How to display a kubectl column using a fraction format (i.e. X/Y) for a Custom Resource

Unfortunately additionalPrinterColumns only supports Simple JsonPath and so we cannot use .status.readyReplicas/.status.replicas in JsonPath and the GET Operation is handled by kube-apiserver so there is no involvement of Operator in GET Operation. So, easiest way I would recommend is to create a new Field in .status called ready with value "readyReplicas/replicas" and update it every time you update readyReplicas and replicas.

Then put below in additionalPrinterColumns

   additionalPrinterColumns:    - jsonPath: .status.ready      name: Ready      type: string

For StatefulSet, they are using below code in https://github.com/kubernetes/kubernetes/blob/master/pkg/printers/internalversion/printers.go to print value of READY Column

    statefulSetColumnDefinitions := []metav1.TableColumnDefinition{        {Name: "Name", Type: "string", Format: "name", Description: metav1.ObjectMeta{}.SwaggerDoc()["name"]},        {Name: "Ready", Type: "string", Description: "Number of the pod with ready state"},        {Name: "Age", Type: "string", Description: metav1.ObjectMeta{}.SwaggerDoc()["creationTimestamp"]},        {Name: "Containers", Type: "string", Priority: 1, Description: "Names of each container in the template."},        {Name: "Images", Type: "string", Priority: 1, Description: "Images referenced by each container in the template."},    }    h.TableHandler(statefulSetColumnDefinitions, printStatefulSet)    h.TableHandler(statefulSetColumnDefinitions, printStatefulSetList)func printStatefulSet(obj *apps.StatefulSet, options printers.GenerateOptions) ([]metav1.TableRow, error) {    row := metav1.TableRow{        Object: runtime.RawExtension{Object: obj},    }    desiredReplicas := obj.Spec.Replicas    readyReplicas := obj.Status.ReadyReplicas    createTime := translateTimestampSince(obj.CreationTimestamp)    row.Cells = append(row.Cells, obj.Name, fmt.Sprintf("%d/%d", int64(readyReplicas), int64(desiredReplicas)), createTime)    if options.Wide {        names, images := layoutContainerCells(obj.Spec.Template.Spec.Containers)        row.Cells = append(row.Cells, names, images)    }    return []metav1.TableRow{row}, nil}func printStatefulSetList(list *apps.StatefulSetList, options printers.GenerateOptions) ([]metav1.TableRow, error) {    rows := make([]metav1.TableRow, 0, len(list.Items))    for i := range list.Items {        r, err := printStatefulSet(&list.Items[i], options)        if err != nil {            return nil, err        }        rows = append(rows, r...)    }    return rows, nil}