POSTIN

/bin/sh

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

POSTUN

/bin/sh

if [ "$1" = 0 ]; then
        grep -v '^/bin/bash' < /etc/shells > /etc/shells.new
        mv /etc/shells.new /etc/shells
fi