enviper

Package enviper is a helper/wrapper for viper with the same API. It makes it possible to unmarshal config to struct considering environment variables.

Problem

Viper package doesn't consider environment variables while unmarshaling. Please, see: 188 and 761

Solution

Just wrap viper instance and use the same Unmarshal method as you did before:

e := enviper.New(viper.New())
e.Unmarshal(&config)

Example

package main

import (
	"github.com/iamolegga/enviper"
	"github.com/spf13/viper"
)

type barry struct {
    Bar int `mapstructure:"bar"`
}
type bazzy struct {
    Baz bool
}
type quxxy struct {
	Qux string
}
type config struct {
    Foo      string
    Barry    barry
    Barries  map[string]barry
    Bazzy    bazzy `mapstructure:",squash"`
    Quxxy    *quxxy
}

// For example this kind of structure can be unmarshaled with next yaml:
//  Foo: foo
//  Barry:
//    bar: 42
//  Baz: true
//  Barries: 
//    key1:
//      Bar: 255
//    key2:
//      Bar: 256
//  Quxxy:
//    Qux: "lorem"
//
// And then it could be overriden by next env variables:
//  FOO=foo
//  BARRY_BAR=42
//  BAZ=true
//  BARRIES_KEY1_BAR=42
//  QUXXY_QUX=ipsum
//
// Or with prefix:
//  MYAPP_FOO=foo
//  MYAPP_BARRY_BAR=42
//  MYAPP_BAZ=true
//  MYAPP_BARRIES_KEY1_BAR=42
//  MYAPP_QUXXY_QUX=ipsum

func main() {    
    var c config

    e := enviper.New(viper.New())
    e.SetEnvPrefix("MYAPP")
    e.AddConfigPath("/my/config/path")
    e.SetConfigName("config")

    e.Unmarshal(&c)
}

Custom Tag Names

In case you want to use custom tag name (something different from mapstructure), you have to set it explicitly via WithTagName function. The wrapper must know custom tag name in order to register all the env vars for viper so you can't just use DecoderConfigOption.

Credits

Thanks to krak3n (issuecomment-399884438) and celian-garcia (issuecomment-626122696) for inspiring.