Unix File System

The Unix file system has a hierarchical (tree-like) structure with its highest level 
directory called root ( /).
Immediately below the root level directory are several sub-directories, most of which contain system files. 

The following figure shows a typical organization of files in Unix system.

bin - short for binaries, this is the directory where many commonly used 
          executable commands reside
dev - contains device specific files
etc - contains system configuration files
home - contains user directories and files
lib - contains all library files
mnt - contains device files related to mounted devices
proc - contains files related to system processes
root - the root users' home directory (note this is different than /)
sbin - system binary files reside here. If there is no sbin directory on your system, these files most likely reside in etc
tmp - storage for temporary files which are periodically removed from the filesystem
usr - also contains executable commands

Unix File Types 

All files in the Unix file system can be loosely categorized into 3 types, specifically:

1. ordinary files
2. directory files
3. device files

Ordinary Files

Ordinary files hold data and executable programs. Executable programs are the commands

(ls) that you enter on the prompt. The data can be anything and there is no specific format

 enforced in the way the data is stored.

The regular files can be visualized as the leaves in the UNIX tree.

Directories

Directories are files that contain other files and sub-directories. Directories are used to

organize the data by keeping closely related files in the same place. The directories are 

just like the folders in windows operating system.

The kernel alone can write the directory file. When a file is added to or deleted from this directory, the kernel makes an entry. 
A directory file can be visualized as the branch of the UNIX tree.

Special Or Device Files

These files represent the physical devices. Files can also refer to computer hardware such as terminals and printers. These device files can also refer to tape and disk drives, CD-ROM players, modems, network interfaces, scanners, and any other piece of computer hardware. When a process writes to a special file, the data is sent to the physical device associated with it. Special files are not literally files, but are pointers that point to the device drivers located in the kernel. The protection applicable to files is also applicable to physical devices.