Update Elixir: Using Environment Variables

Author: mbuffa

content/articles/20210916-elixir-environment-variables.md

@@ -22,11 +22,11 @@ And the same call would work in your application source code.
 
 However, you may want to avoid scattering those calls in every corner, and use a config file instead to centralize your settings and your default values.
 
-Newer versions of Mix no longer generate config files by default, so you'd have to create one of those, like this:
+Mix doesn't generate config files by default, so you'd have to create one of those, like this:
 
 ```sh
-mix new test
-cd test
+mix new foo
+cd foo
 mkdir config
 touch config/config.exs
 ```
@@ -36,16 +36,16 @@ And edit your `config/config.exs` file, like this:
 ```elixir
 import Config
 
-config :test, hello: System.get_env("HELLO")
+config :foo, hello: System.get_env("HELLO")
 ```
 
-And lastly, modify `lib/test.ex` like this, to actually use the key-value pair we defined:
+And lastly, modify `lib/foo.ex` like this, to actually use the key-value pair we defined:
 
 ```elixir
-module Test do
+module Foo do
   # ...
   def hello do
-    Application.get_env(:test, :hello, "default")
+    Application.get_env(:foo, :hello, "default")
   end
 end
 ```
@@ -60,12 +60,12 @@ MIX_ENV=prod mix release
 Now let's run an interactive terminal to our build, and let's override that env variable along the way:
 
 ```sh
-HELLO=bar ./_build/prod/rel/test/bin/test start_iex
+HELLO=bar ./_build/prod/rel/foo/bin/foo start_iex
 ```
 
-There, calling `Test.hello` would return "foo", not "bar".
+There, calling `Foo.hello` would return "foo", not "bar".
 
-You can easily get surprises when releasing your first Elixir app, because this is not something you can reproduce with `iex -S mix`.
+You can easily get surprises when releasing your first Elixir app, because this is not something you can reliably reproduce with `iex -S mix` locally.
 
 ## What happened
 
@@ -93,5 +93,19 @@ _Et voilà !_ Hope this helps.
 
 If you want to know more about Elixir builds and releases, as always, [the official documentation is a great place to start][1].
 
+## 2025 Update
+
+I wanted to make an update a while ago to state one thing: you probably don't need a library such as `env` in your application. You just have to remember the following statements:
+- Your configuration is split between environments (`config/{dev,test,prod}.exs`) which is built at compilation time
+- You have to make sure your environment variables are defined at compilation time if you make calls to `System.get_env`, otherwise, your config would contain empty settings. You can check [this article][3] for the most optimal way to do that locally.
+- There's one exception to that: `config/runtime.exs` is *always* evaluated at runtime, and not only in production. This is where you can reliably call `System.get_env`. Just be cautious and insert production configuration inside the right "block" in there, because it's also used for dev and test.
+
+There's actually another exception you may come across in older Elixir projects, which is `config/releases.exs`, used until Elixir v1.11.0.
+
+Finally, checking both [Elixir][2] and [Mix][4] documentations for your versions is always a good idea.
+
 [0]: https://hex.pm/packages/env
 [1]: https://hexdocs.pm/mix/1.12/Mix.Tasks.Release.html
+[2]: https://hexdocs.pm/elixir/config-and-releases.html
+[3]: /articles/20250227-loading-env-without-leaks
+[4]: https://hexdocs.pm/mix/1.18.4/Mix.Tasks.Release.html