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{{Short description|Computer program that runs as a background process}}
[[File:Free and open-source-software display servers and UI toolkits.svg|thumb|right|400px|Components of some [[Software:Linux|Linux]] desktop environments that are daemons include D-Bus, NetworkManager (here called ''unetwork''), PulseAudio (''usound''), and Avahi.]]

In [[Computer multitasking|multitasking]] computer [[Operating system|operating system]]s, a '''daemon''' ({{IPAc-en|ˈ|d|iː|m|ən}} or {{IPAc-en|ˈ|d|eɪ|m|ən}})<ref name="jargon">{{Cite web|url=http://catb.org/~esr/jargon/html/D/daemon.html |title=daemon |author=Eric S. Raymond |access-date=2008-10-22 |work=[[Jargon File|The Jargon File]] }}</ref> is a [[Computer program|computer program]] that runs as a [[Background process|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, {{mono|syslogd}} is a daemon that implements system logging facility, and {{mono|sshd}} is a daemon that serves incoming [[Secure Shell|SSH]] connections.

In a [[Software:Unix|Unix]] environment, the [[Parent process|parent process]] of a daemon is often, but not always, the [[Software:Init|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 [[Software:Tty (Unix)|terminal]] (tty). Such procedures are often implemented in various convenience routines such as ''daemon(3)'' in Unix.

Systems often start daemons at [[Booting|boot]] time that will respond to network requests, hardware activity, or other programs by performing some task. Daemons such as [[Software:Cron|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 [[Organization:MIT Computer Science and Artificial Intelligence Laboratory#Project MAC|Project MAC]] in 1963, his team was the first to use the term daemon, inspired by [[Physics:Maxwell's demon|Maxwell's demon]], an imaginary agent in physics and [[Physics:Thermodynamics|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".<ref>{{Cite web|title=The Origin of the word Daemon|url=http://ei.cs.vt.edu/~history/Daemon.html}}</ref> [[Software:Unix|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 [[Social:Greek language|Greek]] δαίμων. In the ''Unix System Administration Handbook'' [[Biography:Evi Nemeth|Evi Nemeth]] states the following about daemons:<ref>{{Cite web|title=The BSD Daemon|url=http://www.freebsd.org/copyright/daemon.html|access-date=2008-11-15|publisher=Freebsd.org}}</ref>

{{bquote|
Many people equate the word "daemon" with the word "demon", implying some kind of [[Religion:Satanism|satanic]] connection between UNIX and the [[Unsolved:Underworld|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 {{wipe|Ancient Greece}}' 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.}}

A further characterization of the mythological symbolism is that a daemon is something that is not visible yet is always present and working its will. In the ''Theages'', attributed to [[Biography:Plato|Plato]], [[Biography:Socrates|Socrates]] describes his own personal daemon to be something like the modern concept of a moral conscience: "The favour of the gods has given me a marvelous gift, which has never left me since my childhood. It is a voice that, when it makes itself heard, deters me from what I am about to do and never urges me on".{{citation needed|date=January 2020}}

In modern usage in the context of computer software, the word ''daemon'' is pronounced {{IPAc-en|ˈ|d|iː|m|ən}} {{respell|DEE|mən}} or {{IPAc-en|ˈ|d|eɪ|m|ən}} {{respell|DAY|mən}}.<ref name="jargon" />

Alternative terms for ''daemon'' are ''[[Software:Windows service|service]]'' (used in Windows, from Windows NT onwards, and later also in Linux), ''started task'' (IBM z/OS),<ref>{{cite web|url=https://www.ibm.com/docs/en/zos-basic-skills?topic=glossary-zos-terms-abbreviations|title=Glossary of z/OS terms and abbreviations|website=IBM Documentation|date=31 January 2006 |publisher=[[Company:IBM|IBM]]}}</ref> and ''ghost job'' (XDS [[Software:Universal Time-Sharing System|UTS]]). Sometimes the more general term ''server'' or ''server process'' is used, particularly for daemons that operate as part of [[Client–server model|client-server systems]].<ref>{{Cite web |title=sshd: OpenSSH server process {{!}} SSH Academy |url=https://www.ssh.com/academy/ssh/sshd |access-date=2023-12-04 |website=www.ssh.com |language=en}}</ref>

