Elixir console: Safety first

By Miguel PalhasOn July 23, 2020

I suspect that every developer who has dealt with production systems, has also messed up a production system. At least once.

Whether that was by deleting data accidentally or by manually calling a function thinking you were safely in your own local version, we've all had that mini-heart-attack feeling.

I've done it myself. And since then, I make a point to make sure my environment explicitly tells me where I'm at.

This is how my IEx prompts might look like, depending on which environment I'm in:


Let's see how we can do that in Elixir (or skip ahead for the full snippet if you're in a hurry):

The .iex.exs file

This file can be added to the root of your directory, and works as an Elixir script that gets executed whenever you start an IEx prompt

This actually means you can add any Elixir code you might find useful. Setting up alias, or the usual import Ecto.Query, is a common use-case.

You can also call IEx.configure/1 which, among other things, let's you customize the prompt, which is what we're doing here.

In my example above, prompt color is based on an APP_ENV environment variable, but being Elixir code, you can do it in whatever way you prefer. I tend to already have a similar variable available, to distinguish between staging environments and the real production one.

ANSI escape codes

Terminal colors are achieved via espace codes: special character sequences that tell your terminal to change colors. Elixir supports that natively via the IO.ANSI module.

Go ahead, try out this snippet in an IEx session:

IO.puts "#{IO.ANSI.red_background()}#{IO.ANSI.black()} WOW #{IO.ANSI.reset()}" 

If you inspect the return of these functions, you'll see that IO.ANSI.black() just returns "\e[30m", which is gibberish for "text color is now black".

Full snippet

With these two things in mind, you can write an .iex.exs file that customizes IEx prompts as shown in the beginning:

# .iex.exs

env = System.get_env("APP_ENV")

prompt_reset = "\e[G" prompt_color = case env do
  "production" ->
  "development" ->
  _ ->
    "#{IO.ANSI.white_background()}#{IO.ANSI.black()}" end

prompt = 
  prompt_reset <>
  "\n" <>
  "#{prompt_color} #{env} #{IO.ANSI.reset()}" <>
  " #{IO.ANSI.light_blue()}#%counter#{IO.ANSI.reset()}"

  default_prompt: prompt,
  alive_prompt: prompt

Applying to Elixir releases

The last step is to make this available in production. Elixir is often packaged via mix release, which doesn't consider .iex.exs at all. Luckily, copying the file over to the final release's directory seems to be enough.

Let's say you're using a Dockerfile to publish the release, and /app is the root of your application. Just add:

COPY .iex.exs /app/_build/prod/rel/.iex.exs

Wrapping up

Not much to say here, it's a rather small tip, but one I found very useful since first implementing it.

Drop me a line on Twitter if you have any thoughts or related suggestions. 🌮👋

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