Goでコマンドラインツールを作るときの鉄板ライブラリ ref: https://qiita.com/nasum/items/f1911894802a2b92dc9e

file2.txt

$ go run main.go     
root command

$ go run main.go --help
command line calculator

Usage:
  culc [flags]

Flags:
  -h, --help   help for culc

file5.txt

$ go run main.go help
command line calculator

Usage:
  culc [flags]
  culc [command]

Available Commands:
  help        Help about any command
  sum         sum culc

Flags:
  -h, --help   help for culc

Use "culc [command] --help" for more information about a command.

file6.txt

$ go run main.go sum 1 1
2

file7.txt

green := color.New(color.FgGreen).SprintFunc()
fmt.Println(green(itemOne + itemTwo))

main.go

package main

import (
    "fmt"
    "os"

    "github.com/nasum/culc/cmd"
)

func main() {
    if err := cmd.RootCmd.Execute(); err != nil {
        fmt.Fprintf(os.Stderr, "%s: %v\n", os.Args[0], err)
        os.Exit(-1)
    }
}

root.go

func init() {
    cobra.OnInitialize()
    RootCmd.AddCommand(
        sumCmd(),
    )
}

sum.go

package cmd

import (
    "fmt"
    "log"
    "strconv"

    "github.com/spf13/cobra"
)

func sumCmd() *cobra.Command {
    cmd := &cobra.Command{
        Use:   "sum",
        Short: "sum culc",
        Args:  cobra.RangeArgs(2, 2),
        RunE: func(cmd *cobra.Command, args []string) error {
            if len(args) == 0 {
                return nil
            }

            itemOne, err := strconv.Atoi(args[0])

            if err != nil {
                log.Fatal(err)
                return nil
            }

            itemTwo, err := strconv.Atoi(args[1])

            if err != nil {
                log.Fatal(err)
                return nil
            }

            fmt.Println(itemOne + itemTwo)

            return nil
        },
    }

    return cmd
}