cupsd

cupsd is the daemon core of the Common UNIX Printing System (CUPS), the printer system used on many Unix-like operating systems, including Linux distributions and macOS. It is responsible for receiving print jobs, managing print queues, scheduling jobs, and coordinating communication with printers and print drivers.

The daemon runs in the background as cupsd, typically listening on port 631 via HTTP. It provides a web-based administration interface and exposes a standard API (IPP) for clients to submit jobs, query queue status, and control printers. It can operate with both local and network printers.

CUPS uses a modular architecture. Printers are described by PostScript Printer Description (PPD) files, and a set of filters converts input documents into printer-ready formats. Backends send the data to printers over various transports such as IPP, AppSocket/JetDirect, LPD, or SMB. A spool directory holds pending jobs while they print.

Configuration is controlled by /etc/cups/cupsd.conf (or equivalent) and related files. The system defines access controls, log levels, and port and authentication settings. CUPS stores printers, classes, and options in its internal databases, and status information is exposed through command-line tools (lp, lpr, lpstat, cancel, lpadmin) or the web interface.

CUPS is the de facto printing system on many Unix-like platforms and is the printing backbone in macOS. It supports remote management, authentication, and a wide range of printers through PPDs and drivers, enabling both local and network printing within organizations.