hdiutil detach /Volumes/Install\ OS\ X\ El\ Capitan hdiutil convert /tmp/ElCapitan.cdr.dmg -format UTOI -o ~/Desktop/ElCapitan.iso Use code with caution.
Open Terminal and run these commands one by one to create a blank disk image, mount it, and write the installer files to it: Mac Os X 10.11 El Capitan Iso Download
If you doubt the source, test the ISO in a virtual machine before installing it on your primary hardware. Key Features of OS X 10.11 hdiutil detach /Volumes/Install\ OS\ X\ El\ Capitan hdiutil