Why it might be important

In my organization (Geneva public school system), we manage our system updates by downloading the full installer and then installing the update with the startosinstall command. The reason we are doing it like this is because these are lab computers that do not belong to a specific user, and because we cannot afford to update these computers during the day, as they are in use by students of professors.

Our process looks like this :

1. A script is used to detect that a computer needs an update
2. The full installer package is downloaded
3. The package is installed, which results in a “Install macOS XXX.app” application in the Applications folder (XXX being for example Sonoma or Sequoia)
4. The startosinstall command is executed, which updates the operating system

However, for reporting and verification purposes, after the OS has been updated and the computer reboots, I run a script that compares the actual operating system version with the one that should have been installed (this information is stored in a file before running the startosinstall command).

The reasoning behind this is that there are many reasons for the install to fail: the lack of a secure token for the account that runs the startosinstall command, not enough free space on the disk, or any other random reason not always very well documented by Apple.

Getting the actual version currently running on a computer is easy with sw_vers command :

% sw_vers
ProductName: macOS
ProductVersion: 15.3.1
BuildVersion: 24D70

However, getting the version that will be installed by the application “Install macOS XXX.app” is more complicated (unless I completely missed something, in which case I urge you to enlighten me in the comments).

First attempts

As with every application, in the “Install macOS XXX.app” resides an Info.plist file that contains some information. Here is what it looks like for Sequoia 15.3.1 :

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Application-Group</key>
	<string>AirPort</string>
	<key>BuildMachineOSBuild</key>
	<string>22A380021</string>
	<key>CFBundleDevelopmentRegion</key>
	<string>English</string>
	<key>CFBundleDisplayName</key>
	<string>Install macOS Sequoia</string>
	<key>CFBundleExecutable</key>
	<string>InstallAssistant_springboard</string>
	<key>CFBundleGetInfoString</key>
	<string>Install macOS Sequoia, Copyright © 2007–2025 Apple Inc. All rights reserved.</string>
	<key>CFBundleIconFile</key>
	<string>InstallAssistant</string>
	<key>CFBundleIdentifier</key>
	<string>com.apple.InstallAssistant.macOSSequoia</string>
	<key>CFBundleInfoDictionaryVersion</key>
	<string>6.0</string>
	<key>CFBundleName</key>
	<string>Install mac OS</string>
	<key>CFBundlePackageType</key>
	<string>APPL</string>
	<key>CFBundleShortVersionString</key>
	<string>20.3.03</string>
	<key>CFBundleSignature</key>
	<string>????</string>
	<key>CFBundleSupportedPlatforms</key>
	<array>
		<string>MacOSX</string>
	</array>
	<key>CFBundleURLTypes</key>
	<array>
		<dict>
			<key>CFBundleURLName</key>
			<string>Open Install OS X URL</string>
			<key>CFBundleURLSchemes</key>
			<array>
				<string>x-install-osx-assistant</string>
			</array>
		</dict>
	</array>
	<key>CFBundleVersion</key>
	<string>20303</string>
	<key>DTCompiler</key>
	<string>com.apple.compilers.llvm.clang.1_0</string>
	<key>DTPlatformBuild</key>
	<string>24D62</string>
	<key>DTPlatformName</key>
	<string>macosx</string>
	<key>DTPlatformVersion</key>
	<string>15.3</string>
	<key>DTSDKBuild</key>
	<string>24D62</string>
	<key>DTSDKName</key>
	<string>macosx15.3.internal</string>
	<key>DTXcode</key>
	<string>1600</string>
	<key>DTXcodeBuild</key>
	<string>16A6170g</string>
	<key>LSApplicationCategoryType</key>
	<string>public.app-category.utilities</string>
	<key>LSHasLocalizedDisplayName</key>
	<true/>
	<key>LSMinimumSystemVersion</key>
	<string>10.13</string>
	<key>MinimumOSVersion</key>
	<string>10.13</string>
	<key>NSMainNibFile</key>
	<string>MainMenu</string>
	<key>NSPrincipalClass</key>
	<string>NSApplication</string>
	<key>NSSupportsAutomaticGraphicsSwitching</key>
	<true/>
	<key>ProductPageIconFile</key>
	<string>ProductPageIcon.icns</string>
</dict>
</plist>

Two keys are interesting here :

<key>DTPlatformBuild</key>
<string>24D62</string>
<key>DTPlatformVersion</key>
<string>15.3</string>

One would think the DTPlatformVersion key would contain the necessary information, but no ! Minor versions do not appear here, which means that 15.3, 15.3.1 and 15.3.2 all have the same value for the key DTPlatformVersion : 15.3

That key is therefore not a reliable way to check if the update has been successfully installed. What about using DTPlatformBuild ? Because after all, to check if a computer is running the same version as the one installed by the “Install macOS XXX.app” application, I could simply compare the build numbers.

