In multitasking computer operating systems, a daemon (// or //) is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is the daemon that implements the system logging facility, and sshd is a daemon that serves incoming SSH connections.
In a Unix environment, the parent process of a daemon is often, but not always, the init process. A daemon is usually either created by a process forking a child process and then immediately exiting, thus causing init to adopt the child process, or by the init process directly launching the daemon. In addition, a daemon launched by forking and exiting typically must perform other operations, such as dissociating the process from any controlling terminal (tty). Such procedures are often implemented in various convenience routines such as daemon(3) in Unix.
Systems often start daemons at boot time which will respond to network requests, hardware activity, or other programs by performing some task. Daemons such as cron may also perform defined tasks at scheduled times.
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 thought experiment that constantly works in the background, sorting molecules. Unix systems inherited this terminology. Maxwell's demon is consistent with Greek mythology's interpretation of a daemon as a supernatural being working in the background, with no particular bias towards good or evil. However, BSD and some of its derivatives have adopted a Christian demon as their mascot rather than a Greek daemon.
After the term was adopted for computer use, it was rationalized as a "backronym" for Disk And Execution MONitor.
Daemons which connect to a computer network are examples of network services.
In a strictly technical sense, a Unix-like system process is a daemon when its parent process terminates and the daemon is assigned the init process (process number 1) as its parent process and has no controlling terminal. However, more generally, a daemon may be any background process, whether a child of the init process or not.
On a Unix-like system, the common method for a process to become a daemon, when the process is started from the command line or from a startup script such as an init script or a SystemStarter script, involves:
- Optionally removing unnecessary variables from environment.
- Executing as a background task by forking and exiting (in the parent "half" of the fork). This allows daemon's parent (shell or startup process) to receive exit notification and continue its normal execution.
- Detaching from the invoking session (usually accomplished by a single operation,
- Dissociating from the controlling tty.
- Creating a new session and becoming the session leader of that session.
- Becoming a process group leader.
- If the daemon wants to ensure that it won't acquire a new controlling tty even by accident (which happens when a session leader without a controlling tty opens a free tty), it may fork and exit again. This means that it is no longer a session leader in the new session, and can't acquire a controlling tty.
- Setting the root directory (/) as the current working directory so that the process does not keep any directory in use that may be on a mounted file system (allowing it to be unmounted).
- Changing the umask to 0 to allow open(), creat(), and other operating system calls to provide their own permission masks and not to depend on the umask of the caller.
- Closing all inherited files at the time of execution that are left open by the parent process, including file descriptors 0, 1 and 2 for the standard streams (stdin, stdout and stderr). Required files will be opened later.
- Using a logfile, the console, or /dev/null as stdin, stdout, and stderr.
If the process is started by a super-server daemon, such as inetd, launchd, or systemd, the super-server daemon will perform those functions for the process (except for old-style daemons not converted to run under systemd and specified as Type=forking and "multi-threaded" datagram servers under inetd).
On Microsoft Windows NT systems, programs called Windows services perform the functions of daemons. They run as processes, usually do not interact with the monitor, keyboard, and mouse, and may be launched by the operating system at boot time. In Windows 2000 and later versions, Windows services are configured and manually started and stopped using the Control Panel, a dedicated control/configuration program, the Service Controller component of the Service Control Manager (sc command), the net start and net stop commands or the PowerShell scripting system.
However, any Windows application can perform the role of a daemon, not just a service, and some daemons for Windows have the option of running as a normal process.
On the classic 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 regular system extensions.
macOS, which is a Unix system, uses daemons. (The term "services" is used in macOS for software that performs functions selected from the Services menu, rather than being used for daemons as in Windows.)
According to Fernando J. Corbató who worked on Project MAC in 1963, his team was the first to use the term daemon, inspired by Maxwell's demon, an imaginary agent in physics and thermodynamics that 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 general sense, daemon is an older form of the word demon, from the Greek δαίμων. In the Unix System Administration Handbook, page 403, 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.
A further characterization of the mythological symbolism is that a daemon is something which is not visible yet is always present and working its will. In the Theages, attributed to Plato, Socrates describes his own personal daemon to be something like the modern concept of a moral conscience:
The favour of the gods has given me a marvelous gift, which has never left me since my childhood. It is a voice which, when it makes itself heard, deters me from what I am about to do and never urges me on.
- Eric S. Raymond. "daemon". The Jargon File. Retrieved 2008-10-22.
- Fernando J. Corbató (2002-01-23). "Take Our Word for It". Retrieved 2006-08-20.
- "Merriam-Webster definition of daemon". Merriam-Webster Online. Retrieved 2009-08-05.
- "IBM Knowledge Center - Glossary of z/OS terms and abbreviations". IBM.
- "Daemon Definition". www.linfo.org.
inetd(8)– FreeBSD System Manager's Manual
launchd.plist(5)– Darwin and macOS File Formats Manual
- "systemd.service". freedesktop.org. Retrieved August 25, 2012.
- "The Origin of the word Daemon".
- "The BSD Daemon". Freebsd.org. Retrieved 2008-11-15.