Linux/UNIX treats everything like a file.

1. Linux/UNIX treats everything like a file.
2. When it's writing to your screen it thinks it's just writing to a file.
3. When it sends data through a modem it thinks it's just writing to a file.
4. As a result, all your hardware, including

ports,
hard-drives,
video cards, etc.
on your system must be represented somehow somewhere in the file system.