IO Redirection - Swapping stdout and stderr

Question

Given a shell script:

#!/bin/sh  echo "I'm stdout"; echo "I'm stderr" >&2; 

Is there a way to call that script such that only stderr would print out, when the last part of the command is 2>/dev/null, ie

$ > sh myscript.sh SOME_OPTIONS_HERE 2>/dev/null I'm stderr 

Or, alternatively:

$ > sh myscript.sh SOME_OPTIONS_HERE >/dev/null I'm stdout 

It's a question at the end of a set of lecture slides, but after nearly a day working at this, I'm nearly certain it's some sort of typo. Pivoting doesn't work. 2>&- doesn't work. I'm out of ideas!

Answer 1

% (sh myscript.sh 3>&2 2>&1 1>&3) 2>/dev/null I'm stderr % (sh myscript.sh 3>&2 2>&1 1>&3) >/dev/null  I'm stdout 

Explanation of 3>&2 2>&1 1>&3:

• 3>&2 means make a copy of file descriptor 2 (fd 2) (stderr), named fd 3 (file descriptor 3). It copies the file descriptor, it doesn't duplicate the stream as tee does.
• 2>&1 means that fd 2 of sh myscript.sh becomes a copy of it's fd 1 (stdout). Now, when myscript writes to it's stderr (it's fd 2), we receive it on stdout (our fd 1).
• 1>&3 means that fd 1 of sh myscript.sh becomes a copy of fd 3 (stderr). Now, when myscript writes to it's stdout (it's fd 1), we receive it on stderr (our fd 2).

Answer 2

For sake of completeness, based on a comment by @200_success above, it is probably better to move the file descriptor 3 using 1>&3- :

$ (sh myscript.sh 3>&2 2>&1 1>&3-) 2>/dev/null I'm stderr $ (sh myscript.sh 3>&2 2>&1 1>&3-) >/dev/null  I'm stdout 

Instead of swapping file descriptors on a per-process basis, using exec you can swap stdin & stderr for all following commands launched by the current shell :

$ (exec 3>&2 2>&1 1>&3- ; sh myscript.sh ; sh myscript.sh ) 2>/dev/null I'm stderr I'm stderr $ (exec 3>&2 2>&1 1>&3- ; sh myscript.sh ; sh myscript.sh ) >/dev/null  I'm stdout I'm stdout