While I’ve been working on deploying some elixir releases I’ve been trying to understand how the flags I’m setting end up being seen by erlang. Well, it turns out elixir has an ENV var for this named ELIXIR_CLI_DRY_RUN.

Here is an example:

$ ELIXIR_CLI_DRY_RUN=1 iex
erl -pa /usr/local/Cellar/elixir/1.11.2/bin/../lib/eex/ebin \
               /usr/local/Cellar/elixir/1.11.2/bin/../lib/elixir/ebin \
               /usr/local/Cellar/elixir/1.11.2/bin/../lib/ex_unit/ebin \
               /usr/local/Cellar/elixir/1.11.2/bin/../lib/iex/ebin \
               /usr/local/Cellar/elixir/1.11.2/bin/../lib/logger/ebin \
               /usr/local/Cellar/elixir/1.11.2/bin/../lib/mix/ebin \
       -noshell \
       -user Elixir.IEx.CLI \
       -extra --no-halt +iex

You can see a lot of interesting info here:

• elixir or iex are always just running erl in the end
• -pa is to prepend a module load path and elixir makes sure all it’s modules are accessible at boot
• -noshell means to, uh, not start the erlang shell, since iex is going to boot up it’s own shell in it’s start function with :user_drv (find it in the erl docs)
• -user is the first module erlang will call start of after boot, in this instance it’s Elixir.IEx.CLI
• -extra means “erl won’t look at these, but they are available with :init.get_plain_arguments() which elixir does a few times
• --no-halt tells elixir not to auto-exit after boot, so iex can setup it’s shell and accept input
• +iex is a special flag elixir looks for while starting up it’s CLI (which is also called by IEx.start)

I found this super interesting and it helped me learn a lot about how erlang boots up, how modules are loaded, etc.