Unix is the most widely used computer Operating System in the world. Unix has been ported to run on a wide range of computers, from handheld personal digital assistants to inexpensive home computing systems to some of the worlds' largest super-computers. Unix is a multiuser, multitasking operating system, which enables many people to run many programs on a single computer at the same time. After more than three decades of use, Unix is still regarded as one of the most powerful, versatile, flexible and reliable operating systems in the world of computing.
Unix is a layered operating system. The innermost layer is the hardware that provides the services for the OS. The operating system, referred to in Unix as the kernel, interacts directly with the hardware and provides the services to the user programs. These user programs don't need to know anything about the hardware. They just need to know how to interact with the kernel and it's up to the kernel to provide the desired service. One of the big appeals of Unix to programmers has been that most well written user programs are independent of the underlying hardware, making them readily portable to new systems.
User programs interact with the kernel through a set of standard system calls. These system calls request services to be provided by the kernel. Such services would include accessing a file: open close, read, write, link, or execute a file; starting or updating accounting records; changing ownership of a file or directory; changing to a new directory; creating, suspending, or killing a process; enabling access to hardware devices; and setting limits on system resources. Various Applications
The Unix Operating System has a number of features that account for its flexibility, stability, power, robustness and success. Unix is a multi-user, multi-tasking Operating System, which allows multiple users to access and share resources...