Access to environment variables in Unit config

[cohix]

When writing a Unit config, certain parts may need to be dynamically inserted, and so it would be great to be able to reference env vars that get evaluated at runtime. For example, I'd like to be able to do this:

"applications": {
			"foo": {
					"type": "external",
					"working_directory": "$(HOME)/path/to/thing",
					"executable": "app",
					"environment": {
						"SOME_APP_CONFIG": "$(SOME_ENV_VAR)"
					}
			}
	}

Which currently does not work.

[travisbell]

Oooooh, this is a good one. I use envtpl right now to get this functionality in my configs but would love to drop it as a dependency!

[hongzhidao]

Hi,
There are a few separate functions here.

1. Unit supports variables and njs/JS for options like pass, share, etc.
   https://unit.nginx.org/scripting/
2. It's able to access environment variables with njs.
   https://nginx.org/en/docs/njs/reference.html#process
3. The variables and JS only work on the runtime phase.

But the option environment in the application is applied when the proto process starts, it's a different phase than the runtime phase which runs for the http request.

To make it short, the feature is about whether Unit provides variable engine in the application starting phase.
We'll discuss it with the team, and any thoughts are welcome.

[hongzhidao]

Hi, our thoughts on it:

1. Users can now use a config template to maintain the "environment" options.
2. We agree it's more convenient to provide native variables for the application options like working_directory and environment. Now we are on the 1.32 version and we'll try it when we start the 1.33 version.

Thanks.

[lcrilly]

If we consider that envsubst(1) and similar tools provides a reasonable workaround today, how useful would it be to "pass through" specific environment variables to the application's environment?

{
  "applications": {
    "foo": {
      "type": "external",
      "working_directory": "/path/to/thing",
      "executable": "app",
      "environment_passthrough": ["SOME_ENV_VAR", "OTHER_ENV_VAR"],
      "environment": {
        "HOME": "/path/to/thing"
      }
    }
  }
}

Would this be a worthwhile feature in its own right? Or is full access to env vars, across the entire configuration scope the main thing? If so, we might also consider environment variable substitution a function that the unitc CLI tool can provide.

[razvanphp]

Our use-case would be to configure number of processes, options for applications (like memory_limit), etc. Thank you!

[travisbell]

Yup, same here. Here's what one of our configs looks like and how we use ENV vars in our config:

"applications": {
    "sinatra": {
      "type": "ruby",
      "processes": {{ NUM_WORKERS }},
      "threads": {{ WORKER_THREADS_MAX }},
      "script": "{{ HOME }}/workspace/app/config.ru",
      "hooks": "{{ HOME }}/workspace/app/config/unit/hooks.rb",
      "working_directory": "{{ HOME}}/workspace/app/",
      "limits": {
        "timeout": 30
      }
    }
  }

[gourav-kandoria]

@lcrilly Does this issue needs work. Would be happy to work on it

[lcrilly]

Yes, still open. Would welcome a contribution but makes sense to discuss the approach and design here, first.

For example, are we reading the env vars of the user applying the configuration or the user that owns the unit: controller process?