The UNIX system has been around for a long time, and many people may remember it as it existed in the previous decades. Many IT professionals who encountered UNIX systems in the past found it uncompromising. While its power was impressive, its command-line interface required technical competence, its syntax was not intuitive, and its interface was unfriendly.

  In the UNIX system's early days, security was virtually nonexistent. Subsequently, the UNIX system became the first operating system to suffer attacks mounted over the nascent Internet. As the UNIX system matured, however, the organization of security shifted from centralized to distributed authentication and authorization system.

  Now, a single Graphical User Interface is shipped and supported by all major vendors has replaced command-line syntax, and security systems, up to and including B1, provide appropriate controls over access to the UNIX system.

Task  , Process  &  Thread  are interlinked hierarchy ,  yet they are different .

this  hierarchy can be represented  as...

• A  Task  is set of program instructions that are loaded in memory. 
• A  Process  is an instance of a computer program being executed. It  conatins  the program code and its current  activity . 
• A  Thread  of execution results from a fork of a computer program into two or more concurrently running   task.

 HOW THREADS ARE DIFFERENT FROM PROCESS

A thread is contained inside process.  Multiple threads can exist  witin the same process and share resources such as memory , while different process do not share these resources.

 A thread what CPU actually runs , its about scheduling access to shared resources e.g. CPU .

A process is the allocated memory for instruction data (a process needs one or more threads because that is what is run by CPU ) , simply we hink of thread as a slice of time and a process as a slice of space .

shortly............. :)

• processes are typically independent , while threads exists as subset of process.
•  processes have seprate address spaces , where as threads share their adress spaces .

        

             

what is an OPERATING SYSTEM ????

An operating system is a program designed to run other programs on a computer. A computer’s operating system is its most important program. It is considered the backbone of a computer, managing both software and hardware resources. Operating systems are responsible for everything from the control and allocation of memory to recognizing input from external devices and transmitting output to computer displays. They also manage files on computer hard drives and control peripherals, like printers and scanners.It's a communication between user and machine that connect user to machine.