Mountain Lion Installer

[darkside1709]

The official Apple support(?) page with the Mountain Lion image gives a .dmg file rather than a .app. I'm unsure if that's what's supposed to happen and I have to do some conversion or if I should look for a different source that has the image in .iso format.

[jgrisham]

That .dmg file contains a single file on an HFS+ volume labeled "Install Mac OS X": an installer .pkg file named InstallMacOSX.pkg, which contains (sort of) and will create the actual OS installer, Install OS X Mountain Lion.app, as needed by this script.

To use it with this script:

1. Download the appropriate disk image from the official How to download and install macOS page (Mac OS X 10.7, OS X 10.8, 10.10, 10.11, or macOS 10.12)
2. Mount the InstallMacOSX.pkg disk image [Note1]
3. Open the /Volumes/Install Mac OS X/InstallMacOSX.pkg on macOS; it should open with the system's Installer.app, and after confirmation the OS installer .app should be deposited in your Applications directory. [Note2]
4. Run this script, using the --applicationpath argument to point to wherever you saved the Install OS X Mountain Lion.app installer.

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Notes:

1. Third-party tools such as 7-zip or the java-based HFSExplorer may be needed if not doing this on macOS.)
2. A .pkg file is just a .xar archive and can also be extracted using utilities such as 7-Zip. See 'Details' below

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Details:

1. InstallMacOSX.dmg is a disk image containing a volume named "Install Mac OS X"
2. The "Install Mac OS X" volume contains a single file, a XAR archive for the macOS installer named "InstallMacOSX.pkg"
3. InstallMacOSX.pkg contains installer scripts and more importantly, another disk image, this one named "InstallESD.dmg" and which is a bootable .iso image, which contains a volume named "Mac OS X Install ESD"
4. The "Mac OS X Install ESD" volume contains, among other things, the Install OS X Mountain Lion.app application
5. The "Mac OS X Install ESD" volume is bootable. After the installer copies Install OS X Mountain Lion.app to the /Applications folder, it then copies the InstallESD.dmg file into that app! (i.e. the final .app contains a copy of itself, plus everything needed to created bootable USB drives)
6. Because of all of this, the script probably isn't even needed for this version, as a bootable installer can be directly imaged from the InstallESD.dmg disk image found in Details 3. above.