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Daemon has an interesting usage in computing. From my local dictionary:

a background process that handles requests for services such as print spooling and file transfers, and is dormant when not required

Does anyone know where this came from? I assume its relation to the word demon is notable. My dictionary also lists daemon as an archaic form of demon. Why did computing use daemon instead of demon?

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    I had always ascribed this to the high correlation between Linux programmers and D&D players.
    – JeffSahol
    Jun 24, 2011 at 17:41
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    I find it highly coincidental that this question has been posed so soon after this was posted to the r/programming reddit (and is currently at the very top of my homepage)
    – TJ Ellis
    Jun 24, 2011 at 18:57
  • @TJ Ellis: I don't follow reddit, but I do follow Hacker News, and this appeared at around the same time.
    – John Y
    Jun 25, 2011 at 14:20
  • must just be the zeitgeist :)
    – TJ Ellis
    Jun 26, 2011 at 17:40
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    Its association with computing goes back to the "Pandaemonium architecture" in perceptual research. In this model there are multiple extremely stupid daemons, each searching for one thing -- a corner, a fricative, some alcohol vapor -- that yell when they find it. The more they find, the louder they yell. Loudest daemon defines the sensation. Very primitive stuff, but it works. Apr 18 at 20:06

3 Answers 3

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In this interesting history, which is the actual description of the origin of this use from Professor Corbato, he explains that daemon originally had the connotation of "an attendant ... or indwelling spirit" and that:

By the late 16th century, the general supernatural meaning was being distinguished with the spelling daemon, while the evil meaning remained with demon.

He goes on to share the story of Maxwell's daemon. Maxwell was a 19th century scientist who'd posed a physics problem in which a tiny daemon was the attendant at a gate between two chambers that was large enough for a single molecule to enter. The daemon observed the molecules and chose which ones to allow to pass through the gate.

So now we come to the crux of it. We have established a daemon as an attendant, and a scientist used a hypothetical daemon in a famous problem in which the daemon's job was to monitor molecular movement. Professor Corbato wraps up his explanation with

As you probably know, the "system processes" called daemons monitor other tasks and perform predetermined actions depending on their behavior. This is so reminiscent of Maxwell's daemon watching his molecules that we can only assume that whoever dubbed these "system processes" had Maxwell's daemon in mind.

The history also notes that Professor Saltzer, who also worked on Project MAC with Professor Corbato at the time "daemon" came into use for this purpose, confirms that this is the origin of daemon as it is used in computing.

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    Maxwell's daemon was my first thought here for a "tireless / reliable automated attendant", so I was a bit surprised to see from Google NGrams that even that one is almost always spelt demon. It's always a daemon in my Linux-based computer system. Apr 18 at 17:25
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According to Fernando J. Corbato who worked on Project MAC in 1963 his team is the first to use the term daemon. The use of the term daemon was inspired by Maxwell's daemon, in physics and thermodynamics as an imaginary agent which helped to sort molecules.

"We fancifully began to use the word daemon to describe background processes which worked tirelessly to perform system chores."

(...)

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."

See a full explanation in the Daemon (computing) Wikipedia article.

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  • This explains what the word means, but how did it become associated with computing? Namely, if it was inspired by Maxwell's daemon, what was Maxwell's daemon?
    – MrHen
    Jun 24, 2011 at 17:47
  • @MrHen Project MAC was a Project on Mathematics and Computation, a collaborative computer endeavour in the 1960s that sought to to create a functional time-sharing system. --what was Maxwell's daemon? - From the answer an imaginary agent which helped to sort molecules.
    – Greybeard
    Dec 23, 2021 at 15:12
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According to https://kb.iu.edu/d/aiau, "Daemon" stands for Disk and Execution Monitor.

More information on this meaning here: www.techopedia.com: Disk and Execution Monitor (Daemon)

Extract:
What Does Disk and Execution Monitor (Daemon) Mean?

A disk and execution monitor (daemon) is a background process run in computer multitasking operating systems, usually at bootstrap time, to perform administrative changes or monitor services.

Common daemon processes include email handlers, print spoolers and other programs that perform OS administrative tasks. Daemons also perform specified operations at predefined times in response to events.

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    Your link doesn't cite any sources, while the jargon file not only disagrees but gives some history. The jargon file at least has some pedigree when it comes to documenting Unix/hacker culture.
    – Chris H
    Jan 8, 2016 at 17:40
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    The entry for daemon in The New Hacker's Dictionary, third edition (1996), begins this way: "daemon _n. {from the mythological meaning, later rationalized as the acronym 'Disk And Execution MONitor'} ..."Programmers seem peculiarly susceptible to rationalizing as acronyms computer terms whose origin they don't know.
    – Sven Yargs
    Jan 8, 2016 at 20:17

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