Org Src Block does not return any output

Question

TL;DR: How do I get the stderr of my source block to be part of the #+RESULTS block, without changing the contents of the block? I can only get stdout to appear.

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[Long and winding explanation]

While writing this question I found a solution/reason to it not working. Since I spent quite some time on it, I thought I'd still post it.

Hey there!

I'm relatively new to emacs and org mode especially. Last month, I started writing my bachelor's thesis for applied computer science using org mode (based on dangom's template).

In the thesis, I use org-src blocks to demonstrate stuff. In this case, I've set up an example to demonstrate warnings from a C Compiler:

#+CAPTION: A simple C-Program without errors, but with a Code Smell
#+NAME: qc:c-code
#+ATTR_LATEX: :placement [H]
#+BEGIN_SRC C :tangle ../sourcecodes/foo.c
// foo.c
#include <stdio.h>

int main() {
    int i=10;
}
#+END_SRC

As you can see, this program is stored in a seperate sourcecodes folder:

├── backmatter
├── frontmatter
├── images
├── library.bib
├── literatur
├── mainmatter
│   └── 03-analysis.org <- This is where I am
├── sourcecodes
│   ├── foo
│   └── foo.c <- and here goes the program
└── thesis

Now, I wish to build that file.

#+CAPTION: ...
#+NAME: qc:c-code-build
#+ATTR_LATEX: :placement [H]
#+BEGIN_SRC sh :results output :dir ../sourcecodes
gcc foo.c -Wall -o foo
#+END_SRC

In this example, I use the dir header to change the directory for execution of the code.

When run in a terminal, this results to this:

foo.c: In function 'main':
foo.c:5:9: warning: unused variable 'i' [-Wunused-variable]
    5 |     int i=10;
      |         ^

This is what I would expect. In emacs, it results to this:

#+RESULTS: qc:c-code-build

Whereas this

#+CAPTION: ...
#+NAME: qc:c-code-build
#+ATTR_LATEX: :placement [H]
#+BEGIN_SRC sh :results output :dir ../sourcecodes
ls
gcc --version
gcc foo.c -Wall -o foo
ls
#+END_SRC

returns that:

#+RESULTS: qc:c-code-build
: foo.c
: gcc (GCC) 9.3.0
: Copyright (C) 2019 Free Software Foundation, Inc.
: This is free software; see the source for copying conditions.  There is NO
: warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
: 
: foo
: foo.c

Now, obviously this is not a real problem - just copy and paste the desired output into a src block your own.

I would just like to know if I'm missing something super obvious.

Other stuff I've tried/noticed:

• Using shell instead of sh
• results: output replace
• :exports both
• created a minimal example, which does not load anything from the template
• tried to use scimax (and failed, since by default it doesn't contain the org-babel-execute function for sh/shell)
• some Buffer once gave me errors (Error Output or something)

Now, as promised, the reason:

Org-mode shell source blocks do not capture stderr

My solution for the problem: I don't have a good solution for this as of yet. The solution of John Kitchin suggests use of 2>&1 which writes all stderr to stdout, for example:

#+BEGIN_SRC sh :results output replace :dir ../sourcecodes :exports both
gcc foo.c -Wall -o foo 2>&1
#+END_SRC

But that adds some unnecessary logic to the eye for the reader of the thesis, which is why I'm probably staying with copy-pasting stderr into src-blocks (except someone has a better solution)

Answer 1

Rephrasing and shortening, I guess the question is roughly: "how to obtain stderr output without changing the source block content?"

By skimming the comments section of the John Kitchin's post, adding

#+PROPERTY: header-args:sh :prologue "exec 2>&1" :epilogue ":"

globally, at the top of the file, should do the job (it works in my side).