control the way that information is stored on mass storage devices.
Modern filesystems allow information to be stored in files with
arbitrary names; these files are then arranged in a structured tree
of directories and subdirectories. Most filesystems allow files to be
created, deleted, modified, and moved to different directories. The
whole task is somewhat complex, because filesystems allow
character-by-character control over data, whereas most mass storage
systems allow information to be read or written only block by block.
The filesystem is also the primary tool for enforcing security on a
Unix system. Besides holding the computer's
operating system, programs, and user data, additional information
stored in the filesystem is used to determine what information can be
viewed, what can be modified, and what is inaccessible to the various
users and system processes.