defp aliases do
  [
    …,
    "api_test": &api_test/1
  ]
end

defp api_test(_) do
  Mix.env(:test) # or whatever env you need
  MixTask.run("test" ["--only", "api_test:true"])
end

This is from memory though, can’t test it right now.

there’s also this in mix.exs:

def project do
    [
      …,
      preferred_cli_env: [
        coveralls: :test,
        "coveralls.detail": :test,
        "coveralls.post": :test,
        "coveralls.html": :test,
        "test.reset": :test,
        "test.integration": :test
      ]
    ]
  end

I know I am joining the party quite late, but I have seen a way in which you can actually execute an alias with a given ENV set, without having to manually type “MIX_ENV=blabla” before the command.

You can do it like they do in ecto, use a test_with_env function that does a lot of dark magic :

defp aliases do
    [
      "test.all": ["test.unit", "test.integration"],
      "test.unit": &run_unit_tests/1,
      "test.integration": &run_integration_tests/1
    ]
  end

  def run_integration_tests(args), do: test_with_env("integration", args)
  def run_unit_tests(args), do: test_with_env("test", args)

  def test_with_env(env, args) do
    args = if IO.ANSI.enabled?(), do: ["--color" | args], else: ["--no-color" | args]
    IO.puts("==> Running tests with `MIX_ENV=#{env}`")

    {_, res} =
      System.cmd("mix", ["test" | args],
        into: IO.binstream(:stdio, :line),
        env: [{"MIX_ENV", to_string(env)}]
      )

    if res > 0 do
      System.at_exit(fn _ -> exit({:shutdown, 1}) end)
    end
  end

You can read more about this in the blog:

Basically, the hardcore part is the command:

System.cmd("mix", ["test" | args],
        into: IO.binstream(:stdio, :line),
        env: [{"MIX_ENV", to_string(env)}]
      )

Which executes a command at system level and executes the given command with any variable you pass it, setting the ENV explicitly but without you having to care about it.

A clever use of aliases IMHO.