Introducing UNIX and Linux

Processes and devices

Overview
Processes
      Process status
      Foreground and background
      Process control
      Signals
Environment
      Environment variables
      Global and local variables
      Executable scripts
Program control
      Job control
      Command history list
      Running a job at a specific time
      Running programs periodically
      Big programs
      Timing a program
      Running programs in order
Quotes and escapes
Devices
Backquotes
Summary
Exercises

Processes

So far we have considered a dialogue with a UNIX system as being a sequence of commands entered by the user, and the system taking action at each command. We now explore the mechanisms with which UNIX implements commands. When describing an operating system, we need to remember that the computer system on which it is running contains electronics that run only machine code. Any command a user types in is either translated into machine code directly by compilation, or interpreted by another program that is already in machine code. A machine code program is known as a process. Every command given to a UNIX system, and every program run on a UNIX machine, relates to a process - either it creates one or more processes, or it is interpreted by one that is already running. A UNIX shell has a mechanism for controlling processes that appears to the user to be independent of the electronics inside the machine.