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Daemon definition

A daemon is a computer program that is always running in the background of a multitasking operating system.

The system user doesn’t have to start a daemon program for it to run because daemon programs are not under the user’s control. Instead of the user, a certain event or action starts a daemon program.

For example, if a piece of software is manually installed on a server, a daemon will run in the background. Also, if a certain service is installed on a server, a daemon will run in the background in this case too.

Daemons are Unix or Linux programs that are executed to respond to requests for certain hardware activities and services through other systems on the network and respond to requests from other programs as well.

See also: HTTP, secure shell

Most common daemon programs

  • Cron Daemon (crond): A crond is a job scheduler that triggers specific actions at scheduled times.
  • OpenSSH Daemon (sshd): An OpenSSH daemon processes incoming requests by listening to the secure shell protocol.
  • mysqld: A mysqld daemon can stop and start a MySQL database server.
  • httpd: A httpd waits for and answers incoming requests sent to its web server.
  • SMTP or mailer daemon (sendmail): Sendmail controls the email automatization on a certain server or system.