Selecting Array Elements with the Custom Columns Kubernetes CLI Output - kubernetes

Question

How can I use the --output=custom-columns option for the the Kubernetes CLI (kubectl) to select array elements?
In other words, how can I turn this:
kubectl get services
#=>
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
kafka-manager NodePort 10.3.242.200 146.148.20.235 9000:32619/TCP 0d
spark-master NodePort 10.3.242.209 104.199.21.235 9000:32619/TCP 0d
into this?
kubectl get services \
--output=custom-columns=. . .
#=>
NAME EXTERNAL-ENDPOINT
kafka-manager 146.148.20.225:9000
spark-master 104.199.21.225:7077
What I have so far:
kubectl get services \
--output=custom-columns='NAME:.metadata.name,IP:.spec.clusterIP,PORT:.spec.ports'
#=>
NAME IP PORT
kafka-manager 10.3.242.200 [map[nodePort:32619 port:9000 protocol:TCP targetPort:9000]]
spark-master 10.3.242.209 [map[nodePort:30588 port:7077 protocol:TCP targetPort:7077]]

Answer 1

TLDR
For an element that is in a list use * in square brackets.
So your query should look like this:
kubectl get service \
-n kube-system \
-o=custom-columns='NAME:.metadata.name,IP:.spec.clusterIP,PORT:.spec.ports[*].targetPort'
#=>
NAME IP PORT
kube-dns 10.0.0.10 53,53
kubernetes-dashboard 10.0.0.250 9090
Notice the * in PORT:.spec.ports[*].targetPort.
Details:
So kubectl is expecting a json-path-expr after header. The error I got when playing with expressions was following:
expected <header>:<json-path-expr>
To iterate over all elements in a list instead of putting an index just use *.
Various other json-path expressions can be found here.

Answer 2

You can use * for understanding the data in the JSON. For example:
kubectl get svc gdpr -o custom-columns='svc:*'
As for me, the get command was perfect (due external IP info.) and looks like:
kubectl get svc -o custom-columns='SVC:.metadata.name,IP:.metadata.annotations.domainName,PORT:.spec.ports[*].targetPort'
#=>
SVC IP PORT
event site1.com 9000
gdpr site2.com 3333,8080
svcInt none 80
ui site6.com 80,6123,6124,6125,8081
P.S. About list external IP and hosts:
kubectl get nodes -o jsonpath='{range .items[*]}{.metadata.name} {.status.addresses[?(#.type=="ExternalIP")].address}{"\n"}'
#=>
ip-10-10-40-13.xxxxx.internal xx.xx.xx.175
ip-10-10-40-15.xxxxx.internal xx.xx.xx.236
ip-10-10-40-18.xxxxx.internal xx.xx.xx.207
kubectl get nodes -o jsonpath='{range .items[*]}{.status.addresses[?(#.type=="ExternalIP")].address}{"\n"}'
#=>
xx.xx.xx.175
xx.xx.xx.236
xx.xx.xx.207

Answer 3

If you want to search in a map using custom columns, you can use
[?(#.Key=="Value"]
An important thing if you want to use it, is that the query in the custom-columns must be insider ['] character.
For example, the following query fails:
kubectl get nodes -o=custom-columns=NAME:.metadata.name,CPU:.status.capacity.cpu,MEM:.status.capacity.memory,IP:.status.addresses[?(#.type=="InternalIP")].address
bash: syntax error near unexpected token `('
But if we execute the same query using ['] character it works:
kubectl get nodes -o=custom-columns='NAME:.metadata.name,ROLE:TOBEDEFINED,CPU:.status.capacity.cpu,MEM:.status.capacity.memory,IP:.status.addresses[?(#.type=="InternalIP")].address'
NAME CPU MEM IP
name-node 8 8 10.10.10.10

Answer 4

Would that work for you?
kubectl get service -o=custom-columns=NAME:.metadata.name,IP:.spec.clusterIP,PORT:.spec.ports[0].targetPort