How to build a dmg Mac OS X file (on a non-Mac platform)?

Is it possible to build a .dmg file (for distributing apps) from a non-Mac platform? And if yes, how?

Yep, mkfs.hfsplus does it.

dd if=/dev/zero of=/tmp/foo.dmg bs=1M count=64
mkfs.hfsplus -v ThisIsFoo /tmp/foo.dmg

This creates a dmg file (in this case 64M) that can be mounted on a mac. It can also be mounted on linux, with something like

mount -o loop /tmp/foo.dmg /mnt/foo

after wich you just copy the content you want to it (in /mnt/foo). Unmount it, and the dmg can be copied over to a mac and mounted there.

A project I work on creates DMG files on Linux using genisoimage:

mkdir -p dmgdir/progname.app/Contents/{MacOS,Resources}
...copy your PkgInfo, Info.plist to Contents...
...copy your .icns to Resources...
...copy your other things to where you expect them to go...
genisoimage -V progname -D -R -apple -no-pad -o progname.dmg dmgdir 

If you want to be really fancy, you can steal the .DS_Store file from a DMG made on a Mac with a volume name progname and app bundle called progname.app (i.e., matching what you want to create off the Mac) where you've put a background in .background/background.png and a symbolic link to /Applications in the root dir, and put that in dmgdir along with your own a symbolic link to /Applications.

Finally, if you want to create a compressed DMG, get the dmg tool from libdmg-hfsplus:

dmg uncompressed.dmg compressed.dmg

git clone https://github.com/hamstergene/libdmg-hfsplus
cd libdmg-hfsplus && cmake . && make && cd dmg
./dmg --help

Makefile:

dmg:
    genisoimage -D -V "$(PROJECT) $(VERSION)" -no-pad -r -apple -o project-$(VERSION)-uncompressed.dmg $(DARWIN_DIR)
    ./dmg dmg project-$(VERSION)-uncompressed.dmg project-$(VERSION).dmg

uncompressed works out of the box, compression may cause problems - the origin/master at least produces a 'checksum' error on snow-leopard

It does seem possible to create DMG files with some third party tools. A quick google search reveals at least a few commercial tools:

• TransMac
• MagicISO

Not sure about any OSS/freeware options, but it does at least seem possible if you are so inclined.

Edit: I also forgot about MacDrive, which is another great tool for working with HFS+ filesystems under windows. Since a DMG is basically just a HFS+ filesystem snapshot, it is probably possible with MacDrive to create DMG's as well.

I'm not sure if anyone is still watching this thread, but I tried TransMac as recommended by Nik Reiman.

Using this tool I was able to, running on Windows 7, create dmg files which were mountable on OSX 10.8.3.

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Downside

The only downside for us is that this tool doesn't appear to be command-line friendly; for us that's a deal-breaker as we need to be able to have an automated tool which our build server (Windows based) can use to build dmg files on-the-fly.

See mkfs.hfsplus

If you're distributing Mac apps, then surely you have a Mac to write and test them. Why not simply use that same Mac to create the disk image?

[Edit] Alternatively, if you're distributing a portable app, for example a Java .jar file, why bother with a disk image? Macs understand .zip and .tar.gz archives just fine.

I guess what I'm getting at is, I don't understand how one might need a DMG disk image, but not have a Mac with which to create it.