[QUESTION] - How would I Read from multiple config files and environment variables?

[sujit-baniya]

I've come across a use case where I've to use multiple config files .json, .env and environment variables in such a way.

• First all config from .json is loaded
• All non-empty variables from .env is loaded. Empty variables on .env or non-existing variables wouldn't override values from .json
• All non-empty variables from environment variables of platform is loaded. Empty variables on .env or non-existing variables wouldn't override values from .json

For the same purpose, I tried with following snippet but doesn't seem to work.

package main

import (
	"github.com/spf13/viper"
	"log"
)

type ServerConfiguration struct {
	Port int
}

type DatabaseConfiguration struct {
	ConnectionUri string
}

type Configuration struct {
	Server   ServerConfiguration
	Database DatabaseConfiguration
}

func main() {
	var configuration Configuration
	readFromConfigFile()
	readFromEnvFile()
	viper.AutomaticEnv()

	err := viper.Unmarshal(&configuration)
	if err != nil {
		log.Fatalf("unable to decode into struct, %v", err)
	}
	log.Printf("database uri is %s", configuration.Database.ConnectionUri)
	log.Printf("port for this application is %d", configuration.Server.Port)
}

func readFromConfigFile() {
	viper.SetConfigName("config")
	viper.AddConfigPath(".")
	if err := viper.ReadInConfig(); err != nil {
		log.Fatalf("Error reading config file, %s", err)
	}
}

func readFromEnvFile() {
	viper.SetConfigName(".env")
	viper.SetConfigType("env")
	viper.AddConfigPath(".")
	if err := viper.MergeInConfig(); err != nil {
		log.Fatalf("Error reading config file, %s", err)
	}
}

{
	"database": {
		"connectionUri": "test uri"
	},
	"server": {
		"port": 8283
	}
}

DATABASE_CONNECTION_URI="test2 uri"

Expected result:

2023/03/15 17:07:42 database uri is test2 uri
2023/03/15 17:07:42 port for this application is 8283

Actual result:

2023/03/15 17:07:42 database uri is test uri
2023/03/15 17:07:42 port for this application is 8283

Is it possible to attain as mentioned? If so, how could I achieve the functionality