Three important concepts when it comes to building command line applications are input, output and arguments. These three concepts are foundational for pretty much any command line application, as they define the application's behaviour and its ability to be configured, without needing to recompile code.

So in this module, we're going to cover all three concepts, beginning with modifying our application to first handle errors, which it currently can't do. We'll handle these by communicating to both the operating system and to our users as well. Then, once we've managed to add in support for handling errors, and writing to one of the three file streams that an operating system provides - standard error - we'll then move on to adding in support for our application to be able to handle files of any size.

Which, currently, it's not able to do. This modification will not only teach us how we can read in large files by making use of a binary or byte buffer, but will also introduce us to the concepts of interfaces in Go, which are foundational for writing code that is considered loosely coupled.

In addition to this, we're also going to look at a couple of key concepts related to passing an input to our application. The first of which is through the use of command line arguments, which we can use to configure our CLI application without needing to recompile the code. In our case, we're going to use these CLI arguments to select files that we want to count from, however, we'll also add in support so that we can select from multiple files to count at the same time.

Once we've taken a look at CLI arguments, we'll then take a look at the standard input stream, which in our case we can use in lieu of any files being passed in as arguments. This will allow our application to accept data from other commands as well, through the use of piping, or through prompting our user to enter input when the application is running.

In addition to input, this module will also focus on output, beginning by looking at the difference between standard out and standard error, which are two of the three file streams provided by the operating system to every process. By the end of this module, you should have a decent understanding of how to create a basic CLI application that will correctly handle reading input, writing output, and can be configured using CLI arguments.

So, let's jump in.