Stderr and stdout of external program

Hello there!

I am exploring ways of running external programs via Elixir, and to my surprise I didn’t found a way to read from called program’s stdout and stderr separately.

It seems that this is related to how Erlang primitives are handling that task.
Only thing I found, is open_port/2’s stderr_to_stdout option, to merge both streams, and it seems Elixir’s does the same.

I found some external modules for Erlang supposedly addressing my need, but my question would be: is there elegant / native way to do it in Elixir?

Alternatively, is there a way to call bare sysalls on POSIX platform?

P.S. I wasn’t really sure where to post that question or how exactly label / tag it - my apologies if something went wrong here!

Kind regards,

Welcome to the forum!

Have you had a look at this yet (if you were thinking about pipes)?

1 Like

Hi @peerreynders, thank you for the response!

With my current knowledge of Elixir I guess that I can try to achieve what I want with - for example - 2 named pipes or named pipe plus stderr or stdout? All the moving parts around it seem to me bit intimidating.

Following links mentioned in the thread you’ve shared, I found also how do I communicate with non-Erlang programs, I guess I will go through it, but it will take time :slight_smile:

Thanks again!

While back from 2015 this may still be of some help:

The Erlangelist: Outside Elixir: running external programs with ports

by Saša Jurić, author of Elixir in Action 2e.

1 Like

For note, Port's are more for running with programs designed to work with your BEAM program, it’s less for running ‘other’ things as it doesn’t have this control over file descriptors, PID handling, or anything. If you want to manage programs that aren’t designed to be Port programs then I recommend using something like erlexec or so as it uses a shim C program to act as the binding layer between a Port call and Everything else.


Thank you for the reply, much appreciated @OvermindDL1.
Yes, that’s the module I’ve found, but I was searching something more lightweight, native, so to speak, but after your post I think I understand situation better.

As a side-note - it feels quite exotic to me, I’ve never encountered such design during my adventures with other programming languages; it’s interesting.


Yeah that is the native way here. ^.^

The only way to do mutability on the BEAM (short of some hacks) is via message passing, and Ports are just “Message Passing” back and forth to an external process designed for it, so it fulfills that model.

Multiple paths of streams doesn’t. So a hoisting program or NIF or something can fill that void by wrapping a higher level layer into the message passing structure. Which is what erlexec does. :slight_smile:

Yeah the BEAM is not so much a VM like Java, it’s designed to pretend to be it’s own standalone operating system internally within it. ^.^

1 Like

You can also look at porcelain with or without goon plugin which despite its age still works well for me when I needed stdin control beyond what System.cmd can provide without digging into low-level Ports.

That said erlexec is better maintained and I found (but not tried) a more idiomatic Elixir wrapper for it exexec


There is also rambo which might come in handy as an alternative to porcelain and goon, as it’s wrapper does not use go and it’s runtime…


Thank you for all of the suggestions folks!

I had a similar problem, what I did was to pipe stderr to a temp file. A bit ugly, but it worked from what I remember :stuck_out_tongue:

Yeah, I’ve had good success with erlexec in the past. Just used it directly, wasn’t too bad, although I do remember running into a bump or two configuring it.