Flags, environment variables, and .env file config loading https://play.golang.org/p/pautLXl7yMW

.env

MYSQL_HOST=127.0.0.1
MYSQL_USER=hello
MYSQL_PASS=world
MYSQL_NAME=myapp
MYSQL_PORT=3306

main.go

package main

import (
	"bufio"
	"flag"
	"fmt"
	"io"
	"os"
	"strings"

	"github.com/peterbourgon/ff"
)

func main() {
	_, err := New()
	if err != nil {
		fmt.Println(err)
	}
}

// Config for the root command, including flags and types that should be
// available to each subcommand.
type Config struct {
	MySQL   mysqlConfig
	Verbose bool
}

type mysqlConfig struct {
	User string
	Pass string
	Host string
	Port string
	Name string
}

func New() (*Config, error) {
	c := &Config{}
	fs := flag.NewFlagSet("app", flag.ExitOnError)

	fs.StringVar(&c.MySQL.Host, "MYSQL_HOST", "", "MySQL host")
	fs.StringVar(&c.MySQL.Port, "MYSQL_POST", "", "MySQL port")
	fs.StringVar(&c.MySQL.User, "MYSQL_USER", "", "MySQL username")
	fs.StringVar(&c.MySQL.Pass, "MYSQL_PASS", "", "MySQL password")
	fs.StringVar(&c.MySQL.Name, "MYSQL_NAME", "", "MySQL database name")
	fs.BoolVar(&c.Verbose, "v", false, "log verbose output")

	return c, ff.Parse(fs,
		os.Args[1:],
		ff.WithEnvVarNoPrefix(),
		ff.WithConfigFileParser(EnvParser),
		// ff.WithConfigFileParser(ff.JSONParser),
		// ff.WithConfigFileParser(ff.PlainParser),
		ff.WithConfigFileFlag("config-file"),
	)
}

// EnvParser is a parser for config files in an extremely simple format. Each
// line is tokenized as a single key/value pair. The first whitespace-delimited
// token in the line is interpreted as the flag name, and all remaining tokens
// are interpreted as the value. Any leading hyphens on the flag name are
// ignored.
func EnvParser(r io.Reader, set func(name, value string) error) error {
	s := bufio.NewScanner(r)
	for s.Scan() {
		line := strings.TrimSpace(s.Text())
		if line == "" {
			continue // skip empties
		}

		if line[0] == '#' {
			continue // skip comments
		}

		var (
			name  string
			value string
			index = strings.IndexRune(line, '=')
		)

		if index < 0 {
			name, value = line, "true" // boolean option
		} else {
			name, value = line[:index], strings.TrimSpace(line[index+1:])
		}

		fmt.Println("env:", name, value, index)

		// remove ending comment
		if i := strings.Index(value, " #"); i >= 0 {
			value = strings.TrimSpace(value[:i])
		}

		if err := set(name, value); err != nil {
			return err
		}
	}
	return nil
}


// or you can just wrap what already exists
func EnvTransformParser(r io.Reader, set func(name, value string) error) error {
	b, err := ioutil.ReadAll(r)
	if err != nil {
		return err
	}
	r = bytes.NewReader(bytes.ReplaceAll(b, []byte("="), []byte(" ")))
	return ff.PlainParser(r, set)
}