Apcupsd is an application that runs on Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

, UNIX

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

, Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

 and some Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 machines (using Cygwin

Cygwin

Cygwin is a Unix-like environment and command-line interface for Microsoft Windows. Cygwin provides native integration of Windows-based applications, data, and other system resources with applications, software tools, and data of the Unix-like environment...

). It allows the computer to interact with (almost all recent) APC

American Power Conversion

Schneider Electric's Critical Power and Cooling Services Business Unit, formerly known as American Power Conversion Corporation, is a manufacturer of uninterruptible power supplies, electronics peripherals and data center products....

 Uninterruptible power supplies (UPS). Apcupsd also works with some OEM

Original Equipment Manufacturer

An original equipment manufacturer, or OEM, manufactures products or components that are purchased by a company and retailed under that purchasing company's brand name. OEM refers to the company that originally manufactured the product. When referring to automotive parts, OEM designates a...

 APC power supplies (e.g., Hewlett-Packard

Hewlett-Packard

Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...

's).

Apcupsd must run in daemon mode to keep a live connection with the UPS. This is needed to monitor the UPS via USB

Universal Serial Bus

USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....

 or RS232.

Apcupsd is the free software

Free software

Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 equivalent of the APC's proprietary software called Powerchute

Powerchute

PowerChute is a computer program made by APC to control the uninterruptible power supplies the company produces. It provides unattended shutdown of servers and workstations in the event of an extended power outage. It also monitors and logs the UPS status...

. As of version 3.14.0 Apcupsd has support for the PowerChute Network Shutdown function as well as many other features.