Using env variables in YML with default value

I have the following code to read config files from yml which includes ENV variables too:

confContent, err := ioutil.ReadFile("config.yml")
	if err != nil {
		panic(err)
	}
	// expand environment variables
	confContent = []byte(os.ExpandEnv(string(confContent)))
	conf := &SysConfig{}
	if err := yaml.Unmarshal(confContent, conf); err != nil {
		panic(err)
	}

config.yml

db:
  name: ${DB_NAME:qm}
  host: localhost

It is working but how can I get it to read default values if DB_NAME env is not given?

You can replace the mapper on ExpandEnv using Expand and take into account default values like this:

package main

import (
	"fmt"
	"os"
	"strings"
)

func main() {
	mapper := func(placeholderName string) string {
		split := strings.Split(placeholderName, ":")
		defValue := ""
		if len(split) == 2 {
			placeholderName = split[0]
			defValue = split[1]
		}

		val, ok := os.LookupEnv(placeholderName)
		if !ok {
			return defValue
		}

		return val
	}

	os.Setenv("DAY_PART", "morning")

	fmt.Println(os.Expand("Good ${DAY_PART:test}, ${NAME:Gopher}", mapper))
}

this will render

Good morning, Gopher

This is based on the example from Expand from the os package documentation.

I recommend you use the awesome Viper package by spf13) to read conf files, it can solve your problem gracefully and you can use it to load many other type of conf files.

Solve your problem

1. Get the package

    go get github.com/spf13/viper

2. conf file

Supposed you have conf file named db.yaml:

db:
  name: somedb
  host: localhost

3. code example

As we can see, once Viper loads the conf file, we can get values by key. A Yaml file will be parsed as a nested structure which you can unmarshal into a Golang struct, We should use viper.GetString("db.name") to get the value, you can refer to this page to get more usage info.

import (
	"fmt"

	"github.com/spf13/viper"
)

func InitConf() {
	viper.AutomaticEnv()     // read system env 
	viper.SetConfigName("db")  // conf file name to be load
	viper.AddConfigPath(".")    // conf file path

	viper.SetDefault("db.name", "mysqldb") // you can set default da name value here 

    // do read conf file and handle error
	if err := viper.ReadInConfig(); err != nil {
		if _, ok := err.(viper.ConfigFileNotFoundError); ok {
			// Config file not found; ignore error if desired
		} else {
			// Config file was found but another error was produced
		}
	}

	fmt.Printf("db.name: %s", viper.GetString("db.name"))
}

viper.AutomaticEnv() this method will read system env automatically, suppose you have an env named ENV_XXX, then you can get it by viper.GetString("ENV_XXX"). If the env name is the same as one of conf file key, viper uses the env first

• when we do not set the name in db.yaml the output will be db.name: mysqldb
• when we set the name value: somedb in db.yaml the output will be db.name: somedb

Hope you will find it useful! more details on usage see the Viper README file

I guess you had a struct for your conf like this:

Db *struct{
		Name string
		Host string
	}

so for retrieve the default variable of Dbname, you must check it like this:

if conf.Db == nil || conf.Db.Name == "" {
        conf.Db.Name = "test"
    }