Files are central to Unix in ways that are not true for some other operating systems. Commands are executable files, usually stored in standard locations in the directory tree. System privileges and permissions are controlled in large part via access to files. Device I/O and file I/O are distinguished only at the lowest level. Even most interprocess communication occurs via file-like entities. Accordingly, the Unix view of files and its standard directory structure are among the first things a new administrator needs to know about.
Like all modern operating systems, Unix has a hierarchical (tree-structured) directory organization, know collectively as the filesystem . The base of this tree is a directory called the root directory. The root directory has the special name / (the forward slash character). On Unix systems, all user-available disk space is transparently combined into a single directory tree under /, and the physical disk a file resides on is not part of a Unix file specification. We’ll discuss this topic in more detail later in this chapter.
Access to files is organized around file ownership and protection. Security on a Unix system depends to a large extent on the interplay between the ownership and protection settings on its files and the system’s user account and group structure (as well as factors like physical access to the machine). The following sections discuss the basic principles of Unix file ownership and protection.