11.1. Introduction to Processes

Every program, server, or command running on a Linux system is a process. At any time, there are dozens of processes running on your system, some for programs that you are interacting with graphically, some for commands that you have started at a shell prompt, some for servers running in the background, and some that perform system tasks. Every time you type a command like ls or vi at the shell prompt, a new process is created, only to exit as soon as its job is done.

Each process is identified by a unique ID known as the PID, or process ID. Each is owned by a single user and is a member of multiple groups, which determine the privileges that the process has. And each has a priority (also known as the nice level), which controls how much CPU time the process can use up on a busy system. Almost every process has a parent, which is the process that started it, and from which it inherits ownership, priority, and other settings.

A process will run until it chooses to exit, or until it is killed by a signal from another process.