The Root Command

Inside the cmd directory ("package") that cobra-cli scaffolded for you should be a file called root.go. Let's look at some key parts of this file (I've truncated a few sections with ellipsis, ..., so that it appears cleaner in this book.)

// rootCmd represents the base command when called without any subcommands
var rootCmd = &cobra.Command{
	Use:   "math",
	Short: "A brief description of your application",
	Long: `...`,
	// Uncomment the following line if your bare application
	// has an action associated with it:
	// Run: func(cmd *cobra.Command, args []string) { },
}

Here we define the variable rootCmd at the global scope to be a *cobra.Command. the cobra.Command struct is the building block of cobra-based applications, and is how we define almost everything we need (metadata, logic, etc.) to execute our business logic. Every command and subcommand will be defined as one of these cobra.Commands. We'll use these to build out our tree of subcommands.

As we saw, our main() function calls the Execute() function in this package, which itself just wraps our rootCmd.

// Execute adds all child commands to the root command and sets flags appropriately.
// This is called by main.main(). It only needs to happen once to the rootCmd.
func Execute() {
	err := rootCmd.Execute()
	if err != nil {
		os.Exit(1)
	}
}

As the scaffolded comment suggests, this is called by main(), and only ever needs to happen here.

As mentioned in the kubectl context, the root command of a non-trivial CLI application is typically organizational, and typically doesn't have any logic associated with it. That way, when a user runs our command with no flags, subcommands, args, or otherwise, they get the help output in return to help guide them.