Apple typically distributes its operating systems as .app files downloaded from the Mac App Store or as .dmg disk images. These are not directly compatible with many tools or for creating bootable media on non-Apple systems. An ISO file is needed for several specific tasks:
mv ~/Desktop/ElCapitan10.11.1.cdr ~/Desktop/ElCapitan10.11.1.iso macosxelcapitan10111imageiso work
The previous step creates a file named ElCapitan.iso.cdr . We just need to rename it to have a standard .iso extension. Apple typically distributes its operating systems as