service
also: daemon, background process, systemd unit
A service is a background process or daemon that runs continuously on a Linux system to provide specific functionality, such as web serving, email delivery, or SSH access. Services are typically managed by init systems like systemd and can be started, stopped, or restarted on demand.
A service is a long-running program that performs a particular function in the background without requiring direct user interaction. Services are essential to Linux system administration—they handle everything from network communication to file sharing to system logging.
Services are controlled through init systems. On modern systems using systemd, you manage services with commands like systemctl start nginx, systemctl stop apache2, or systemctl restart ssh. Each service typically has a unit file defining how it should behave—whether it starts at boot, what user it runs as, and what dependencies it needs.
For example, a web server like Nginx runs as a service in the background, listening on port 80 for incoming HTTP requests. You don't run it manually each time; instead, the init system starts it automatically when the system boots and keeps it running.
Services can be enabled (start automatically at boot) or disabled, and their status can be checked with systemctl status service-name. Log output from services is typically sent to the system journal, viewable with journalctl.