env var substitution inside config file

[PepijnK]

Considering the following example YAML file excerpt:

---
version: 1
timeout: 3000ms
token_introspection_endpoint: ${DOWNSTREAM_SERVICES_URL}/auth/token

While reading the config, the value ${DOWNSTREAM_SERVICES_URL} should be replaced with env var value of DOWNSTREAM_SERVICES_URL.

Is there some decoder hook I can use to implement this substitution?

[natemurthy]

I'm looking for a similar solution. My current application has config files that look like this:

database:
  host: $HOST
  dbname: $DBNAME
  username: $USERNAME
  password: $PASSWORD

and I would like to continue to use these configuration files as-is with the viper library. Do you have examples of using environment variable substitution?

[leandro-lugaresi]

This will help me to improve an application. I need some way to set the base URL for n HTTP consumers:

    consumer-name-1:
      connection: default
      queue:
        name: "wololo"
        options:
          durable: true
      runner:
        type: http
        options:
          url: https://myurl/app/message/wololo-handler"
    consumer-name-2:
      connection: default
      queue:
        name: "fooo"
        options:
          durable: true
      runner:
        type: http
        options:
          url: https://myurl/app/message/fooo-handler"

I can help if needed!

[lvqingan]

try godotenv

[chen56]

Why not support this feature？

[sagikazarmark]

There are better ways for this that doesn't require a direct integration in Viper:

• There are solutions that can do that for you (eg. dockerize, gomplate, etc)
• You can do it yourself by reading the file separately and doing any replacement necessary

Personally, I think if you use env vars anyway, it's just better to load them using Viper instead of replacing values in a config file.

Since there are alternatives available, I'm going to close this as won't fix. Feel free to open a new issue if you think you have a strong case for implementing this feature.

[hjizettle]

This would be super convenient actually. I hope you can reconsider.

[pkit]

@PepijnK

Is there some decoder hook I can use to implement this substitution?

Yes
Obviously it will work only if you use string -> string substitution which is probably what you want in 99% of cases

func StringExpandEnv() mapstructure.DecodeHookFuncKind {
	return func(
		f reflect.Kind,
		t reflect.Kind,
		data interface{}) (interface{}, error) {
		if f != reflect.String || t != reflect.String {
			return data, nil
		}

		return os.ExpandEnv(data.(string)), nil
	}
}

err := viper.Unmarshal(&config, viper.DecodeHook(StringExpandEnv()))