Colouring the 'iex' prompt

9 Jan 2022 12:17 elixir

If you’ve got separate dev, test and prod environments, it can help to use colour to tell you which one you’re logged into. Here’s how to colour the iex prompt appropriately.

Create .iex.exs containing the following:

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

get_colour = fn ->
  case System.get_env("WHICH_ENVIRONMENT") do
    nil -> :white
    "dev" -> :green
    "test" -> :yellow
    _ -> :red

default_prompt = ["%prefix(%counter)>"]
alive_prompt = ["%prefix(", get_colour.(), "%node", :reset, ")%counter>"]

  colors: [enabled: true],
  default_prompt: default_prompt |> IO.ANSI.format() |> IO.chardata_to_string(),
  alive_prompt: alive_prompt |> IO.ANSI.format() |> IO.chardata_to_string()

This applies the colour to only the node name, so it only works with --sname or --name (or in a remote console). Feel free to edit it for your needs.

The name of the environment variable (WHICH_ENVIRONMENT), and the possible values for it (dev, test, etc.), are completely up to you (or your ops team; go and talk to them). I’ve assumed that the variable will be unset on your local machine, and that unrecognised values should be treated as if they were prod (because potentially dangerous).

Elixir searches for this file in the current directory, then in $HOME, so you should just put it at the top level of your project. If you’re using containers, copy it to $HOME with something like this:

WORKDIR /home/app
# ... other stuff
COPY .iex.exs .