Overview of Processes, Sessions and Daemons
A process can be thought of as being a program, and the environment necessary for that program to run. At process creation, time the process is normally given three files, the standard input, the standard output, and the standard error.
Linux is a modern multitasking operating system, which has many different programs or processes, all running at the same time. These processes are made to appear to be running simultaneously through mechanism known as time slicing or sharing the CPU among the various running programs for small periods.
A few types of different processes will be discussed, first those that are started at and controlled by or through a terminal session. A user must have logged on to the system to start any of these types of processes. They may require interaction with a terminal. Secondly, a user can start background processes, which are typically long running programs that do not require interaction with a terminal. Their standard input and output is probably a file or some other device. Using commands such as: jobs, fg, bg or kill to control these processes was discussed in the previous section.
Second groups of processes are those that might be referred to as automatically started processes or potentially batch processes. These are not connected to a terminal but are typically queued into some area to be executed later or at a specific time only (e.g. printing at nighttime). The at command can queue tasks for execution at a later time or the cron process can be used to start tasks on a repetitive basis at designated times. This allows the administrator to queue tasks that may require a large number of system resources to run at a time in the future when system resource demands are low.
Finally, there is a type of process referred to as a daemon. These processes run continuously in the background and are typically server processes. Processes of this nature are usually started when the system starts up and waits in the background until they are needed. A Web server (httpd) or the systems logging function (syslogd) are two types of daemons. Daemon process names typically end in the letter d.
Each process has several bits of information associated with it. This includes: the process ID (PID), the parent process ID (PPID, this is the process that started or forked the process), the process priority (which is calculated from several factors), the name of the owner of the process (typically root or some other userID), and several other important pieces of information.
Home - Table Of Contents - Contact Us
CertiGuide to A+ (A+ 4 Real) (http://www.CertiGuide.com/apfr/) on CertiGuide.com
Version 1.0 - Version Date: March 29, 2005
Adapted with permission from a work created by Tcat Houser et al.
CertiGuide.com Version © Copyright 2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.