Role of `?` when accessing environment variables in HOCON files

Question

The docs say that I can access environment variables like this:

database_name = "${?DB_NAME}"

Where DB_NAME is an environment variable. Do I have to put the question mark ? or is that there to prevent a crash if it doesn't exist?

Also, are we allowed to reference a variable in a file before it is declared?

foo = "hello-${bar}" bar = "baz"

Better to ask two separate questions, one for each of your concerns. — Nader Ghanbari
– Nader Ghanbari, Commented Feb 15, 2017 at 4:10

Nader Ghanbari · Accepted Answer · 2019-11-30 17:36:26Z

18

1- ? means optional. If the environment variable does not exist at runtime, the whole line will be ignored, as if it didn't exists in the first place. So use it when you want optional overriding.

See Optional Env Vars section of the docs for more info.

Note that ? works for all substitutions and not just env vars.

2- Yes, you can reference a variable which is defined later in the config file (forward referencing). As long as the value exists and there is no cycle (circular dependency), the substitution will be done successfully.

Think of it like this: first the whole file will be parsed at runtime and the value of the literals will be bound to their vars, and then the substitutions will take place, so the order doesn't really matter.

edited Nov 30, 2019 at 17:36

answered Feb 15, 2017 at 4:20

Nader Ghanbari

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5 Comments

Blankman Over a year ago

so do I HAVE to use ? with environmental variables? if not, how do I access them normally?

Nader Ghanbari Over a year ago

No, you don't have to use them. Only if you want optional overriding. Normal use case would be myVar = ${MY_ENV_VAR}.

Blankman Over a year ago

I see, just confused how it knows it is a ENV VAR then, caps lock? :)

Nader Ghanbari Over a year ago

No, by default it takes all system properties and environment variables into account, the format doesn't really matter. BTW, be aware that even if you just have myVar = 2 but in runtime there is an env var with the same name, say export myVar = 10 your config value will be overridden! Personally I don't like this so try to use namespaced config params (e.g. app.settings.feature1.myVar or with curly braces app { settings { feature { myVar = ....)

Nader Ghanbari Over a year ago

By the way, the docs mentions everything in a much clearer way :) So I'd read the whole docs (it's just a few pages) to grasp a better understanding. You can also look at their code and debug it: github.com/typesafehub/config

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