Example CLI, Env & Default Variables in Go using Viper and Cobra

cli_env_example.go

// Example Usage of Viper & Cobra to parse CLI, Env & Default Vars.
//
// ------------------------------------------------------------
// $ FIRST=aaaa NUM=123 go run cli_env_example.go --first bbb
//
//	Default: first=default-first, second=default-second, num=1
//	Params: first=bbb, second=default-second, num=123
// ------------------------------------------------------------
// $ FIRST=aaaa NUM=123 go run cli_env_example.go --help
//
//  My Sample CLI Application
//
//  Usage:
//    myapp [flags]
//
//  Flags:
//    -f, --first string    First Param (default "aaaa")
//    -h, --help            help for myapp
//    -n, --num int         Num Param (default 123)
//    -s, --second string   Second Param (default "default-second")
// ------------------------------------------------------------

package main

import (
	"fmt"

	"github.com/spf13/cobra"
	"github.com/spf13/viper"
)

type AppParams struct {
	first  string
	second string
	num    int64
}

var AppDefaults = AppParams{
	first:  "default-first",
	second: "default-second",
	num:    1,
}

var params = AppParams{}

var rootCmd = &cobra.Command{

	Use:   "myapp",
	Short: "My Sample CLI Application",

	Run: func(cmd *cobra.Command, args []string) {
		
		fmt.Printf("Default: first=%s, second=%s, num=%d\n", AppDefaults.first, AppDefaults.second, AppDefaults.num)
		fmt.Printf("Params: first=%s, second=%s, num=%d\n", params.first, params.second, params.num)
		
		// Do some important stuff ...
	},
}

func init() {
	viper.AutomaticEnv()
	viper.SetDefault("first", AppDefaults.first)
	viper.SetDefault("second", AppDefaults.second)
	viper.SetDefault("num", AppDefaults.num)

	rootCmd.PersistentFlags().StringVarP(&params.first, "first", "f", viper.GetString("first"), "First Param")
	viper.BindPFlag("first", rootCmd.PersistentFlags().Lookup("first"))

	rootCmd.PersistentFlags().StringVarP(&params.second, "second", "s", viper.GetString("second"), "Second Param")
	viper.BindPFlag("second", rootCmd.PersistentFlags().Lookup("second"))

	rootCmd.PersistentFlags().Int64VarP(&params.num, "num", "n", viper.GetInt64("num"), "Num Param")
	viper.BindPFlag("num", rootCmd.PersistentFlags().Lookup("num"))

}

func main() {
	if err := rootCmd.Execute(); err != nil {
		fmt.Errorf("Encountered error: %v", err)
	}
}

When going this route, consider rather the following:

Sting of the Viper - Post
Sting of the Viper - GitHub