command output not captured with org-babel bash

Question

Try the source block below with android device connected, the output is empty:

#+begin_src bash :results output
  out=$(adb shell date)
  echo $out
#+end_src

But if I change bash to sh, it outputs the date correctly:

#+begin_src sh :results output
  out=$(adb shell date)
  echo $out
#+end_src
    
#+RESULTS:
: Wed Dec 14 13:02:29 CST 2022

What's wrong with the bash code block?

Answer 1

I don't quite understand the details yet, but it turns out that for both ssh and adb, you need to prevent them from reading stdin - otherwise, they block waiting for input. The confusing part is that it is not always necessary, but it is necessary in some (most?) cases, so it's better to be safe than sorry.

In the case of ssh, I got the same behavior (blocking) when I tried to ssh to localhost. ssh has a flag -n (or a setting StdinNull) that allows you to tell it to ignore stdin.

It turns out that adb has a similar flag in its shell subcommand, also denoted by -n (surely, not a coincidence).

So the final outcome is that both source blocks (sh and bash) will work fine, as long as you provide the flag:

#+begin_src sh :results output
   echo Start
   out=$(ssh -n localhost date)
   echo $out
   echo Done
#+end_src

#+RESULTS:
: Start
: Thu Dec 15 01:24:31 AM EST 2022
: Done

#+begin_src bash :results output
   echo Start
   out=$(ssh -n localhost date)
   echo $out
   echo Done
#+end_src

#+RESULTS:
: Start
: Thu Dec 15 01:24:36 AM EST 2022
: Done

#+begin_src sh :results output
   echo Start
   out=$(adb shell -n date)
   echo $out
   echo Done
#+end_src

#+RESULTS:
: Start
: Thu Dec 15 01:25:14 EST 2022
: Done

#+begin_src bash :results output
   echo Start
   out=$(adb shell -n date)
   echo $out
   echo Done
#+end_src

#+RESULTS:
: Start
: Thu Dec 15 01:25:19 EST 2022
: Done