Where / How does the Mix environment variable get set?

https://github.com/elixir-lang/elixir/blob/50293b46f13a86328f0ffabdcbb8592e29ac24c6/lib/mix/lib/mix/state.ex#L11

it gets set here. It’s a compile-time variable, gets fetched fron shell environment, and default value is “dev”.

And for anyone who isn’t already aware (like me until relatively recently), this means you can set the environment on each mix command.

Unix-like:

MIX_ENV=prod mix {task}

Windows:

set "MIX_ENV=prod" && mix {task}

At least for UNIXoid systems, that is not possible on windows.

As it happens I was at the mix intro doc tonight so I came across the Windows equivalent and edited my reply.

It is not 100% equivalent and one needs to know the differences when using the windows version:

$ echo %MIX_ENV%
%MIX_ENV%

$ set MIX_ENV=test && mix deps.get
Running dependency resolution...
All dependencies up to date

$ echo %MIX_ENV%
test

Linux:

$ echo $MIX_ENV

$ MIX_ENV=test mix deps.get
Running dependency resolution...
All dependencies up to date

$ echo $MIX_ENV

As you can see, in linux the value of MIX_ENV is only overwritten for that single command and recovered afterwards, while the windows version changes the value permanently.

I just use a bash/cygwin shell on Windows, much more uniform. ^.^

aside from inlining them with command calls in Linux these can be set on a user level by adding them to .profile or .bashrc or for all users in /etc/environment or /etc/profile, this might be useful when provisioning servers

A related concept: you may have noticed that when you run mix test, it assumes MIX_ENV=test, but most tasks assume MIX_ENV=dev unless otherwise specified.

To set the default environment for a Miix task (eg, a task to run JS tests) or task alias, you can use :preferred_cli_env. For example:

defmodule MyProject.Mixfile do
  use Mix.Project

  def project do
    [
      # ...
      aliases: aliases(),
      preferred_cli_env: [
        # note that all task names are atoms
        test: :test,
        jstest: :test,
        "test.all": :test
      ]
    ]
  end

  # ...

  defp aliases do
    [
      "test.all": ["test", "jstest"]
    ]
  end
end

See also this answer https://stackoverflow.com/a/35888676/119790