Certified Kubernetes Administrator (CKA) — Tips and Tricks — Part 4
Mastering “Field Selectors”, “Custom Column” and “Sort By” will help us in a few questions at the CKA exam. In this blog, we will look into a framework that will help us to construct any kubectl “Field Selectors”, “Custom Column” and “Sort By” commands without memorizing much. If you are lucky you will get two questions from the below examples.
Tip 1: Reach Documentation And Escape Memorizing
You will be allowed to refer the Kubernetes documentation page during the exam. From the Kubernetes documentation page (doc page) search for “kubectl Cheat Sheet” & “Custom Column” respectively, then from the results click the first link.
Scan through the resulting pages to get hold of the Field Selectors, Custom columns and Sort By commands.
kubectl get nodes -o jsonpath=’{.items[*].status.addresses[?(@.type==”ExternalIP”)].address}’
kubectl get services — sort-by=.metadata.name
kubectl get pods <pod-name> -o custom-columns=NAME:.metadata.name,RSRC:.metadata.resourceVersion
Tip 2: Looking at the output structure
To start using the JSON path in all these commands, we should know the output structure from kubectl commands. You can either use “-o yaml” or “-o json” on any kubectl command to understand the structure. I prefer “-o yaml”, because the output from “-o json” is less readable than the output of “-o yaml”. You can choose the option which looks better readable for you.
kubectl get node -o yaml
kubectl get node -o json
Tip 3: Find Arrays and Map
The next step is to know, how to find arrays and maps from the YAML structure. Concerning YAML, it’s the same as the YAML’s that we use to create any Kubernetes resource. The one that starts with “-” is an array. If you look at the below image
- “addresses” is an Array
- “allocatable” is a Map
Tip 4: Field Selectors
Now we know the base command, how to look and interpret the output structure, the next step is to learn how to construct the given requirement into a kubectl command. We can look into two different examples moving from simple to complex scenarios. so, that we will be able to answer any exam question.
Challenge 1: List all pods name, lastProbeTime from status where the type is ready.
Execute “kubectl get pod -o yaml” and look for the needed fields in the YAML output.
“pods name:: items : Array → metadata : Map → name”
Command below will give you all the pod names.
kubectl get pod -o jsonpath=’{.items[*].metadata.name}’
Learning 1: Use [*] for getting all array items.
lastTransitionTime:: items : Array → status : map → conditions : Array → lastProbeTime
Below command will give all the pod names and lastTransitionTime
kubectl get pod -o jsonpath=’{.items[*].metadata.name}{.items[*].status.conditions[?(@.type==”Ready”)].lastTransitionTime}’
Learning 2: Use [?(@.type==”Ready”)] to apply “where condition” with the array.
- “?” represents an if condition.
- “@” represents the current element in the array.
Challenge 2: Get all schedulable nodes. Each node should be displayed in a new row.
kubectl get nodes -o jsonpath=”{range .items[*]}{.metadata.name} {.spec.taints[*].effect}{\”\n\”}{end}”
Learning 3: To display every node in an individual row, we have to use a loop to process node by node. Use {range .items[*]} …… {end} to loop through the list of items.
Learning 4: Within the loop, directly refer the items from the looping node. For example, use {.metadata.name} under {range .items[*]} for referring name. Should not use {.items[*].metadata.name}.
Learning 5: At the end of every iteration use {\”\n\”} for new line.
kubectl get nodes -o jsonpath=”{range .items[*]}{.metadata.name} {.spec.taints[*].effect}{\”\n\”}{end}” | grep -v NoSchedule
Learning 6: Use “grep -v NoSchedule” to exclude node with taint ‘NoSchedule’. Note: “grep -v” is inverse grep.
Tip 4: Custom Columns
We can achieve the same requirement of challenge 2 with Custom Columns.
Get all schedulable nodes. Each node should be displayed in a new row.
kubectl get node -o custom-columns=NAME:.metadata.name,TAINT:.spec.taints[*].effect | grep -v NoSchedule
Learning 7: For “custom columns” and “sort by” we will use the same yaml structure, excluding the outer “items array” (i.e exclude — .items[*] )
Learning 8: Custom columns can have titles. NAME, TAINT in the above example are titles.
Tip 5: Sort By
Sort by will help us to order the output based on an attribute.
Challenge 3: Get all persistence volume from kube-system namespace ordered with capacity.
Look at the json structure to locate the capacity.
“capacity:: items : Array → spec : Map → capacity : storage”
The command for both sorted and unsorted output,
kubectl get pv -n kube-system — sort-by=.spec.capacity.storage
kubectl get pv -n kube-system
With this you should be able to handle any “Field Selectors”, “Custom Column” and “Sort By” related question. Also, visit other tips and tricks at
We will look into more tips and tricks in an upcoming article. Let you pass with flying colors :)
