Save stdout, stderr and stdout+stderr synchronously

Question

For testing purposes, I would like to save stdout and stderr separately for inspection by subsequent code. For example, a test run with erroneous input should resul

Answer 1

For the last iteration, where the value of bar is set to the contents of stderr, try:

# restore original stdout & stderr at the end by adding: 1>&2 2>&1
bar="$(
{
   {
      echo foo >&2 | tee stdout.log 2>&3 3>&-
   } 2>&1 >&4 4>&- | tee stderr.log 2>&3 3>&-
} 3>&2 4>&1 1>&2 2>&1
)"

echo "$bar"