In this tutorial, we run a Bash script on a Kubernetes Pod in three different ways.

Prerequisites

You need a Kubernetes cluster to complete this tutorial. If you do not have access to a Kubernetes cluster, you can deploy one easily with Minikube.

You also need to be familiar with using the Kubernetes CLI.

Why run Bash on Kubernetes

We use Bash scripts for automating and batch processing tasks like collecting logs, archiving backups, analyzing text files, etc. We can use Kubernetes to run these tasks at scale.

Let's check out the three methods now.

Method #1: Single line bash script with Kubernetes CLI

The quickest way to run a bash script in a Kubernetes Pod is to use the --command option in the kubectl run.

$ kubectl run math-pod --image=ubuntu --restart=Never --command -- "/bin/bash" "-c" "echo $((100+100))"

The --command accepts a series of commands and arguments to be run in the container in the Pod.

/bin/bash runs the Bash shell in the container with arguments -c and echo $((100+100)).

This single-line Bash script adds two numbers 100 and 100 prints the output.

Since the script completes instantly the Pod enters into Completed state immediately after creation.

$ kubectl get pods
NAME       READY   STATUS      RESTARTS   AGE
math-pod   0/1     Completed   0          5s

When creating the Pod we set the restart policy to Never so Kubernetes will not try to restart the Pod.

To check the output of the Bash script, we can use kubectl log command.

$ kubectl logs math-pod

Delete math-pod before proceeding to the next method.

$ kubectl delete pod math-pod

Method #2: Single line bash script in yaml manifest

Instead of creating a Pod directly from the Kubernetes CLI, we can use a Kubernetes YAML manifest.

Create file math-pod.yml in your working directory.

#math-pod.yml

apiVersion: v1
kind: Pod
metadata:
  labels:
    run: math-pod
  name: math-pod
spec:
  containers:
  - image: ubuntu
    name: math-pod
    command: ["/bin/bash", "-c", "echo $((100+100))"]
  restartPolicy: Never

We use spec.command to run the Bash script in the Pod. spec.command accepts an array which defines a command with arguments to be run in the Pod.

Let's create the Pod by applying the manifest.

$ k apply -f math-pod.yml

Again the Pod will enter the Completed state immediately after creating. Since we have set spec.restartPolicy to Never Kubernetes will not try to restart the Pod.

You can check the output of the Bash script using kubectl logs command as we did in Method #1.

Before going to the next step delete math-pod. Otherwise you'll get an error when trying to create a Pod named math-pod again.

Method #3: Multiline bash script in YAML manifest

Now, we are going to define a multi-line Bash script.

#math-pod.yml

apiVersion: v1
kind: Pod
metadata:
  labels:
    run: math-pod
  name: math-pod
spec:
  containers:
  - image: ubuntu
    name: math-pod
    command:
    - /bin/bash
    - -c
    - |
        i=0
        c=0
        while ((i<=100)) do
          ((c=c+i))
          ((i=i+1))
          echo $c
        done
  restartPolicy: Never

Now we are using the YAML notation - to define an array.

The pipe character (|) defines a multi-line String which is our Bash script. This Bash script adds numbers from 1 to 100 and prints the output at each iteration.

You can check the output with kubectl logs as we did in previous methods.

Wrapping up

In this tutorial, we run Bash scripts on Kubernetes by creating standalone Pods.

But to run Bash scripts in production on Kubernetes, it's better to use a Kubernetes Workload type such as a Job or a CronJob without creating Pods directly. When you create a Kubernetes Job or CronJob it takes care of the entire lifecycle of the Pod including failure recovery.

We will explore more about Kubernetes Jobs in the next tutorial.