Unix Power Tools, 3rd Edition

Chapter 23. Job Control

Job Control in a Nutshell

As has been said many times in this book, Unix is a multiprocessing system. Unlike some historic systems such as MS-DOS, all flavors of Unix run more than one process at a time. In fact, when Unix boots, the first program executed is called init , which is the parent of all future processes. init immediately creates a new process in which other programs can run, such as getty and the various rc setup scripts. At some point when a user logs into the system, the getty program creates a new shell for that session. Even when the system is in single-user mode, Unix is still capable of running multiple processes. Multiprocessing is pervasive in Unix.

But multiprocessing isn’t just for system daemons. It’s also there to make your interactive shell session just a little bit more productive. Often, you will need to execute a program that takes a long time to run. For instance, you might be downloading a file with FTP or Lynx. It is possible to have that task put into the background so that you may execute new commands while the previous ones are running to completion. Just as you may have several piles of work on your desk, you often need to set aside one stack to work on another. A process is said to be in the foreground when it is receiving your keyboard input and is writing to your screen. Using the desk analogy, the foreground process is that pile of work currently in front of you. Only one process can be in the foreground at a time. ...