8.3. Using Access Control Lists
Unix/Linux permission modes are very simple; they don't cover all security needs. But, because they are simple, they are actually used , which is more than can be said for many other access control technologies.
But sometimes permissions just don't cut it, and a better system of discretionary access control is needed. Access control lists (ACLs) enable you to specify exactly which users and groups can access a file and in what ways.