Dir and files permissions in AOSP filesystem

I’m currently working on the native part of Android using the 5.x AOSP release made by Intel for the Minnowboard MAX. During one of my activities I had the need to put a shell script inside a directory of the system root. Since most of the AOSP filesystem is mounted as read only, my question was: how are directories and files permissions set in the AOSP filesystem?
Source: Ziobudda.net