How to redirect stdout+stderr to one file while keeping streams separate?

Question

Redirecting stdout+stderr such that both get written to a file while still outputting to stdout is simple enough:

cmd 2>&1 | tee output_file
         


        

Answer 1

{ { cmd | tee out >&3; } 2>&1 | tee err >&2; } 3>&1

Or, to be pedantic:

{ { cmd 3>&- | tee out >&3 2> /dev/null; } 2>&1 | tee err >&2 3>&- 2> /dev/null; } 3>&1

Note that it's futile to try and preserve order. It is basically impossible. The only solution would be to modify "cmd" or use some LD_PRELOAD or gdb hack,