UNIX Operating System

UNIX is a multitasking, Multiuser computer Operating System that exists in many variants.

The original Unix was developed at AT&T's Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. From the power user's or programmer's perspective, Unix systems are characterized by a modular design that is sometimes called the "Unix philosophy," meaning the OS provides a set of simple tools that each perform a limited, well-defined function, with a unified file-system as the main means of communication and a shell scripting and command language to combine the tools to perform complex workflows.

The C programming language was designed by Dennis Ritchie as a systems programming language for UNIX.

Architecture

Kernel is the core of UNIX Operating system. Its main functions are the process management, memory management, file system, and I/O system. Many application subsystems and programs that provide a high-level performance of the system, such as the shell and editors, have gradually been seen as features of the UNIX operating system. However, they all use lower level Services ultimately provided by the kernel, and they avail themselves of these services via a set of system calls.

Shell is a Program which provides an interface for the Users to communicate with the kernel. The shell is a command line interpreter; when a user types a command and press Enter key on the keyboard, the shell interprets and executes the command. The shell searches for commands in a given sequence of directories changeable by user request per invocation of the shell. The shell usually executes a command synchronously, which means that it waits for the command to terminate before reading the next command line. But, it also allows asynchronous execution, where it reads the next command line and executes it without waiting for the prior command to terminate. Commands executed asynchronously can be implemented by putting them to execute in the background. it translates commands entered by the user and converts them into a language that is understood by the kernel.

     UNIX operating system comes with a variety of shells, among which the Bourne, Korn, and C shells are the most popular When logging on, one particular type of shell executes. This shell is the login shell. To use a different shell, do so by running a corresponding command available on the UNIX operating system.

The applications contain all the applications (such as, compilers, editors, word processors, ftp, telnet, Web browser, etc.) that are available for users. A typical application is usually related to a program that can be executed by typing a command from the command line and pressing Enter key. When an application that needs to manipulate a system resource (e.g., an editor from which to read or write a file), it needs to invoke the kernel that performs the task. A user can use any command or application that is available on the system in the UNIX operating system through an interface known as Application User Interface (AUI).

In Short

• Kernel is the heart of UNIX operating system which is responsible for most of the task such as Memory Management, File System, I/O System and it interacts with the hardware.
• Shell is the Programming utility which  interpreters the commands from command line and interacts  with the Kernel.
• File System: 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.

Enjoy Unixing...!!!