Let's get the next dan of bash with the text processing.
The 1st note is the greatest idea of the whole unix is in that everything is file - classic file, directory, devices, file systems and so on.
Many commands expects the file for its input or it could be a simple standard input also known as stdin (refers as stream #0).
As well many commands put their output to standard output aka stdout (refers as stream #1) or to standard error output aka stderr (refers as stream #2).
There are several key text processing command and several techniques like stream redirection.
And once more! the most powerful command is man means manual.