Apple Disk Image#Data format

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A disk image file (extension. Dmg or. Sparseimage or. Sparsebundle ) is a memory map data format on Mac OS X.

Such a disk image is an image of a disk and can act like a hard drive are subjected to, for example, a formatting and divided into partitions.

The associated MIME type is application / octet-stream. A disk image file can be both password-protected and compressed and is therefore used primarily for the protection of sensitive data and the distribution of software over the Internet. When opening a disk image file is mounted and appears as a drive in the Finder. Before opening can sometimes appear a license agreement.

Disk image files can either have a fixed size, such as the size of a DVD (4.7 GB ), or grow automatically when you fill it with data. The latter are called " sparse disk image " (English "sparse image" ) called ( "sparse " = sparse, see also sparse file ). Was a translation error from the German version of Mac OS 10.3, a " Simple Image ", this mistake was corrected in newer operating system versions. " Grows with images" can initially only grow, but not be smaller when you delete files in it, but a reduction to the minimum size is via the command line (see hdiutil ).

The " grows with bundle - image" ( engl. "sparse bundle" ) was introduced in Mac OS X v10.5 "Leopard" and disassembled, transparent for the user, the image into smaller files of 8 MiBytes. This has the advantage that when an incremental backup (eg via "Time Machine" ), only the changed parts of the image need to be backed up, rather than at every little change, the entire image file to a single large file. Furthermore, can thus be large images, especially those with more than 4 GiByte, place it on FAT32 volumes.

Disk image files can be created in earlier versions of Disk Copy with or without encryption (AES 128-bit key length, from Mac OS 10.5 Leopard with 256 bit key length) using Disk Utility ( Disk Utility ) or. These programs use this file format also for burning CDs and DVDs. At the command line, there is the program hdiutil. Other programs are FileStorm and FreeDMG.

Open Windows

To open disk image files in Windows, there is the commercial program TransMac and the free program HFSExplorer. Also, various packers such as 7 -Zip can read the format.

With the mkdosfs Linux program you can create VFAT formatted disk image files to simplify the exchange of data between Mac and ( virtual ) PC. Then, to mount the disk image on Windows ( "mount" ) is still vdk.exe requires the utility.

Open on Linux

Under Linux disk image files can be in principle directly integrated into the system. However, this is only for uncompressed files, compressed disk images must first be converted. The versions can not be seen directly, but via the Unix file command allows the MIME type of the file to determine. About this then, may be a distinction.

$ File unkomprimiert.dmg unkomprimiert.dmg: Macintosh HFS Extended version 4 data load mounted by [ ... ] block size: 4096, number of blocks: 6400, free blocks: 218 $ File komprimiert.dmg komprimiert.dmg: VAX COFF executable not stripped - version 376 An uncompressed file can then mount directly.

$ Mount -t hfsplus - o loop unkomprimiert.dmg / tmp / dmg / The compressed version must be converted before including. To this end, the program must be installed separately dmg2img in most Linux distributions. The resulting file can then be integrated in the usual way.

$ Dmg2img -i -o eingabedatei.dmg ausgabedatei.dmg $ Mount -t hfsplus - o loop ausgabedatei.dmg / tmp / dmg / Conversion on Mac OS X

Disk image files can be converted to an ISO image on Mac OS X. The Programme will hdiutil used in the command line, which is accessed from the Terminal utility.

$ Hdiutil convert Datei.dmg format UDTO -o Datei.iso

If necessary, the Datei.iso.cdr in Datei.iso must be renamed.

$ Mv imagefile.iso.cdr imagefile.iso

The subsequently created. Iso file can be burned to a CD or DVD.

Images of type "sparse image" or "sparse bundle" can shrink to the minimum size by deleting unused areas by using the following command:

$ Hdiutil compact Datei.dmg

This reduction by removing unused areas will now depend on the software used during delete of image content often occur automatically.

In Mac OS 10.5 "Leopard" can be converted to another format disk images not only from the command line, but also with the Disk Utility; so, for example, from a "sparse image" a "sparse bundle" are made. Nor can it with the Disk Utility, the size of an image change.