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

post_upgrade() {
  post_install
}

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