How to make bootable El Capitan ISO image

After downloading El Capitan installer app from Apple Store, you can either upgrade your existing MAC OS to El Capitan, make a bootable USB using \”createinstlallmedia\” from the package, or you can make a bootable ISO image to use either with virtual CDROM or to burn your own installation DVD. I will show you how to create a bootable ISO image of the El Capitan.

First, we need to mount InstallESD.dmg from /Applications/Install El Capitan.app

$ hdiutil attach "/Applications/Install OS X El Capitan.app/Contents/SharedSupport/InstallESD.dmg" -noverify -nobrowse -mountpoint /Volumes/esd
/dev/disk5     GUID_partition_scheme\n/dev/disk5s1   EFI\n/dev/disk5s2   Apple_HFS              /Volumes/esd

-noverify switch speeds up mounting process and -nobrowse prevents Finder poping up windows after mount

Now we will create a placeholder for our ISO image:

$ hdiutil create -o ElCapitan3.cdr -size 7613m -layout SPUD -fs HFS+J\n.....................................................................
created: /Users/amnesia/ElCapitan.cdr.dmg

After the placeholder is created, we need to mount it:

$ hdiutil attach ElCapitan3.cdr.dmg -noverify -nobrowse -mountpoint /Volumes/iso
/dev/disk6     Apple_partition_scheme
/dev/disk6s1   Apple_partition_map
/dev/disk6s2   Apple_HFS               /Volumes/iso

Now use asr tool to populate ISO image:

$ asr restore -source /Volumes/esd/BaseSystem.dmg -target /Volumes/iso -noprompt -noverify -erase
Validating target...done\nValidating source...done\nRetrieving scan information...done
Validating sizes...done\nRestoring  ....10....20....30....40....50....60....70....80....90....100
Remounting target volume...done

\nasr tool will automatically create new mountpoint /Volumes/OS X Base System. We need to place some files in til directory:\n\nFirst remove invalid link:\n

$ rm /Volumes/OS\\ X\\ Base\\ System/System/Installation/Packages

Now we will copy proper directory that contains package files. This is done from the ESD media:

$ cp -rp /Volumes/esd/Packages /Volumes/OS\ X\ Base\ System/System/Installation

\nThe next step is to copy some additional dependencies:\n

$ cp -rp /Volumes/esd/BaseSystem.chunklist /Volumes/OS\ X\ Base\ System/
$ cp -rp /Volumes/esd/BaseSystem.dmg /Volumes/OS\ X\ Base\ System/

Now unmount the unnecessary volumes:

$ hdiutil detach /Volumes/esd/
"disk5" unmounted.
"disk5" ejected.
bash-3.2# hdiutil detach /Volumes/OS\ X\ Base\ System/
"disk6" unmounted.
"disk6" ejected.

Now we need to convert our ISO to UDTO format:

$ hdiutil convert ElCapitan3.cdr.dmg -format UDTO -o ElCapitan.iso
Reading Driver Descriptor Map (DDM : 0)…
Reading Apple (Apple_partition_map : 1)…
Reading disk image (Apple_HFS : 2)…
.........................................................................
Elapsed Time: 50.898s
Speed: 149.6Mbytes/sec
Savings: 0.0%
created: /Users/amnesia/ElCapitan.iso.cdr

Rename ElCapitan.iso.cdr to ElCapitan.iso and use it as needed