POSTIN

/bin/sh

if [ ! -f /etc/shells ]; then
        echo "/bin/ksh" > /etc/shells
else
        if ! grep -q '^/bin/ksh$' /etc/shells ; then
                echo "/bin/ksh" >> /etc/shells
        fi
fi

POSTUN

/bin/sh

if [ ! -f /bin/ksh ]; then
	sed -i '/^\/bin\/ksh$/ d' /etc/shells
fi