How do you capture logs and write them to `stderr`?

Can you give one full example on what you are trying to achieve, please? It will be easier to help you.

To capture logs you can mark the pid with a special metadata and then log out only logs that contain that metadata.

so basically my script outputs the result of Code.eval_string in json format.

But when I run the following:

mix run script.exs 'IO.puts("foobar")'

I get:

foobar
{"result":"ok"}

I would like to catch foobar as well, not ok only.

Here is a way to do it. Keep in mind it shouldn’t’ be used in actual systems, as there is a race condition here since you won’t have a stderr registered for a brief second. It should be fine for scripts like yours though:

iex(1)> Process.unregister(:standard_error)
true
iex(2)> {:ok, device} = StringIO.open("")
{:ok, #PID<0.109.0>}
iex(3)> Process.register(device, :standard_error)
true
iex(4)> IO.warn "nothing will show"
:ok

@josevalim is there a way to turn off warnings like
warning: variable "a" does not exist and is being expanded to "a()", please use parentheses to remove the ambiguity or change the variable name nofile:1

I’m trying to json encode it and then write it to stderr, but I’m only able to write it as it is without encoding. any other ideas if turning it off isn’t an option ?

The code above should hide the warnings, exactly as you asked. Without the code above:

x(1)> Code.eval_string("a")
warning: variable "a" does not exist and is being expanded to "a()", please use parentheses to remove the ambiguity or change the variable name
  nofile:1

** (CompileError) nofile:1: undefined function a/0
    (elixir) lib/code.ex:232: Code.eval_string/3
    (stdlib) erl_eval.erl:677: :erl_eval.do_apply/6
    (iex) lib/iex/evaluator.ex:249: IEx.Evaluator.handle_eval/5
    (iex) lib/iex/evaluator.ex:229: IEx.Evaluator.do_eval/3

With the code above (notice no warnings):

iex(1)> Process.unregister(:standard_error)
true
iex(2)> {:ok, device} = StringIO.open("")
{:ok, #PID<0.107.0>}
iex(3)> Process.register(device, :standard_error)
true
iex(4)> Code.eval_string("a")
** (CompileError) nofile:1: undefined function a/0
    (elixir) lib/code.ex:232: Code.eval_string/3
    (stdlib) erl_eval.erl:677: :erl_eval.do_apply/6
    (iex) lib/iex/evaluator.ex:249: IEx.Evaluator.handle_eval/5
    (iex) lib/iex/evaluator.ex:229: IEx.Evaluator.do_eval/3

yea that is exactly why I asked again, cause I think it doesn’t answer my original question in this post which was, how do I retrieve the output of IO.puts ? as I described in this post

so basically, how do I capture the output of the following line Code.eval_string('IO.puts("test")')

@josevalim would ExUnit do that job ?

Did you try it?

The warning doesn’t appear, but neither does the undefined function. I get nothing