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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 '11 at 17:41
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Very interesting question, MrHen. –  KitFox Jun 24 '11 at 18:13
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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 '11 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 '11 at 14:20
    
must just be the zeitgeist :) –  TJ Ellis Jun 26 '11 at 17:40
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2 Answers

up vote 18 down vote accepted

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 would 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. 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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+1, really good answer. –  Ankur Banerjee Jun 24 '11 at 18:28
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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 full explanation here in the wikipedia.

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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 '11 at 17:47
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