minus=-

# standard input
fd=0

exec 3<&$fd

read line <&3
echo from stdin: $line

# close fd 3
exec 3<&${minus}

# should give `bad fd', but exact error messages vary
# read line <&3

# standard output
fd=1

exec 4>&$fd

echo to stdout >&4

exec 4>&$minus

# should give `bad fd', but exact error messages vary
# echo to stdout >&4

unset fd

# these are ambiguous redirects
exec 3<&$fd
exec 4>&$fd

exec 3>&1 4>&2

exec >&/tmp/err-and-out
echo to stdout
echo to stderr >&2

exec 1>&3 2>&4
echo /tmp/err-and-out:
cat /tmp/err-and-out

rm /tmp/err-and-out

fd=/tmp/err-and-out
exec >&$fd
echo to stdout
echo to stderr >&2

exec 1>&3 2>&4
echo /tmp/err-and-out:
cat /tmp/err-and-out

rm /tmp/err-and-out