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

Answer 3

Moving a file descriptor (1>&3-) is not portable, not all POSIX shell implementations support it. It is a ksh93-ism and bash-ism. (more info here https://unix.stackexchange.com/questions/65000/practical-use-for-moving-file-descriptors)

It is also possible to close FD 3 instead after performing the redirections.

ls 3>&2 2>&1 1>&3 3>&-

prints the contents of the current working directory out stderr.

The syntax 3>&- or 3<&- closes file descriptor 3.

Answer 4

The bash hackers wiki can be very useful in this kind of things. There's a way of doing it which is not mentioned among these answers, so I'll put my two cents.

The semantics of >&N, for numeric N, means redirect to the target of the file descriptor N. The word target is important since the descriptor can change target later, but once we copied that target we don't care. That's the reason why the order in which we declare of redirection is relevant.

So you can do it as follows:

./myscript.sh 2>&1 >/dev/null

That means:

1. redirect stderr to stdout's target, that is the stdout output stream. Now stderr copied stdout's target

2. change stdout to /dev/null. This won't affect stderr, since it "copied" the target before we changed it.

No need for a third file descriptor.

It is interesting how I can't simply do >&-, instead of >/dev/null. This actually closes stdout, so I'm getting an error (on stderr's target, that is the actual stdout, of course :D)

line 3: echo: write error: Bad file descriptor

You can see that order is relevant by trying to swap the redirections:

./myscript.sh >/dev/null 2>&1

This will not work, because:

1. We set the target of stdout to /dev/null
2. We set the target of stderr to stdout's target, that is /dev/null again.