main.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"reflect"
	"strings"

	"github.com/kelseyhightower/envconfig"
	"gopkg.in/yaml.v2"
)

type Input struct {
	Path string `envconfig:"PATH" required:"true"`
}

func main() {
	fmt.Println("Ready to compile ...")

	cfg := Input{}
	if err := envconfig.Process("input", &cfg); err != nil {
		fmt.Printf("process env var: %s", err)
		os.Exit(1)
	}

	filename, _ := filepath.Abs(cfg.Path)
	yamlFile, err := ioutil.ReadFile(filename)
	if err != nil {
		panic(err)
	}

	var mapResult map[interface{}]interface{}
	err = yaml.Unmarshal(yamlFile, &mapResult)
	if err != nil {
		panic(err)
	}

	fmt.Println(fmt.Sprintf("Env variables will be replaced: %v", mapResult["env_variables"]))

	for k, any := range mapResult {
		if k == "env_variables" {
			err := checkIsPointer(&any)
			if err != nil {
				panic(err)
			}
			valueOf := reflect.ValueOf(any)
			val := reflect.Indirect(valueOf)
			switch val.Type().Kind() {
			case reflect.Map:
				envMap := any.(map[interface{}]interface{})
				for in, iv := range envMap {

					envName := in.(string)
					envVal := iv.(string)

					env := strings.Replace(strings.TrimSpace(envVal), "$", "", -1)
					envMap[envName] = os.Getenv(env)
				}
			default:
				panic(fmt.Sprintf("This is not supposed to happen, but if it does, good luck"))
			}
		}
	}

	fmt.Println(fmt.Sprintf("Compiled env variables: %v", mapResult["env_variables"]))

	out, err := yaml.Marshal(mapResult)
	// write the whole body at once
	err = ioutil.WriteFile(cfg.Path, out, 0644)
	if err != nil {
		panic(err)
	}
}

func checkIsPointer(any interface{}) error {
	if reflect.ValueOf(any).Kind() != reflect.Ptr {
		return fmt.Errorf("You passed something that was not a pointer: %s", any)
	}
	return nil
}