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Daemon and daimon are distinctive, older spellings of demon. This spelling is used purposely today to distinguish the daemon of Greek mythology, "supernatural beings between mortals and gods, such as inferior divinities and ghosts of dead heroes", from the Judeo-Christian usage demon, "a malignant spirit that can possess humans".

Daemons ("replete with knowledge", "divine power", "fate" or "god") were not necessarily evil. The Greek divided daemons into good and evil categories: eudaemons and cacodaemons, respectively.
Eudaemons were somewhat similar to the modern idea of the guardian angel. They watched over ordinary mortals to help keep them out of trouble. Socrates claimed to have a daimon that warned him and gave him advice but never coerced him into following it. He claimed that his daimon was more accurate than any of the forms of divination practiced at the time.
See also eudaimonia.

In computer science, a daemon is a particular class of computer programs. The term was coined by the inventors of UNIX, to refer to a process which runs in the background, rather than under the direct control of a user. Daemons are often launched at system start-up time, and often serve the function of responding to network requests, hardware activity, or other programs by performing some task.

On Microsoft Windows systems, such functions are performed by programs called "services", though the term "daemon" is creeping into common usage on that platform as well.

In a strictly technical sense, a daemon on UNIX is defined to be any process whose parent process is process number 1, init. Any process whose parent dies without waiting for the child's status is adopted by init, so the common idiom for launching a demon is to fork once or twice, and make the parent (and grandparent) die while the child (or grandchild) process begins performing its normal function. The idiom is sometimes summarized with the phrase "fork off and die". In common usage a daemon may be any background process, whether or not it is the child of init.

Daemons can also be used to configure hardware (like devfsd on some Linux systems), run scheduled tasks (like crond), and a variety of other tasks.

UNIX users sometimes spell daemon as demon, and most usually pronounce the word that way.

Daemon is an acronym for 'disk and execution monitor'

In the His Dark Materials trilogy, Philip Pullman describes a fictional world where everyone has a daemon, although he spells it 'Dæmon'. This takes the form of an animal, which has a separate identity even though it is an integral part of a person in that world; a physical manifestation of their soul. This concept is sometimes referred to as a "Familiar."

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