Let's look at some basic kubectl output options.
Our intention is to list nodes (with their AWS InstanceId) and Pods (sorted by node).
We can start with:
kubectl get no
and
kubectl get po -o wide
I've found the internal data structures easier to explore using the -o json
output with jid
and jq.
Once both jq
and jid
are installed (assuming OSX), we can quickly discover the data with the following command:
kubectl get no -o json | jid -q | pbcopy
This allows us to explore the json data interactively and keep our final jq
query on the clipboard:
note: jid
currently implements it's own query parser to allow powerfull autocompletion, the drawback is
a lack of support for all the jq
constructs (i.e.: we have to specify an index for array elements during discovery).
As can be seen in the recording:
once done with jid
, getting rid of the index on the items
array in jq
, did gave us the full listing.
jq
gives us a lot more power for example:
Boxing the result into it's own array and constructing a new object combining several nested attributes gives us the following query:
kubectl get no -o json | jq -r '[.items[] | {name:.metadata.name, id:.spec.externalID, unschedulable:.spec.unschedulable}]'
Here is how the above query was built up using jid
and jq
:
Converting the json array into a tabular output with jq
can be done using @tsv
as follows:
kubectl get no -o json | jq -r '.items[] | select(.spec.unschedulable!=true) | [.metadata.name,.spec.externalID] | @tsv'
Jq also allows us to sort:
kubectl get po -o json | jq -r '.items | sort_by(.spec.nodeName)[] | [.spec.nodeName,.metadata.name] | @tsv'
The input for the sort_by
command must be an array, we iterate the elements after the sorting.
If all we need is a nicely formatted, sorted tabular report, kubectl
has built-in support for powerfull sorting:
kubectl get po -o wide --sort-by=.spec.nodeName
Using jid
to list pods sorted by node:
The usage of Custom Columns with the knowledge of the data structure gained from jid
, is also much easier:
kubectl get no -o=custom-columns=NAME:.metadata.name,AWS-INSTANCE:.spec.externalID,UNSCHEDULABLE:.spec.unschedulable
Note: apart from using grep
, there is no easy way to filter.
If we do not wish to use jq
(or have no access to jq
) need filtering and powerfull output control,
we may use Kubectl's built-in support for golang templates (inline or from a template file on disk):
kubectl get no -o go-template='{{range .items}}{{if .spec.unschedulable}}{{.metadata.name}} {{.spec.externalID}}{{"\n"}}{{end}}{{end}}'
or
kubectl get no -o go-template="{{range .items}}{{if .spec.unschedulable}}{{.metadata.name}} {{.spec.externalID}}:{{end}}{{end}}" | tr ":" "\n"
I could not find an easy way to print newline characters with inline golang template, so used a trick
printing colons and using tr
to convert colons to newlines.
Golang templates can be complicated and verbose - an alternative, if you are more familiar with jq
-style queries, or awscli
,
is to use JSONPath.
kubectl get no -o jsonpath="{.items[?(@.spec.unschedulable)].metadata.name}"
Internally, this seems tightly coupled to the golang templates.
Kubectl supports a superset of JSONPath, with a special range
keyword to iterate over ranges,
using the same trick to add newlines:
kubectl get no -o jsonpath="{range.items[?(@.spec.unschedulable)]}{.metadata.name}:{end}" | tr ":" "\n"
More examples of using jsonpath can be found in the Kubernetes tests for the JSONPath utility
EDIT - this also works: