3. Basics of a CLI application

1. What is a CLI application?

CLI (Command Line Interface) - an application that allows users to interact with it by typing commands on the terminal
- These applications run entirely in the terminal and do not have a graphical user interface (GUI)

Argument support
- CLI applications usually accept arguments (or data) when they are ran, these allow the user to specify configuration options or data to the app
- Arguments are usually passed after the command (i.e. npm install express)
  - In the example above there are two arguments being provided, install and express
- Arguments are almost always split on a space
Error and help messages
- Like any app, a CLI application should have basic error handling as well
- Usually this comes in the form of help messages when a user enters incorrect, or insufficient arguments
  - i.e. If your application requires two arguments to run and only one is specified an error message could be displayed telling the user why the program cannot continue
- Many applications will support some kind of help argument (often one of: help, -h, --help,)
  - When this argument is provided a help message is displayed which guides the user on how to use the app (i.e. what arguments are valid, how many are expected, etc.)

npm is an easy example of a CLI application
It contains no GUI, but instead is controlled entirely via the command line
Running npm with the help argument (or any invalid argument) will get you a help message displaying a list of valid commands
If an incorrect command is provided to npm an error message is displayed telling the user the comamnd was wrong and that they can use the help argument if necessary to discover other commands
```
$ npm not-a-command
Unknown command: "not-a-command"

To see a list of supported npm commands, run:
  npm help
```