Mac 101: DMG files are Disk Images

Thanks very much for this info - I just bought my first mac(book) and spent the last two days trying to figure out how DMGs worked and why my apps kept disappearing. I'm no computer neophyte - I've been using Windows - for 17 years, but this was new and really had me baffled. I can well believe that every newbie gets stumped on this. Thanks again!

Another Quick TIP:

If the disk image doesn't have an alias to the Applications folder (or even if it does), click the little blue pill on the right side of the title bar. That will open the links panel on the left side which has the link to the Applications folder, and you can just drag the app over into it.

Actually, disk images are useful for keeping infrequently-used applications available, but in a compressed format.

At some point (10.3, I think) Apple changed the OS to automatically mount an image file if it contains the only available copy of a requested application - that is, if you double-click on a document which has as it's creator an application in an unmounted (but available), Mac OS X will mount the image file and launch the application.

Until I moved to Intel-based Macs, I had a Classic image file which contained the entire Classic environment - including the few Classic applications I used. Just double-clicking a document which needed GeoQuery (for example) would cause Mac OS X to mount the Classic.dmg volume, launch Classic, launch GeoQuery and open my document.

Pretty slick, no?

------

Mac Help (available from the Finder) has the following regarding .dmg

Seeing the contents of a disk image
A disk image is a copy of the contents of a disk, volume, or folder. Disk images have the filename extension ".dmg" at the end of their names. To see the contents of a disk image, you must first open, or "mount," the disk image, which makes it appear on the desktop or in a Finder window.

To open a disk image, double-click the image on your desktop or in a Finder window.

Once a disk image is opened, you can double-click it to access its contents. To learn more about disk images, open Disk Utility, in the /Applications/Utilities folder, and choose Help > Disk Utility Help.

The first app I ever downloaded when I switched from PC to Mac in 2004 was Firefox

It downloaded as a .dmg file, which, coming from the windows world of '.exe's, I assumed was some sort of mac executable.

Imagine my confusion when I clicked it and another icon appeared, this time looking like a disk drive.

Then imagine my confusion when I clicked on that, and a new window opened, with a firefox icon.

Then imagine my confusion when I learned that, in order to install it, I had to drag it to the applications folder.

But the finder window that was displaying the contents of the disk image did not have a side bar. So I clicked the Finder icon... nothing happened of course. Finder was already running.

So then I figured I had to click on File > New Finder Window just to see my applications folder.

All this, just to install an app.

Since I got a Mac, three of my friends have also got them. And the one thing that confuses them more than anything else (and there isn't much else), is disk images.

It's not obvious what to do with them. When there's an alias, then that makes things a lot better. But it's not always the case.

Sure, you can lay the blame at the door of the developer, for not including an alias. But for many, especially switchers, it seems a cumbersome way to install an app.

An icon, which when clicked opens another icon, which you then have to drag another icon out of, is not as simple as it sounds for a newbie.

By comparison, .exe files on windows are easy. Just double click it, and the installer starts.

Personally, I prefer .zip files. The fact that the zip file icon looks like a box suggests there's something inside, to unpack.

When you double click it, the contents are shown in a folder. The folder is familiar; it's obvious that the application is inside.

Installers are even easier. No need to drag anything anywhere.

There's my thoughts, anyway.... ;-)

Gosh, I thought the topic of this article couldn't be more obvious and that most of Erica's posts were just annoying filler, but I forgot that there are people who recently adopted the OS X platform who don't know this stuff (no offense meant)! Go Mac 101? :

I think what was thought of as the main advantage of disk images was that they could be "internet enabled".

Otherwise, one might as well tar and bzip (or gzip) an application bundle to distribute it.

Internet-enabling it means, to quote Apple, that:

"The browser launches Disk Copy with the .dmg file. The user watches a progress bar while Disk Copy mounts the disk image, copies the contents out of the image, unmounts the image, and moves the .dmg file to the Trash."

However, that advantage is lost, if you uncheck the "Open Safe files after downloading" in Safari's Preferences - as you probably should, since it has been used as an attack vector before now.

It's interesting to note that Apple now has a "Legacy Document" notice on the page I quoted from:

http://developer.apple.com/documentation/DeveloperTools/Conceptual/SoftwareDistribution4/Concepts/sd_disk_images.html

Panic, who always used to distribute software in this way, just put out their new application Coda in a zip (a Windows-type zip file, rather bzip or gzip). I'd guess they are preparing for 10.5.

In general, all the indications are that Apple is deprecating this method of software distribution and will be making new arrangements for 10.5.

Hi guys...thanks for a visit to my Blog!

http://www.lucianobove.blogspot.com

As a recent switcher I appreciate the native capability to run disk images. With a PC you hace to install extra software to mount disk images :(

Can anyone tell me the best way to create disk images (I know about the mac disk utility) that have the alias and icon.

dwakefield -

Thanks for the reply, the application package system on OS X is so transparent that I forget that things aren't as simple as they look.


Julen -

It doesn't hurt a thing to run apps from the disk image, I do it all the time. The problem is that when you restart the image is unmounted. This used to drive me nuts when I first started using macs, before I understood what disk images were.

As a self-taught Mac user who is still learning some of the "basics,", I really appreciate when sites like this don't assume everyone reading is a Mac wiz. If it's too elementary for you, keep scrolling, but people like me turn to sites like this to learn.