core/shadow/shadow.install

post_upgrade() {
  grpck -r >/dev/null 2>&1
  if [ $? -eq 2 ]; then
    printf '%s\n' \
      "==> Warning: /etc/group or /etc/gshadow are inconsistent." \
      "    Run 'grpck' to correct this."
  fi
  return 0
}