more about the image file:

file image.dmg

If you get image.dmg: x86boot sector that means it’s probably using a GUID Partition Map and isn’t supported. This isn’t good, however, it’s also not too terribly common.

What’s more common is to see something like this:

image.dmg: bzip2 compressed data

If mounting isn’t working, and this is what you’re seeing with “file image.dmg”, then you’re luck!. Our problems are being caused by compression. Linux doesn’t like to mount compressed DMG images. To get around this little obstacle, we’ll use dmg2img (see below).

Option 2: Use dmg2img for compressed images

So you have a DMG image that you can’t mount because it’s compressed. You’ve done “file compressed_image.dmg” and you got “compressed_image.dmg: bzip2 compressed data”. The fix? That’s easy: use dmg2img to convert it to an uncompressed image. Once you run the image through dmg2img you should be able to mount it no problem.

Don’t have dmg2img? It’s usually pretty easy to get using your distribution’s package management. On Ubuntu, you’d do:

sudo apt-get install dmg2img

Using dmg2img isn’t very difficult. Type “dmg2img” into the command line followed by the name of the DMG file you want to decompress. The Mac OS X version of Firefox is a good example of a compressed DMG file.

hfs-test$ dmg2img "Firefox 33.1.1.dmg"
dmg2img v1.6.5 ...
Firefox 33.1.1.dmg --> Firefox 33.1.1.img
decompressing:
opening partition 0 ...     100% ok
opening partition 1 ...     100% ok
opening partition 2 ...     100% ok
Archive successfully decompressed as Firefox 33.1.1.img

Now mount the resulting .img file:

mount -t hfsplus "Firefox 33.1.1.img" /mnt

Option 3: Extract DMG contents with P7ZIP

P7ZIP is awesome. It’s the Linux/BSD version of 7-Zip. Check out their SourceForge page here With it you can literally extract files from any kind of image or archive. Just kidding… It doesn’t really work with every format conceivable. However, it can handle (in alphabetical order): ARJ, CAB, CHM, CPIO, CramFS, DEB, DMG, FAT, HFS, ISO, LZH, LZMA, MBR, MSI, NSIS, NTFS, RAR, RPM, SquashFS, UDF, VHD, WIM, XAR and Z. Impressed? I certainly am!

Installing p7zip is pretty easy using your distribution’s package management system. On Ubuntu with apt-get:

sudo apt-get install p7zip

In addition to being able to extract data from compressed and uncompressed images alike, P7ZIP doesn’t require the HFS kernel modules at all. In the example below, we’re going to extract all of the files from “Firefox 33.1.1.dmg”. When we’re done, we’ll have a tidy little folder called “Firefox”.

Invoke P7ZIP to extract archives and images with “7z x”.

hfs-test$ 7z x "Firefox 33.1.1.dmg"
7-Zip [64] 9.20...
p7zip Version 9.20 (...,utf16=on,HugeFiles=on,2 CPUs)
Processing archive: Firefox 33.1.1.dmg
Extracting 0.ddm
Extracting 1.Apple_partition_map
Extracting 2.hfs
Everything is ok
Files: 3
Size: 158246912
Compressed: 67396319

Notice that 7z extracted three files: “0.ddm”, “1.Apple_partition_map”, and “2.hfs”. To actually get to the files, we’ll need to run 7z again on “2.hfs”.

7z x 2.hfs

We picked “2.hfs” because it was the biggest of the three, meaning it was probably the one with the data. Simple but effective logic. After a few moments, you should have a folder called “Firefox” with all of the files from the original DMG.