#   This program is free software: you can redistribute it and/or modify
#   it under the terms of the GNU General Public License as published by
#   the Free Software Foundation, either version 3 of the License, or
#   (at your option) any later version.
#
#   This program is distributed in the hope that it will be useful,
#   but WITHOUT ANY WARRANTY; without even the implied warranty of
#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#   GNU General Public License for more details.
#
#   You should have received a copy of the GNU General Public License
#   along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
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 >&${TMPDIR}/err-and-out
echo to stdout
echo to stderr >&2

exec 1>&3 2>&4
echo err-and-out:
cat ${TMPDIR}/err-and-out

rm ${TMPDIR}/err-and-out

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

exec 1>&3 2>&4
echo err-and-out:
cat ${TMPDIR}/err-and-out

rm ${TMPDIR}/err-and-out