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

Program control

A process can create other processes. Consider a shell script containing one line, which is date. When the script is executed, two processes will be created, one for the invocation of the shell, and one for the command date. The shell is referred to as the parent process, and date as a child process. Some UNIX commands that appear simple may create child processes, and it is seldom of interest to users how many are created, and how they relate to the original command - the user is only interested in the original command.

The shell allows us to control programs without needing to concern ourselves with the finer details of which process is involved in which activity The concept of a job, and associated shell facilities, will be helpful.