Again, this is a negative answer. For macOS Sequoia 15.3.1, the Info.plist in installer application reports a build number 24D62, but after the update, the sw_vers command reports (as seen above) a build number 24D70 !

As it turns out, no easily accessible file contains the necessary information. I had to dig further.

The solution

The bulk of the macOS installer is actually a disk image file named SharedSupport.dmg that resides in Install macOS XXX.app/Contents/SharedSupport. By mounting this dmg, one will find two folders and one file:

Sorry for the French interface…

One would hope the InstallInfo.plist file would contain something interesting, but no…. it is empty.

However, in the com_apple_MobileAsset_MacSoftwareUpdate folder resides a xml file called com_apple_MobileAsset_MacSoftwareUpdate.xml which is the key to success !

This file is actually a plist file and among a lot of things, it contains two keys:

<key>OSVersion</key>
<string>15.3.1</string>
<key>Build</key>
<string>24D70</string>

This is what I could use to determine exactly which os version and which build number is going to be installed, as this corresponds to what the sw_vers command will ultimately report after the update.

Scripting the process

The last step consisted in putting all of this in a script that I could run prior to executing the startosinstall command.

Here is what I came up with :

getMacOSVersion() {
	local PATH_TO_INSTALLER="$1"
	local PATH_TO_SHARED="Contents/SharedSupport"
	local SHARED_DMG_NAME="SharedSupport.dmg"

	local FULL_PATH_TO_DMG="$PATH_TO_INSTALLER/$PATH_TO_SHARED/$SHARED_DMG_NAME"

	# Mount the Shared Support volume
	local MOUNT_OUTPUT
	MOUNT_OUTPUT="$(/usr/bin/hdiutil attach "$FULL_PATH_TO_DMG" -nobrowse)"
	if [ $? -eq 0 ]; then
		# The Shared support volume has been correclty mounted, get the mount point
		local DMG_VOLUME="$(echo "$MOUNT_OUTPUT" | tail -n 1 | awk -F"\t" '{print $3}')"
		if [ -n "$DMG_VOLUME" ] && [[ "$DMG_VOLUME" =~ "^/Volumes" ]]; then
			# The mount point exists and is in /Volumes, get the version from the xml file
			local PATH_TO_XML="$DMG_VOLUME/com_apple_MobileAsset_MacSoftwareUpdate/com_apple_MobileAsset_MacSoftwareUpdate.xml"
			local MACOS_VERSION="$(/usr/bin/plutil -extract Assets.0.OSVersion raw -o - "$PATH_TO_XML")"
			/usr/bin/hdiutil detach "$DMG_VOLUME" -quiet
			if [ -n "$MACOS_VERSION" ]; then
				# Print the macOS version number
				echo "$MACOS_VERSION"
				return 0
			fi
		fi
	fi
	# If one of the tests failed, fall back to using the Info.plist file
	/usr/bin/defaults read "$PATH_TO_INSTALLER/Contents/Info.plist" DTPlatformVersion
}

This function takes one argument: the full path to the macOS Installer (e.g. /Applications/Install macOS Sequoia.app).

It will mount the SharedSupport.dmg disk image, retrieve the macOS version number from the xml file, unmount the disk image and finally it will then print the macOS version.

If anything goes wrong with the disk image (unable to mount, unable to read the xml file), it will fall back to using the Info.plist file, even though we now know it does not report a reliable version.

One can easily adapt/improve to report the build number as well.

Conclusion

Knowing in advance which version of macOS will be installed by an Install macOS XXX.app application is not as trivial as one might expect. Thanks to this method, I can finally have a reliable answer.

Of course this process takes more time that simply reading the Info.plist, as it requires a 15GB disk image to be mounted. In my case, I don’t really care , since this whole process is taking place during the night, so as not to bother the users.

I hope this can come handy to others.