Introduction to File Permissions

Chapter 11. Using File Permissions and Access Control Lists

IN THIS CHAPTER

• Introduction to File Permissions

• Applying Advanced Resource Control Using ACLs

• Being Someone Else for a While: su, sudo

Because Unix is an inherently multiuser environment, it's important for there to be a way to record who owns what, who can look at it, run it, or change it. In Unix, this is accomplished with metadata flags that tell the system what permissions exist for each file. Because the system doesn't actually know a text file from a word processor, these flags are also used to indicate whether the file should be considered executable (and therefore a runnable program or application).

The basic philosophy of the Unix file permission system revolves around the idea of controlling access at three levels: access that's possible for the owner of the file; access that's possible for a group of owners (users who aren't the file's individual owner, but who (may) have enhanced privileges with respect to it); and access that's possible for everyone else on the system.

Apple is also working to incorporate Access Control Lists into Mac OS X, but as we are writing this, it's uncertain whether this feature will make it into the release version of Tiger client.