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      In multitasking computer operating systems, a daemon ( or ) is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections.
      In a Unix environment, the parent process of a daemon is often, but not always, the init process. A daemon is usually created either by a process forking a child process and then immediately exiting, thus causing init to adopt the child process, or by the init process directly launching the daemon. In addition, a daemon launched by forking and exiting typically must perform other operations, such as dissociating the process from any controlling terminal (tty). Such procedures are often implemented in various convenience routines such as daemon(3) in Unix.
      Systems often start daemons at boot time that will respond to network requests, hardware activity, or other programs by performing some task. Daemons such as cron may also perform defined tasks at scheduled times.


      Terminology


      The term was coined by the programmers at MIT's Project MAC. According to Fernando J. Corbató, who worked on Project MAC around 1963, his team was the first to use the term daemon, inspired by Maxwell's demon, an imaginary agent in physics and thermodynamics that helped to sort molecules, stating, "We fancifully began to use the word daemon to describe background processes that worked tirelessly to perform system chores". Unix systems inherited this terminology. Maxwell's demon is consistent with Greek mythology's interpretation of a daemon as a supernatural being working in the background.
      In the general sense, daemon is an older form of the word "demon", from the Greek δαίμων. In the Unix System Administration Handbook Evi Nemeth states the following about daemons:

      Many people equate the word "daemon" with the word "demon", implying some kind of satanic connection between UNIX and the underworld. This is an egregious misunderstanding. "Daemon" is actually a much older form of "demon"; daemons have no particular bias towards good or evil, but rather serve to help define a person's character or personality. The ancient Greeks' concept of a "personal daemon" was similar to the modern concept of a "guardian angel"—eudaemonia is the state of being helped or protected by a kindly spirit. As a rule, UNIX systems seem to be infested with both daemons and demons.
      In modern usage in the context of computer software, the word daemon is pronounced DEE-mən or DAY-mən.
      Alternative terms for daemon are service (used in Windows, from Windows NT onwards, and later also in Linux), started task (IBM z/OS), and ghost job (XDS UTS). Sometimes the more general term server or server process is used, particularly for daemons that operate as part of client-server systems.
      After the term was adopted for computer use, it was rationalized as a backronym for Disk And Execution MONitor.
      Daemons that connect to a computer network are examples of network services.


      Implementations




      = Unix-like systems

      =
      In a strictly technical sense, a Unix-like system process is a daemon when its parent process terminates and the daemon is assigned the init process (process number 1) as its parent process and has no controlling terminal. However, more generally, a daemon may be any background process, whether a child of the init process or not.
      On a Unix-like system, the common method for a process to become a daemon, when the process is started from the command line or from a startup script such as an init script or a SystemStarter script, involves:

      Optionally removing unnecessary variables from environment.
      Executing as a background task by forking and exiting (in the parent "half" of the fork). This allows daemon's parent (shell or startup process) to receive exit notification and continue its normal execution.
      Detaching from the invoking session, usually accomplished by a single operation, setsid():
      Dissociating from the controlling tty.
      Creating a new session and becoming the session leader of that session.
      Becoming a process group leader.
      If the daemon wants to ensure that it will not acquire a new controlling tty even by accident (which happens when a session leader without a controlling tty opens a free tty), it may fork and exit again. This means that it is no longer a session leader in the new session, and cannot acquire a controlling tty.
      Setting the root directory (/) as the current working directory so that the process does not keep any directory in use that may be on a mounted file system (allowing it to be unmounted).
      Changing the umask to 0 to allow open(), creat(), and other operating system calls to provide their own permission masks and not to depend on the umask of the caller.
      Redirecting file descriptors 0, 1 and 2 for the standard streams (stdin, stdout and stderr) to /dev/null or a logfile, and closing all the other file descriptors inherited from the parent process.
      If the process is started by a super-server daemon, such as inetd, launchd, or systemd, the super-server daemon will perform those functions for the process, except for old-style daemons not converted to run under systemd and specified as Type=forking and "multi-threaded" datagram servers under inetd.


