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I am not experienced with OSX but I have been using it recently.

It seems to me the equivalent of an .exe (executable) in Windows is a .dmg (disk image) in OSX.

And you don't install it, you mount it.

I am trying to wrap my head around these terms, why do they call it this?

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While not explicitly stated in Daniel Beck's answer, .exe is closer to .app, but both of them have very dramatic differences. It may be easier if you learn what it is (what you're doing), and why it is, instead of trying to associate similarities between OSes. –  Jason Salaz Jan 5 '12 at 21:47
    
Yes, Jason is right (as he always is...); .exe files are like .app files, not .dmg files. –  daviesgeek Jan 5 '12 at 21:58
    
DMGs are just containers for the program (the .app inside). And the .app is actually just a folder in OSX (right click and select "Show Package Contents" and you'll see the resources of any program, along with it's true executable). –  cksum Jan 5 '12 at 22:11
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To be honest, the "exe" is not the app either. The app is just a container for the exe along other resources. The real binary sits inside the .app bundle. Finder does some magic by "hiding" all this. ;) –  Martín Marconcini Jan 5 '12 at 22:17

2 Answers 2

up vote 15 down vote accepted

.dmg files are disk images, similar to an .iso file.

You can think of them like a virtual CD. You put it into the CD drive (mount it), and its contents appear on your computer.

When used for software downloads, they are simply a way to bundle up files, like in a ZIP archive. Their advantage is that it's possible to customize the design (e.g. Finder window background and icon positions, see example below).


The actual program is in a special bundle or package directory with the usually invisible extension .app. It contains an application's resources, like image files, and the actual binary (the ".exe equivalent") that is executed when you start the program. If you want to compare it to a Windows equivalent, the closest would probably be the folder in C:\Program files\ created by a program's installer.

To install such an application from a disk image, you drag and drop it to your hard drive, e.g. the Applications directory. That's what a lot of disk images try to convey in their disk image design: You grab the application, Adium in this case, and drag it onto the Applications folder:

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Related topic on Super User. Might be a bit outdated, as the App Store was brand new at the time. –  Daniel Beck Jan 5 '12 at 21:50

Straight answer.

.dmg files are not equivalent to executables on Windows. A .dmg is also known as an Apple Disk Image. It is a compressed storage and delivery bundle format often used for installer packages, although it can have other functions. Very loosely speaking, a .dmg is more like a .zip, .rar, .gz or .tar file.

The Mac OS X equivalent of an Executable is an application, and it ends in .app

There is a Wikipedia entry on Apple Disk Image at this link.

It says, in part:

The [.dmg] format allows secure password protection as well as file compression and hence serves both security and file distribution functions; it is most commonly used to distribute software over the Internet. Universal Disk Image (UDIF) is a flat file format, and is the native image format for Mac OS X.

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