Hello World - Shell

Hello World - Shell

A simple web app that executes a shell script. The shell script reads an env variable TARGET and prints Hello ${TARGET}!. If the TARGET environment variable is not specified, the script uses World.

Follow the steps below to create the sample code and then deploy the app to your cluster. You can also download a working copy of the sample, by running the following commands:

git clone -b "{{< branch >}}" https://github.com/knative/docs knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-shell

Before you begin

A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions if you need to create one.
Docker installed and running on your local machine, and a Docker Hub account configured (we’ll use it for a container registry).

Recreating the sample code

Create a new file named script.sh and paste the following script:
```
#!/bin/sh
echo Hello ${TARGET:=World}!
```

Create a new file named invoke.go and paste the following code. We use a basic web server written in Go to execute the shell script:

package main
import (
  "fmt"
  "log"
  "net/http"
  "os"
  "os/exec"
)
func handler(w http.ResponseWriter, r *http.Request) {
  log.Print("helloworld: received a request")
  cmd := exec.CommandContext(r.Context(), "/bin/sh", "script.sh")
  cmd.Stderr = os.Stderr
  out, err := cmd.Output()
  if err != nil {
    w.WriteHeader(500)
  }
  w.Write(out)
}
func main() {
  log.Print("helloworld: starting server...")
  http.HandleFunc("/", handler)
  port := os.Getenv("PORT")
  if port == "" {
    port = "8080"
  }
  log.Printf("helloworld: listening on %s", port)
  log.Fatal(http.ListenAndServe(fmt.Sprintf(":%s", port), nil))
}

Create a new file named Dockerfile and copy the code block below into it.

# Use the official Golang image to create a build artifact.
# This is based on Debian and sets the GOPATH to /go.
# https://hub.docker.com/_/golang
FROM golang:1.13 as builder
# Create and change to the app directory.
WORKDIR /app
# Retrieve application dependencies using go modules.
# Allows container builds to reuse downloaded dependencies.
COPY go.* ./
RUN go mod download
# Copy local code to the container image.
COPY invoke.go ./
# Build the binary.
# -mod=readonly ensures immutable go.mod and go.sum in container builds.
RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly -v -o server
# Use the official Alpine image for a lean production container.
# https://hub.docker.com/_/alpine
# https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
FROM alpine:3
RUN apk add --no-cache ca-certificates
# Copy the binary to the production image from the builder stage.
COPY --from=builder /app/server /server
COPY script.sh ./
# Run the web service on container startup.
CMD ["/server"]

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"

Use the go tool to create a go.mod manifest.

go mod init github.com/knative/docs/docs/serving/samples/hello-world/helloworld-shell

Building and deploying the sample

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.

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

After the build has completed and the container is pushed to docker hub, you can deploy the app into your cluster. 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
```
Now that your service is created, Knative performs the following steps:
- Create a new immutable revision for this version of the app.
- Network programming to create a route, ingress, service, and load balance for your app.
- Automatically scale your pods up and down (including to zero active pods).

Run the following command 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

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.
```
curl http://helloworld-shell.default.1.2.3.4.xip.io
```
Example:
```
curl http://helloworld-shell.default.1.2.3.4.xip.io
Hello Shell Sample v1!
```
Note: Add -v option to get more detail if the curl command failed.

Removing the sample app deployment

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

kubectl delete --filename service.yaml