      = MS-DOS

      =
      In the Microsoft DOS environment, daemon-like programs were implemented as terminate-and-stay-resident programs (TSR).


      = Windows NT

      =
      On Microsoft Windows NT systems, programs called Windows services perform the functions of daemons. They run as processes, usually do not interact with the monitor, keyboard, and mouse, and may be launched by the operating system at boot time. In Windows 2000 and later versions, Windows services are configured and manually started and stopped using the Control Panel, a dedicated control/configuration program, the Service Controller component of the Service Control Manager (sc command), the net start and net stop commands or the PowerShell scripting system.
      However, any Windows application can perform the role of a daemon, not just a service, and some Windows daemons have the option of running as a normal process.


      = Classic Mac OS and macOS

      =
      On the classic Mac OS, optional features and services were provided by files loaded at startup time that patched the operating system; these were known as system extensions and control panels. Later versions of classic Mac OS augmented these with fully fledged faceless background applications: regular applications that ran in the background. To the user, these were still described as regular system extensions.
      macOS, which is a Unix system, uses daemons but uses the term "services" to designate software that performs functions selected from the Services menu, rather than using that term for daemons, as Windows does.


      See also




      References




      External links


      Unix Daemon Server Programming at the Wayback Machine (archived 2019-10-30)

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    Daemon (computing) - Wikipedia

    In multitasking computer operating systems, a daemon (/ ˈdiːmən / or / ˈdeɪmən /) [1] is a computer program that runs as a background process, rather than being under the direct control of an interactive user.

    What is a Daemon? - TechTarget

    In computing, a daemon (pronounced DEE-muhn) is a program that runs continuously as a background process and wakes up to handle periodic service requests, which often come from remote processes.

    Understanding Daemons: Their Role and Function in Computing

    Mar 1, 2024 · What is a daemon? A daemon is a computer program that runs in the background, performing various tasks or services without direct user interaction. It is usually initiated as part of the system boot process and continues to run until the system is shut down.

    Daemon Definition - What is a software daemon? - TechTerms.com

    May 9, 2023 · A daemon (also known as a "background process") is a program that runs in the background and performs tasks without any input from the computer's user. Instead, they wait for specific triggering events or conditions before performing their functions.

    What are Daemons and How to Use Them - Liquid Web

    If you ever manually installed software or a service on your server, a daemon runs in the background. This article will discuss what daemons are, the difference between a daemon and a process, the most common daemons, and how to use daemons on CentOS 6 and CentOS 7.

    Daemons in Computing: Types and Examples | NordVPN

    Feb 5, 2024 · A daemon is a computer program running in the background of a multitasking operating system (OS). Typically found in Unix or Linux programs, daemons open automatically and run in the background without the user’s input or control. Namely, they often open when the computer is booted up.

    What Is a Daemon? | phoenixNAP IT glossary

    Mar 20, 2024 · A daemon is a computer program that runs in the background instead of being under the user’s direct control. It performs tasks from the moment the operating system boots until the computer is turned off.

    What is a Daemon? - Computer Hope

    Jul 18, 2024 · In Unix and Linux, a daemon is a program that runs in the background without requiring user interaction. The file name of a software daemon usually ends in the letter d. For example, httpd (HyperText Transfer Protocol Daemon) runs as a background process on many web servers, including those created by Apache, CERN, and NCSA. Httpd waits for and ...

    Definition of daemon - PCMag

    Functioning like an extension to the operating system, a daemon is usually an unattended process that is initiated at startup. Typical daemons are print spoolers and email handlers or a scheduler...

    What Is a Daemon? - MUO

    Sep 15, 2021 · A daemon or Disk And Execution MONitor is a program that runs as a background process rather than an interactive one. Daemons are common among network tools and system administration tasks. You can control some daemons by sending signals to them.