#! /bin/sh

TTYS=`who | grep $USER | tr -s ' ' | cut -d' ' -f2 | grep -v :`

if [ -z "$1" ]; then
	if ! tty > /dev/null; then
		echo "stdin is not a tty" >&2
		exit 1
	fi
	HERE=`tty`
elif [ -c "$1" ]; then
	HERE=$1
elif [ -c "/dev/$1" ]; then
	HERE=/dev/$1
else
	echo "No such device: $1"
	exit 1
fi

BASEHERE=`echo $HERE | sed 's|^/dev/||'`

if [ -c $HERE ] && echo $TTYS | grep '\<'$BASEHERE'\>' > /dev/null; then
	for tty in $TTYS; do [ "$BASEHERE" != "$tty" -a -c /dev/$tty ] && chmod g-w /dev/$tty; done
	chmod g+w $HERE
else
	echo "You are not logged in on $BASEHERE" >&2
fi