Building a Go Command Line Tool

Our project needs to be created. We'll do so by creating a directory for us to add our source files.

Our list of users has a very specific shape, and we can represent that shape using a struct. Let's create a main.go file and add a User struct to it that handles the name, id, home, and shell values.

We're going to create a function called collectUsers that parses the /etc/passwd file and returns a slice of User structs. Now we'll be able to use the user information in the main portion of our program.

To keep things manageable, we're going to define our flags in a separate function that performs the following steps:

1. Defines the path and format flags
2. Parses flags
3. Validates the user-provided information. If there is an error, then exits and displays an error.
4. Returns path and format values

We want to write the content to either a file or stdout, and thankfully they both adhere to the io.Writer interface. Because of this interface, we can keep our writing logic ignorant of whether we're writing to a file or stdout. Here's what we need to implement, in order:

1. Parse flags into variables using parseFlags.
2. Gather users using collectUsers.
3. Create an io.Writer variable.
4. Assign a value to the io.Writer variable based on the path variable. This variable should be a file if the path variable is set or os.Stdout otherwise.
5. If the format variable is "json", utilize the json.MarshalIntent function to pretty-print the users to a slice of bytes before using the Write function on our io.Writer variable. OR
6. If the format variable is "csv", write the header line to the io.Writer, create a new csv.Writer from the io.Writer, and write a line for each of the users.