macos-high-sierra-iso-creation.txt

First, run the following command to create a blank disk image:

$ hdiutil create -o /tmp/HighSierra.cdr -size 7316m -layout SPUD -fs HFS+J

Next, mount your blank image:

$ hdiutil attach /tmp/HighSierra.cdr.dmg -noverify -nobrowse -mountpoint /Volumes/install_build

Now you’re going to restore BaseSystem.dmg from the installer over to the newly mounted image:

$ asr restore -source /Applications/Install\ macOS\ High\ Sierra.app/Contents/SharedSupport/BaseSystem.dmg -target /Volumes/install_build -noprompt -noverify -erase

Note that, after doing this, the name of our destination mount point has changed to “OS X Base System/System.” You’re almost done! Unmount the image:

$ hdiutil detach /Volumes/OS\ X\ Base\ System

And, finally, convert the image you created into an ISO file:

$ hdiutil convert /tmp/HighSierra.cdr.dmg -format UDTO -o /tmp/HighSierra.iso

Move the ISO to the desktop:

$ mv /tmp/HighSierra.iso.cdr ~/Desktop/HighSierra.iso

And you’ve got a bootable High Sierra ISO file!