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In a UNIX-like environment, the Parent Process of a daemon is normally Init ( PID =1). Processes usually become daemons by Forking a child process and then having their parent process immediately exit, thus causing init to adopt the child process. This is a somewhat simplified view of things, naturally, as other operations are generally performed (such as disassociating the daemon process from any controlling Tty ), convenience routines such as daemon(3) existing in some UNIX systems for that purpose.

Systems often start (or "launch") daemons at on some Linux systems), run scheduled tasks (like Cron ), and perform a variety of other tasks.


TERMINOLOGY

The term was coined by the programmers of MIT's Project MAC . They took the name from Maxwell's Demon , an imaginary being from a famous thought experiment that constantly works in the background, sorting molecules.1 Unix systems inherited this terminology. Daemons are also 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. BSD and some of its derivatives have adopted a daemon as Its Mascot , although this mascot is actually a Cute Stereotypical depiction of a Demon from Christianity . (The alternative expansion of "daemon" as "disk '''a'''nd '''e'''xecution '''mon'''itor" is also sometimes used, but is a Backronym .)


Pronunciation


The word ''daemon'', taken out of its computer science context, is universally pronounced as 234, i.e., as a homonym of the word ''demon''. Perhaps due to the relative obscurity of the word in other contexts, the alternate pronunciation of has popularity in its computer science context5.


Humor

The term daemon often leads to humorous connections with its mythical BSD-derived Mac OS X is Satan ic due to its use of daemons.


TYPES OF DAEMONS

In a strictly technical sense, in the Unix world, a process comprises a daemon when it has process number 1 ( Init ) as its parent process and no controlling terminal. The init process adopts any process whose parent process terminates. The common method for a process to become a daemon involves:
  • Disassociating from the controlling Tty

  • Becoming a session leader

  • Becoming a Process Group leader

  • Staying in the Background by Forking and Exiting (once or twice). This is required sometimes for the process to become a session leader. It also allows the parent process to continue its normal execution. This Idiom is sometimes summarized with the phrase "fork off and die"

  • Setting the Root Directory ("/") as the current Working Directory so that the process will not keep any directory in use

  • Changing the Umask to 0 to allow open(), creat(), et al. calls to provide their own permission masks and not to depend on the umask of the caller

  • Closing all inherited open files at the time of execution (required files will be opened later) that are left open by the parent process. Those include file descriptors 0, 1 and 2 (stdin, stdout, stderr) too.

  • Using a Logfile , the Console , or /dev/null as Stdin, Stdout, And Stderr


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.

"DAEMON" are named after "Data Event Monitors", referring to any system-based program (as opposed to a user application) that ran all day long, recording and reporting on various system statistics, such as number of users, time spent online, input/output processes, file accesses, etc. DAEMONs were used in Digital Equipment Corporation (DEC)
computers.


ETYMOLOGY


In the general sense, daemon is an older form of the word demon. In the Unix System Administration Handbook, Evi Nemeth http://www.freebsd.org/copyright/daemon.html, has this to say 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. (p.403)



WINDOWS EQUIVALENT

In the Microsoft DOS environment, such programs were written as Terminate And Stay Resident (TSR) 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. They run as processes, usually do not interact with the monitor, keyboard, and mouse, and are launched (or not launched) by the operating system at boot time.


MAC OS EQUIVALENT

On the original Mac OS , optional features and services were provided by files loaded at startup time that patched the operating system; these were known as System Extensions and ''' Control Panels '''. Later versions of classic Mac OS augmented these with fully-fledged ''' Faceless Background Applications ''': regular applications that ran in the background. To the user, these were still described as, and disguised as, regular system extensions.

Mac OS X , being a Unix-like system, has daemons. (There are '' Services '' as well, but these are completely different in concept.)


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