Daemon (computing)
In multitasking computer operating systems, a daemon 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 a daemon that implements 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 created either 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. Such procedures are often implemented in various convenience routines such as daemon 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.
Terminology
The term was coined by the programmers at 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.The word daemon is an alternative spelling of demon, and is pronounced . In the context of computer software, the original pronunciation has drifted to for some speakers.
Alternate terms for daemon are service, started task, and ghost job.
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.
Implementations
Unix-like systems
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 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. This allows daemon's parent to receive exit notification and continue its normal execution.
- Detaching from the invoking session, usually accomplished by a single operation,
setsid: - * 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, 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.
- 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. - Redirecting file descriptors 0, 1 and 2 for the standard streams to /dev/null or a logfile, and closing all the other file descriptors inherited from the parent process.
MS-DOS
In the Microsoft DOS environment, daemon-like programs were implemented as terminate and stay resident software.Windows NT
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, 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 Windows daemons have the option of running as a normal process.
Classic Mac OS and macOS
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. Note that macOS uses the term "services" to designate software that performs functions selected from the Services menu, rather than using that term for daemons as Windows does.
Etymology
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, stating, "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 Evi Nemeth states the following about daemons:
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".