UNIX Operating System

UNIX is a popular time−sharing operating system originally intended for program development and document preparation, but later widely accepted for a number of implementations. UNIX is today's most ubiquitous multi−user operating system, with no indication of any diminishment in the near future.
Today, when a period of several years represents the lifetime of many successful IT
products, UNIX is still considered the most stable and the most secure operating system on the
market, three decades after its appearance. Of course, during 30 years of existence UNIX has
changed a great deal, adapting to new requirements; it is hard to compare today's modern UNIX
flavors with initial (now obsolete) UNIX versions. In fact, these changes and adaptations are unique to the UNIX operating system; no other operating system has so successfully evolved, time and again, to meet modern needs. The concept and basic design of UNIX deserve the credit for this remarkable longevity, as they provide the necessary flexibility for the permanent changes required to make UNIX suitable for many new applications.
UNIX, like any other operating system, is an integrated collection of programs that act as links
between the computer system and its users, providing three primary functions:

1.Creating and managing a filesystem (sets of files stored in hierarchical−structured
directories)
2. Running programs
3. Using system devices attached to the computer

UNIX was written in the C computer language, with careful isolation and confinement of
machine−dependent routines, so that it might be easily ported to different computer systems. As a result, versions of UNIX were available for personal computers, workstations, minicomputers,
mainframes, and supercomputers. It is somewhat curious to note that portability was not a design objective during UNIX development; rather, it came as a consequence of coding the system in a higher−level language. Upon realizing the importance of portability, the designers of UNIX confined hardware−dependent code to a few modules within the kernel (coded in assembler) in order to facilitate porting.
The kernel is the "core" of the UNIX operating system. It provides services such as a filesystem,
memory management, CPU scheduling, and device I/O for programs. Typically, the kernel interacts directly with the underlying hardware; therefore, it must be adapted to the unique machine architecture. However, there were some implementations of UNIX in which the kernel interacted with another underlying system that in turn controlled the hardware. The kernel keeps track of who is logged in, as well as the locations of all files; it also accepts and enables instruction executions received from the shell as the output of interpreted commands. The kernel provides a limited number (typically between 60 and 200) of direct entry points through which an active process can obtain services from the kernel. These direct entry points are system calls (also known as UNIX internals).
The actual machine instructions required to invoke a system call, along with the method
used to pass arguments and results between the process and the kernel, vary from machine to
machine.