Hello World - Shell

This guide describes the steps required to create the helloworld-shell sample app and deploy it to your cluster.

The sample app reads a TARGET environment variable, and prints Hello ${TARGET}!. If TARGET is not specified, World is used as the default value.

You can also download a working copy of the sample, by running the following commands:

git clone -b "release-0.19" https://github.com/knative/docs knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-shell

Prerequisites

  • A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions.
  • Docker installed and running on your local machine, and a Docker Hub account configured.
  • Optional: You can use the Knative CLI client kn to simplify resource creation and deployment. Alternatively, you can use kubectl to apply resource files directly.

Building

  1. Create a new file named script.sh and paste the script below. This will run netcat (nc) in an endless loop, returning a friendly welcome message.
#!/bin/sh
while true ; do
  echo -e "HTTP/1.1 200\n\n Hello ${TARGET:=World}!\n" | nc -l -p 8080 -q 1;
done
  1. Create a new file named Dockerfile and copy the code block below into it.

    # Use the official Alpine image for a lean production container.
    # https://hub.docker.com/_/alpine
    FROM alpine:3
    
    # Update & install netcat (nc)
    RUN apk update \
     && apk add netcat-openbsd
    
    # Copy over the service script
    COPY script.sh /
    
    # Start up the webserver
    CMD [ "/bin/sh", "/script.sh" ]
    

Once you have recreated the sample code files (or used the files in the sample folder) you’re ready to build and deploy the sample app.

  1. Use Docker to build the sample code into a container. To build and push with Docker Hub, run these commands replacing {username} with your Docker Hub username:

    # Build the container on your local machine
    docker build -t {username}/helloworld-shell .
    
    # Push the container to docker registry
    docker push {username}/helloworld-shell
    

Deploying

  1. After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster.

    1. Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

      apiVersion: serving.knative.dev/v1
      kind: Service
      metadata:
        name: helloworld-shell
        namespace: default
      spec:
        template:
          spec:
            containers:
              - image: docker.io/{username}/helloworld-shell
                env:
                  - name: TARGET
                    value: "Shell Sample v1"
      

    Ensure that the container image value in service.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename service.yaml
    

    With kn you can deploy the service with

    kn service create helloworld-shell --image=docker.io/{username}/helloworld-shell --env TARGET="Shell Sample v1"
    

    This will wait until your service is deployed and ready, and ultimately it will print the URL through which you can access the service.

    The output will look like:

    Creating service 'helloworld-shell' in namespace 'default':
    
     0.035s The Configuration is still working to reflect the latest desired specification.
     0.139s The Route is still working to reflect the latest desired specification.
     0.250s Configuration "helloworld-shell" is waiting for a Revision to become ready.
     8.040s ...
     8.136s Ingress has not yet been reconciled.
     8.277s unsuccessfully observed a new generation
     8.398s Ready to serve.
    
    Service 'helloworld-shell' created to latest revision 'helloworld-shell-kwdpt-1' is available at URL:
    http://helloworld-shell.default.1.2.3.4.xip.io
    

    During the creation of your service, Knative performs the following steps:

    • Creates of a new immutable revision for this version of the app.
    • Programs the network to create a route, ingress, service, and load balance for your app.
    • Automatically scales your pods up and down (including to zero active pods).

Verification

  1. Run one of the followings commands to find the domain URL for your service.

    kubectl get ksvc helloworld-shell  --output=custom-columns=NAME:.metadata.name,URL:.status.url
    

    Example:

    NAME                URL
    helloworld-shell    http://helloworld-shell.default.1.2.3.4.xip.io
    
    kn service describe helloworld-shell -o url
    

    Example:

    http://helloworld-shell.default.1.2.3.4.xip.io
    
  2. Now you can make a request to your app and see the result. Replace the URL below with the URL returned in the previous command.

    Example:

    curl http://helloworld-shell.default.1.2.3.4.xip.io
    Hello Shell Sample v1!
    
    # Even easier with kn:
    curl $(kn service describe helloworld-shell -o url)
    

    Note: Add -v option to get more details if the curl command failed.

Removing

To remove the sample app from your cluster, delete the service record.

kubectl delete --filename service.yaml
kn service delete helloworld-shell