post_install() {
  grep -q '/usr/bin/ash' etc/shells || echo '/usr/bin/ash' >> etc/shells
  grep -q '/usr/bin/dash' etc/shells || echo '/usr/bin/dash' >> etc/shells
}

post_upgrade() {
  post_install
}

pre_remove() {
  sed -i '/^\/usr\/bin\/ash/d' etc/shells
  sed -i '/^\/usr\/bin\/dash/d' etc/shells
}