The file system structure is the most basic level of organization in an
operating system. Almost all of the ways an operating system interacts
with its users, applications, and security model are dependent upon the
way it organizes files on storage devices. Providing a common file
system structure ensures users and programs are able to access and write
files.
File systems break files down into two logical categories:
Shareable files are those that can be accessed
locally and by remote hosts; unsharable files are
only available locally. Variable files, such as
documents, can be changed at any time; static
files, such as binaries, do not change without an action from the system
administrator.
The reason for looking at files in this manner is to help correlate the
function of the file with the permissions assigned to the directories
which hold them. The way in which the operating system and its users
interact with a given file determines the directory in which it is
placed, whether that directory is mounted with read-only or read/write
permissions, and the level of access each user has to that file. The top
level of this organization is crucial. Access to the underlying
directories can be restricted or security problems could manifest
themselves if, from the top level down, it does not adhere to a rigid
structure.
