Cron  is a daemon available as a package for most Unix/Linux operating systems and is a time-based job scheduler.
Linux/Unix Cron Jobs and Cron Tab
A so-called Cron Job is represented as an entry in a Cron Tab describing when a particular command is to be executed. A Cron Tab entry may look like this :
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │ 7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * command to execute
The following entry, for example, writes
Hello World to
/var/log/hello.log at 00:01 o'clock every day.
1 0 * * * printf "Hello World\n" >> /var/log/hello.log
Kubernetes Cron Jobs
In Kubernetes a Cron Job is a Kubernetes Job that is executed according to a given schedule described in the classical Cron Tab format. The Cron Tab format is quite expressive and there are online tools helping to express a particular schedule such as .
Respectively, the pattern
1 0 * * * also schedules a CronJob for daily execution at 00:01 (12:01 a.m.).
The following example runs a CRON Job every 2 minutes. This frequency is most likely inappropriate for a Kubernetes CRON Job but good to observe what CRON Jobs do.
Create the file
# See https://crontab.guru/
# At every 2nd minute.
schedule: '*/2 * * * *'
# See https://kubernetes.io/docs/concepts/workloads/controllers/jobs-run-to-completion/#writing-a-job-spec
- name: simple-one-off-job-container
- 'I represent a very important maintenance task'
And apply it:
kubectl apply -f 90-cron-job.yaml
Verify its creation:
kubectl get cronjobs
And obtain CRON Job details:
kubectl describe cronjob cron-job-hello-world
Events section will list a number of Jobs. Apparently, CRON Jobs are used to derive Jobs which in turn create Pods.
The CRON Job should be executed periodically every 2 minutes.
So let's look for the Jobs:
kubectl get jobs
NAME COMPLETIONS DURATION AGE
cron-job-hello-world-1583929080 1/1 3s 5m18s
cron-job-hello-world-1583929200 1/1 3s 3m18s
As expected Jobs are created regularly and their names contain a suffix to uniquely distinguish them from another.
You can retrieve the Pods corresponding to a particular Job:
kubectl get pods -l job-name=cron-job-hello-world-1583929440
Or watch Pods being created:
kubectl get pods -w
NAME READY STATUS RESTARTS AGE
cron-job-hello-world-1583928480-mqxqj 0/1 Completed 0 5m20s
cron-job-hello-world-1583928600-5l8gb 0/1 Completed 0 3m20s
cron-job-hello-world-1583928720-x7bpj 0/1 Completed 0 80s
Kubernetes CRON Jobs are a convenient way to execute containerized workloads as Jobs, periodically.
Delete the CRON Job:
kubectl delete cronjob cron-job-hello-world
Have a look at Jobs and Pods:
kubectl get jobs
kubectl get pods
You will notice that deleting the CRON Job will also delete corresponding Jobs and Pods.