kubernetes cronjob logs

How to get logs from Cron Job in kubernetes (last completed job)

Question is how do I get logs from my Crone job? Ideally, I should be able to do this :

If I have Cron Job “cjob” in namespace “cj2” I can run a command like this

kubectl logs -n cj2 cronjob.batch/cjob

But as of Kubernetes 1.18, I get below Error

error: cannot get the logs from *v1beta1.CronJob: selector for *v1beta1.CronJob not implemented

let’s do the hard way

This is Cron job Yaml

apiVersion: v1
kind: Namespace
metadata:
  creationTimestamp: null
  name: cj2
spec: {}
status: {}
---
apiVersion: batch/v1beta1
kind: CronJob
metadata:
  creationTimestamp: null
  name: cjob
  namespace: cj2
spec:
  jobTemplate:
    metadata:
      creationTimestamp: null
      name: cjob
    spec:
      template:
        metadata:
        spec:
          containers:
          - image: busybox
            name: cjob
            command:
            resources: {}
            args:
            - /bin/sh
            - -c
            - date; echo Hello from the Kubernetes cluster
          restartPolicy: OnFailure
  schedule: "*/1 * * * *"
status: {}

It basically prints timestamp and ‘Hello from kubernetes cluster’ every 1 minute

Thu Jul  9 20:39:12 UTC 2020
Hello from the Kubernetes cluster

Now the hard way is to get a log from Cron job — for that first, we need to get all pods ran for this job and pick the pod that ran the last and get logs

$ kubectl get pods -n cj2
NAME                    READY   STATUS      RESTARTS   AGE
cjob-1594327920-6n4fk   0/1     Completed   0          3m
cjob-1594327980-jt8xp   0/1     Completed   0          2m
cjob-1594328040-rd2mz   0/1     Completed   0          60s

In the above case, it is the latest pod is cjob-1594328040-rd2mz

$ kubectl logs -n cj2 cjob-1594328040-rd2mz
Thu Jul  9 20:54:03 UTC 2020
Hello from the Kubernetes cluster

But that is a really hard way (above all can’t be automated) and I need to run multiple commands to get that — what if I want to get this all one command?

Proposed (Easy way)

First, we add labels to each pod that Cron job creates like this :

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  creationTimestamp: null
  name: cjob
  namespace: cj2
spec:
  jobTemplate:
    metadata:
      creationTimestamp: null
      name: cjob
    spec:
      template:
        metadata:
         labels:
          cj2job: cronjob
        spec:
          containers:
          - image: busybox
            name: cjob
            command:
            resources: {}
            args:
            - /bin/sh
            - -c
            - date; echo Hello from the Kubernetes cluster
          restartPolicy: OnFailure
  schedule: "*/1 * * * *"
status: {}

If you feel adventitious you can use below patch command to patch existing Cron job — but best apporach will be to update origianl yaml file

kubectl patch cronjobs.batch cjob -p '{"spec":{"jobTemplate":{"spec": {"template":{"metadata":{"labels":{"cj2job": "cronjob"}}}}}}}}}' -n cj2

Then now with the above modification, it is easy to get all pods that are created by this Cron job

$ kubectl get pods -n cj2 -l cj2job
NAME                    READY   STATUS      RESTARTS   AGE
cjob-1594328160-btbzx   0/1     Completed   0          2m52s
cjob-1594328220-w8zv2   0/1     Completed   0          112s
cjob-1594328280-8xd9f   0/1     Completed   0          52s

Now we just need last pod’s name that was completed we can do this via this JSON path trick

$ kubectl get pods -n cj2 -l cj2job --sort-by=.metadata.creationTimestamp -o 'jsonpath={.items[-1].metadata.name}'
cjob-1594328400-cwq5l
$

There are two things to pay attention

--sort-by=.metadata.creationTimestamp

This gives us pods sorted by timestamp — the latest pod to complete will be at the end of the list.

We need that pod’s name so we can use this JSON path trick

-o 'jsonpath={.items[-1].metadata.name}'

This will give us the last in list Pod’s name

Now we can feed this value to ‘kubectl logs’ command get our latest logs of Cron Job

$ kubectl logs -n cj2 $(kubectl get pods -n cj2 -l cj2job --sort-by=.metadata.creationTimestamp -o 'jsonpath={.items[-1].metadata.name}')Thu Jul  9 21:03:05 UTC 2020
Hello from the Kubernetes cluster

An alternative way to run above command is to use Xargs so you can add a watch on it

watch "kubectl get pods -n cj2 -l cj2job --sort-by=.metadata.creationTimestamp -o 'jsonpath={.items[-1].metadata.name}' | xargs kubectl logs -n cj2"