Periodically I need to create a bootable installer for OS X out of a USB thumb drive. I wrote a script to handle the process back when OS X Lion was released. Since then I have been maintaining this script - with the intention of post this here, which I am doing now because Patrick needs it to create some El Capitan boot disks for support reasons.
This is pretty easy to use if you have a Mac. You simply download the necessary installer from the App Store, plug in a GUID formatted USB drive titled “Untitled” and run this script
create_osx_boot.sh < Mavericks | Yosemite | ElCapitan | Sierra >
…and viola, a few minutes later you will have a bootable USB drive. I have a 32Gig drive that can boot up to three different versions of OS X.
Here is the whole script
UPDATE - 2014-10-2: Updated for Yosemite
UPDATE - 2016-01-04: Updated for El Capitan finally
UPDATE - 2016-09-22: Updating for macOS Sierra