What is a computer process?

A process is an instance of running a program. If, for example, three people are running the same program simultaneously, there are three processes there, not just one. In fact, we might have more than one process running even with only person executing the program, because (you will see later) the program can "split into two," making two processes out of one. Keep in mind that all UNIX commands, e.g. cc and mail, are programs, and thus contribute processes to the system when they are running. If 10 users are running mail right now, that will be 10 processes. At any given time, a typical UNIX system will have many active processes, some of which were set up when the machine was first powered up. Every time you issue a command, UNIX starts a new process, and suspends the current process (the C-shell) until the new process completes (except in the case of background processes, to be discussed later). UNIX identifies every process by a Process Identification Number (pid) which is assigned when the process is initiated. When we want to perform an operation on a process, we usually refer to it by its pid. Unix is a timesharing system, which means that the processes take turns running. Each turn is a called a timeslice; on most systems this is set at much less than one second. The reason this turns-taking approach

is used is fairness: We don't want a 2-second job to have to wait for a 5-hour job to finish, which is what would happen if a job had the CPU to itself until it completed.