In this lesson, you were introduced to the Unix model of processing input and output, standard I/O, STDIN, STDOUT, STDERR, as well as piping. Here's a review of some of the key points:
Every program has a STDIN, a STDOUT, and a STDERR. Not all programs use them for user interaction (programs such as Photoshop just don't lend themselves to command-line control), but for the vast majority that do, these input and output connections can be manipulated.
You can provide the input data that a program expects on STDIN by hand, from a file, or from another program.
You can send the STDOUT and STDERR of a program into a file if you want to collect it for future use rather than viewing it as it is produced.
You can pipe the STDOUT of one program into ...