Page 154 - DCAP403_Operating System

Unit 8: File Management

8.5 File System Mounting Notes

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 fi les on storage devices. Providing a common fi le system
structure ensures users and programs are able to access and write fi les.
File systems break files down into two logical categories:

1. Shareable vs. unsharable fi les

2. Variable vs. static fi les

Shareable files are those that can be accessed locally and by remote hosts; unsharable fi les are

only available locally. Variable files, such as documents, can be changed at any time; static fi les,
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.

It is important to understand the difference between a file system and a directory. A file system is

a section of hard disk that has been allocated to contain files. This section of hard disk is accessed

by mounting the file system over a directory. After the file system is mounted, it looks just like

any other directory to the end user.
However, because of the structural differences between the file systems and directories, the data

within these entities can be managed separately.
When the operating system is installed for the first time, it is loaded into a directory structure, as

shown in the following illustration.
Figure 8.5 File System Tree. This tree chart shows a directory structure with the/(root) fi le system
at the top, branching downward to directories and file systems. Directories branch to/bin, /dev,

/etc, and /lib. File systems branch to /usr, /tmp, /var, and /home.
Figure 8.5: File System Tree

/(root)
File System

Directories File Systems

/bin /dev /etc /lib /usr /tmp /var /home

The directories on the right (/usr, /tmp, /var, and /home) are all file systems so they have separate
sections of the hard disk allocated for their use. These file systems are mounted automatically

when the system is started, so the end user does not see the difference between these fi le systems
and the directories listed on the left (/bin, /dev, /etc, and /lib).

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