The final purpose of every operating system is to allow the user and the applications to be able to use and interact with hardware. Unix also aims to provide access to the attached hardware in an abstract and transparent way, using the same set of utility programs and commands in all existing and future platforms.
By having this transparency and abstraction, Unix abstracts all of the different hardware to be a number of devices attached to a system. So, the term device is centric in Unix, and every connected piece of hardware is considered to be a device connected to the Unix system.
The hardware attached to a computer can be categorized into two different categories: mandatory and peripheral. The CPU and main memory are mandatory devices attached to a Unix system. All other hardware such as the hard drive, network adapter, mouse, monitor, graphics card, and Wi-Fi adapter, are peripheral devices.
A Unix machine cannot work without mandatory hardware, but you can have...