The Main Concepts that unites all versions of UNIX in Four Layers.
1. Application Programming
Is the layer where the commands and Programming are specified. There are various command and utilities which you would use in your day to day activities. cp, mv, cat and grep etc. are few examples of commands and utilities. There are over 250 standard commands plus numerous others provided through 3rd party software. All the commands come along with various optional options
The shell is the utility that processes your requests. When you type in a command at your terminal, the shell interprets the command and calls the program that you want. The shell uses standard syntax for all commands. C Shell, Bourne Shell and Korn Shell are most famous shells which are available with most of the UNIX variants.
The kernel is the heart of the operating system. It interacts with hardware and most of the tasks like memory management, task scheduling and file management.
It is the Configuration to the system. Contains the Filesystem. All data in UNIX is organized into files. All files are organized into directories. These directories are organized into a tree-like structure called the filesystem.