Essential System Administration, 3rd Edition

Processes

In simple terms, a process is a single executable program that is running in its own address space.^[10] It is distinct from a job or a command, which, on Unix systems, may be composed of many processes working together to perform a specific task. Simple commands like ls are executed as a single process. A compound command containing pipes will execute one process per pipe segment. For Unix systems, managing CPU resources must be done in large part by controlling processes, because the resource allocation and batch execution facilities available with other multitasking operating systems are underdeveloped or missing.

Unix processes come in several types. We’ll look at the most common here.

Interactive Processes

Interactive processes are initiated from and controlled by a terminal session. Interactive processes may run either in the foreground or the background. Foreground processes remain attached to the terminal; the foreground process is the one with which the terminal communicates directly. For example, typing a Unix command and waiting for its output means running a foreground process.

While a foreground process is running, it alone can receive direct input from the terminal. For example, if you run the diff command on two very large files, you will be unable to run another command until it finishes (or you kill it with CTRL-C).

Job control allows a process to be moved between the foreground and the background at will. For example, when a process is moved from the foreground ...