As an exercise to fully understand file descriptor redirection, I'm trying to store a command's STDOUT and STDERR in separate variables.

Thanks to improvements made in this answer, I can store STDERR:

{ err=$(exec 2>&1 >&3 3>&-; ls -ld /x /bin | tr o Z); } 3>&1

I'm trying to also store STDOUT:

{ err=$(exec 2>&1 >&3; ls -ld /x /bin); exec 0<&3; out=$(cat); } 3>&1;

However, the cat still wants to read from my keyboard.

How do I get my desired output:

$ printf 'OUT: %s\nERR: %s\n' "$out" "$err"
OUT: lrwxrwxrwx 1 root root 7 Aug 22 15:44 /bin -> usr/bin/
ERR: ls: cannot access '/x': No such file or directory

If this can't be done, why not?

I'm trying to understand file descriptors in the context of shell redirection.

Why can't I have cat read from FD 3, which is being written to by ls's STDOUT?

{ err=$(exec 2>&1 >&3; ls -ld /x /bin); exec 0<&3; out=$(cat); } 3>&1;

When try this, cat still wants to read from my keyboard.

If this can't be done, why not?

Differentiation: This question is about reading / writing to the same file descriptor, using the problem presented by Redirect STDERR and STDOUT to different variables without temporary files as an example.