Elixir - Logger output is colorful when run from command line, not so when run from iex

Question

When running tests in my Elixir mix app, I have a few instances where I include some calls to Logger.debug() to monitor behavior. I find that, when I call mix test from the command line, the Logger output is colored light-blue, and if I do something like add a custom color to the .debug() call, the output is colored accordingly -

test "some behavior" do
  Logger.debug("foo", ansi_color: :yellow)
  assert true
end

writes the log lines in yellow, as expected.

However, I've noticed that if I run my tests from within an iex session, the colors are not applied, just written to the terminal in the default font color (gray, in my case) -

iex(1)> Mix.shell().cmd(
          "mix test --color",
          env: [{"MIX_ENV", "test"}]
        )

Why is it that running the tests from the bash command line results in colorful output, while running them from within iex results in my default font color?

For context - I'm running this on a Mac (Mojave), from the bash command line (as mentioned), Elixir version 1.8.1.

Admittedly, this is a fairly trivial matter in itself - but I'm trying to get a better understanding of the Elixir shell and Mix's various interactions with the shell and system, and cracking this might help me to get a better understanding of what's going on under the hood in general. It's not what I would expect to happen, I'd like to know why.

Koziołek Koziołek · Accepted Answer · 2019-10-21T11:24:59

Explanation is in this and this article, but TL;DR below:

You need to enable colors in iex via:

iex> Application.put_env(:elixir, :ansi_enabled, true)

but this is session scope only. If you want to make it permanent you need to create .iex.exs file in current directory or your home directory:

timestamp = fn ->
  {_date, {hour, minute, _second}} = :calendar.local_time
  [hour, minute]
  |> Enum.map(&(String.pad_leading(Integer.to_string(&1), 2, "0")))
  |> Enum.join(":")
end

IEx.configure(
  colors: [
    syntax_colors: [
      number: :light_yellow,
      atom: :light_cyan,
      string: :light_black,
      boolean: :red,
      nil: [:magenta, :bright],
    ],
    ls_directory: :cyan,
    ls_device: :yellow,
    doc_code: :green,
    doc_inline_code: :magenta,
    doc_headings: [:cyan, :underline],
    doc_title: [:cyan, :bright, :underline],
  ],
)

IEx.configure(
  default_prompt:
    "#{IO.ANSI.green}%prefix#{IO.ANSI.reset} " <>
    "[#{IO.ANSI.magenta}#{timestamp.()}#{IO.ANSI.reset} " <>
    ":: #{IO.ANSI.cyan}%counter#{IO.ANSI.reset}] >"
)

And that's all

Elixir - Logger output is colorful when run from command line, not so when run from iex

2 Answers