After the term was adopted for computer use, it was rationalized as a [[Social:Backronym|backronym]] for Disk And Execution MONitor.<ref>{{cite web|url=http://www.linfo.org/daemon.html|title=Daemon Definition|website=www.linfo.org}}</ref><ref name="jargon" />

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 [[Software:Init|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 [[Software:Init|init]] script or a [[SystemStarter]] script, involves:
* Optionally removing unnecessary variables from environment.
* Executing as a background task by [[Fork (system call)|forking]] and [[Exit (system call)|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, <code>setsid()</code>:
** Dissociating from the controlling [[Software:Tty (Unix)|tty]].
** Creating a new session and becoming the session leader of that session.
** Becoming a [[Process group|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 ({{mono|/}}) as the current working directory so that the process does not keep any directory in use that may be on a [[Mount (computing)|mounted]] file system (allowing it to be unmounted).
* Changing the umask to 0 to allow <code>open()</code>, <code>creat()</code>, and other operating system calls to provide their own permission masks and not to depend on the umask of the caller.
* Redirecting [[File descriptor|file descriptor]]s 0, 1 and 2 for the [[Standard streams|standard streams]] (stdin, stdout and stderr) to {{mono|/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|super-server]] daemon, such as {{mono|[[Software:Inetd|inetd]]}}, {{mono|[[Software:Launchd|launchd]]}}, or {{mono|[[Software:Systemd|systemd]]}}, the super-server daemon will perform those functions for the process,<ref name=inetd>{{man|8|inetd|FreeBSD}}</ref><ref>{{man|5|launchd.plist|Darwin}}</ref><ref name=systemd_service>{{cite web|title=systemd.service|url=http://www.freedesktop.org/software/systemd/man/systemd.service.html|publisher=freedesktop.org|access-date=August 25, 2012}}</ref> except for old-style daemons not converted to run under {{mono|systemd}} and specified as {{mono|Type{{=}}forking}}<ref name=systemd_service/> and "multi-threaded" datagram servers under {{mono|inetd}}.<ref name=inetd/>

===MS-DOS===
In the [[Engineering:DOS|Microsoft DOS]] environment, daemon-like programs were implemented as [[Terminate-and-stay-resident program|terminate-and-stay-resident program]]s (TSR).

===Windows NT===
On Microsoft Windows NT systems, programs called [[Software:Windows service|Windows service]]s 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 [[Software:Windows 2000|Windows 2000]] and later versions, Windows services are configured and manually started and stopped using the [[Software:Control Panel (Windows)|Control Panel]], a dedicated control/configuration program, the Service Controller component of the [[Software:Service Control Manager|Service Control Manager]] ({{mono|sc}} command), the {{mono|net start}} and {{mono|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 [[Software:Classic Mac OS|classic Mac OS]], optional features and services were provided by files loaded at startup time that patched the operating system; these were known as [[Software:Extension (Mac OS)|system extensions]] and control panels. Later versions of classic Mac OS augmented these with fully fledged [[Software:Extension (Mac OS)#Faceless background applications|faceless background applications]]: regular applications that ran in the background. To the user, these were still described as regular system extensions.

[[Software:MacOS|macOS]], which is a [[Software:Unix|Unix]] system, uses daemons but uses the term "services" to designate software that performs functions selected from the [[Software:Services menu#macOS|Services menu]], rather than using that term for daemons, as Windows does.

==See also==
{{Div col|colwidth=20em}}
* List of computer term etymologies
* [[Software:List of Unix daemons|List of Unix daemons]]
* [[Software:Service wrapper|Service wrapper]]
* [[Software bot]]
* [[User space]]
* Web service
* [[Software:Windows service|Windows service]]
{{div col end}}

==References==
{{Reflist}}

==External links==
* {{Webarchive|url=https://web.archive.org/web/20191030233137/http://www.enderunix.org/docs/eng/daemon.php |title=Unix Daemon Server Programming |date=2019-10-30 }}

[[Category:Process (computing)]]
[[Category:Servers (computing)]]

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