type Config struct {
    BindPort int `mapstructure:"port" yaml:"port,omitempty"`
}

var c Config

// ...
viper.AutomaticEnv()

if err := viper.ReadInConfig(); err != nil {
    // ...
}

if err := viper.Unmarshal(&c); err != nil {
    // ...
}

// I want c.BindPort == viper.Get("PORT") == os.Getenv("PORT")