Everything is a File (Wikipedia)

Everything is a file is an idea that Unix, and its derivatives, handle input/output to and from resources such as documents, hard-drives, modems, keyboards, printers and even some inter-process and network communications as simple streams of bytes exposed through the filesystem name space. […] The advantage of this approach is that the same set of tools, utilities and APIs can be used on a wide range of resources and a number of file types.