POSTIN

/bin/sh

if [ -f /etc/shells ]
then
	grep -q '^/bin/yash$' /etc/shells || echo '/bin/yash' >> /etc/shells
else
	echo '/bin/yash' > /etc/shells
fi
exit 0

POSTUN

/bin/sh

[ "$1" = 0 ] || exit 0
[ -f /etc/shells ] || exit 0
sed -i -e '\@/bin/yash$@d' /etc/shells
exit 0