Building Interactive CLI Applications with Cobra

Introduction

This tutorial aims to educate you on how to use the Cobra library to build interactive Command Line Interface (CLI) applications using Go. We will learn how to create commands, subcommands, and add flags to our commands using Cobra.

By the end of this tutorial, you will be able to:

• Understand the basics of Cobra
• Build a CLI application using Cobra
• Create commands, subcommands, and flags

Prerequisites:
- Basic knowledge of Go programming
- Go installed on your system

Step-by-Step Guide

Cobra is a library that provides a simple interface to create powerful modern CLI interfaces, similar to git & go tools. Cobra is also an application that will generate your application scaffolding to rapidly develop a Cobra-based application.

In this guide, we will start by installing Cobra, creating a new project, and then creating our commands.

Installing Cobra

go get -u github.com/spf13/cobra/cobra

Creating a new project

mkdir mycli && cd mycli
cobra init --pkg-name mycli

This will create a new Cobra application.

Creating Commands

Commands represent actions. In cobra, we can create a new command using cobra.Command.

var cmd = &cobra.Command{
   Use:   "command_name",
   Short: "Short description of the command",
   Long: `Long description of the command`,
   Run: func(cmd *cobra.Command, args []string) {
     // Do Stuff Here
   },
}

Code Examples

Let's create a hello command for our CLI app.

var helloCmd = &cobra.Command{
   Use:   "hello",
   Short: "Prints out 'Hello, World!'",
   Long: `A command to print out 'Hello, World!'`,
   Run: func(cmd *cobra.Command, args []string) {
     fmt.Println("Hello, World!")
   },
}

func init() {
   rootCmd.AddCommand(helloCmd)
}

In the above code:
- helloCmd is the command we are creating
- Use is the name of the command
- Short and Long are the descriptions of the command
- Run is the function that gets executed when the command is called
- rootCmd.AddCommand(helloCmd) adds the hello command to the root command

When you run your cli app with the hello command, it prints out Hello, World!.

Summary

In this tutorial, we learned about the Cobra library, how to create a new Cobra application, and how to create commands. We also saw a code example of creating a hello command.

Next, you can learn about creating subcommands, adding flags to commands, and more. Refer to the Cobra Documentation for more information.

Practice Exercises

1. Create a goodbye command that prints out Goodbye, World!
2. Create a greet command that takes in a name as a flag and prints out Hello, [name]!

Solutions

1. The goodbye command:

var goodbyeCmd = &cobra.Command{
   Use:   "goodbye",
   Short: "Prints out 'Goodbye, World!'",
   Run: func(cmd *cobra.Command, args []string) {
     fmt.Println("Goodbye, World!")
   },
}

func init() {
   rootCmd.AddCommand(goodbyeCmd)
}

1. The greet command:

var name string

var greetCmd = &cobra.Command{
   Use:   "greet",
   Short: "Greets a user",
   Run: func(cmd *cobra.Command, args []string) {
     fmt.Printf("Hello, %s!\n", name)
   },
}

func init() {
   greetCmd.Flags().StringVarP(&name, "name", "n", "", "Name to greet")
   rootCmd.AddCommand(greetCmd)
}

In the greet command, we added a name flag using greetCmd.Flags().StringVarP(). This allows us to pass in a name when calling the greet command.