envconf

package main

import (
	"fmt"

	"github.com/jackc/envconf"
)

func main() {
	config := envconf.New()

	// Register configuration items.
	config.Register(envconf.Item{
		Name:        "FOO",
		Default:     "default-foo",
		Description: "the foo",
	})

	config.Register(envconf.Item{
		Name:        "BAR",
		Default:     "default-bar",
		Description: "the bar",
	})

	// You can override the source of environment variables by setting LookupEnvFunc. Defaults to os.LookupEnv.
	config.LookupEnvFunc = func(key string) (string, bool) {
		if key == "FOO" {
			return "foo from environment", true
		}
		return "", false
	}

	// You can use a config to create help output.
	fmt.Println("Configuration items:")
	for _, item := range config.Items() {
		fmt.Printf("%s: %s (default: %s)\n", item.Name, item.Description, item.Default)
	}

	fmt.Println()

	value := config.Value("FOO")
	fmt.Println("FOO", value)

	value = config.Value("BAR")
	fmt.Println("BAR", value)

}

Output:

Configuration items:
BAR: the bar (default: default-bar)
FOO: the foo (default: default-foo)

FOO foo from environment
BAR default-bar