The three standard files, seen as files

Answer

ls -l /dev/fd/ echo hello > /dev/fd/1 echo trouble > /dev/fd/2

The listing shows one entry per open descriptor of the process that ran ls. Writing to /dev/fd/1 and /dev/fd/2 by name does exactly what an ordinary write to stdout and stderr does — both arrive on your terminal.

Linux proc(5), fd(4)