CLI.md

Basic Command Line Program

Go allows you to quickly and easily create a command line program.

In this guide, we will cover the basic concepts command line arguments using the [http://golang.org/pkg/flag/](Flags Package).

Simple CLI

In our first program, we will see what we get out of the box with very little effort.

Create the following program by opening a file called cli.go and adding the following contents:

// https://play.golang.org/p/DPigLqZ5Co

package main

import (
	"flag"
	"fmt"
)

func main() {
	var cmd string

	flag.StringVar(&cmd, "cmd", cmd, `cmd can be either "hello" or "bye"`)
	flag.Parse()

	switch cmd {
	case "hello":
		fmt.Println("Hello!")
	case "bye":
		fmt.Println("Bye!")
	default:
		flag.Usage()
	}
}

To run the program, issue this command:

go run cli.go

You should get something like this:

Usage of /var/folders/l7/3s7z7s1s4n72lvj4w6g_fdmm0000gn/T/go-build844850686/command-line-arguments/_obj/exe/basic:
  -cmd="": cmd can be either "hello" or "bye"

Breaking it down.

As you can see, if we don't provide any arguments, it prints out the Usage of the program.

Let's pass it an argument:

go run cli.go -cmd=hello

Now you should see that it prints

Hello!

flag.StringVar

This method allows us to tell the flag package to look for specific argument names, in this case, cmd.

For more information, see the definition for http://golang.org/pkg/flag/#FlagSet.StringVar.

Compiling a binary

But wait, what if I want to actually compile the binary? Easy enough, run this command:

go build cli.go

You will now have a file called cli that is an executable. To run that, issue this command:

./cli -cmd=hello

Summary

Congratulations, you just wrote your first command line program! We only scratched the surface, but I hope you enjoyed the quick start.