"If you've never dabbled with filters and pipes before, or
perhaps you've just been too scared, we want to help you out, so
read on to learn how you can make powerful Linux commands just by
stringing smaller bits together...
"A filter is a program that reads a single input stream,
transforms it in some way, and writes the result to a single output
stream, as shown in Figure 1, below. By default, the output stream
(called standard output or just stdout) is connected to the
terminal window that the program is running in, and the input
stream (standard input, or just stdin) is connected to the
keyboard, though in practice filters are rarely used to process
data typed in manually at the keyboard."
