: This article is about a class of computer software. For other uses, see Daemon.

In Unix and other computer computer multitasking operating systems, a daemon is a computer program that runs in the background (computer software), rather than under the direct control of a user; they are usually instantiated as Computer processes. Typically daemons have names that end with the letter d ; for example, syslogd is the daemon which handles the system log.

Systems often start (or launch ) daemons at ), and perform a variety of other tasks.

The programmers of CTSS coined the term by analogy to Maxwell s demon, and all the systems descended from it, including Unix, have inherited the terminology. Daemon (mythology) are characters in Greek mythology, some of whom handled tasks that the gods couldn t be bothered with, much like computer daemons often handle tasks in the background that the user can t be bothered with. Berkeley Software Distribution and some of its derivatives have adopted a daemon as BSD Daemon, although this mascot is actually a cute stereotypical depiction of a Christianity demon. (The alternative expansion of daemon as disk and execution monitor is also sometimes used, but it appears to be a backronym.)

In a strictly technical sense, Unix recognises as a daemon any process that has process number 1 (Init) as its parent process. The init process adopts any process whose parent dies without waiting for the child s status, so the common method for launching a daemon involves forking once or twice, and making the parent (and possibly the 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 Unix usage a daemon may be any background process, whether a child of init or not. UNIX users sometimes spell daemon as demon , and most usually pronounce the word that way.

In the DOS environment, such programs were written as Terminate and Stay Resident software.

On Microsoft Windows systems, programs called services perform the functions of daemons, though the term daemon has started to creep into common usage on that platform as well.

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