Chapter 11: Processes and Scheduling

Overview

The notion of a process is one of the most important aspects of the UNIX system, along with files and directories and the shell. All UNIX variants use this notion in the same way. A process, or task, is an instance of an executing program. It is important to make the distinction between a command and a process; you generate a process when you execute a command. UNIX is a multitasking system because it can run many processes at the same time. At any given time there may be tens or even hundreds or thousands of processes running on your system.

This chapter will show you how to monitor the processes you are running by using the ps command and how to terminate running processes by using the kill command, for instance, to kill runaway processes that are taking inordinate amounts of time. You will also see how to use the ps command with options to monitor all the processes running on a system.

In this chapter, you will learn how to schedule the execution of commands. You will see how to use the at command to schedule the execution of commands at particular times and how to use the batch command to defer the execution of a command until the system load permits.

You will also be introduced to a special type of process called the daemon. This process can invoke other processes. Some daemon processes are loaded automatically at system startup or when a certain event occurs, while others may be started manually by your system administrator.

Support for real-time processing is an important feature of UNIX that makes it possible to run many applications requiring predictable execution. This chapter describes many of the capabilities of UNIX that support real-time processing. You will see how to set the priorities of processes, including giving processes real-time priority

You will also learn about the /proc file system, a virtual file system that provides a wealth of information to both system administrators and users.