Redirect STDOUT to STDERR when exit status isn't 0 in Bash

Question

The programming language Crystal currently doesn't write to STDERR when there's a compile time failure. I need to redirect STDOUT to STDERR if the exit status is not 0 and still return the exit status.

Answer 1

Taken literally, your request is literally impossible: Redirections are performed before a command has started, whereas exit status is only known when it exits.

However, you can unconditionally redirect into a buffer, and then write that buffer to either stdout or stderr after exit status is known.

Consider a wrapper akin to the following:

#!/bin/sh
if output=$("$@"); then
  printf '%s\n' "$output"
else
  retval=$?
  printf '%s\n' "$output" >&2
  exit "$retval"
fi

...invoked as (if this is saved as stderr-wrapper):

stderr-wrapper your-program arg1 arg2 ...