Laura Liparulo's blog

Tag: docker

Docker commands – Getting started

Posted on
Posted inCONTAINERS

To run one of the Docker official image, you can simply use “docker run”:

$ docker run centos

The first time you run it, the image can´t be found and must be downloaded:

Once downloaded, the image is saved in the docker cache and can be reused if necessary.

If it´s an official image, you would only need to enter the image name only.

But it´s taken from a user´s repo the format is: <username>/<image>.

The 100% Free software linux image Trisquel, for example, is taken from the repository of a user called “kpengboy“.

In this case, even if you are logged in, you can´t run “docker run”. You need to pull it first:

$docker pull kpengboy/trisquelbash

To see the list of the images donwload so far, run:

$ docker images

If you also want to start a bash in the container, use “-it” for the interactive mode:

$ docker run -it kpengboy/trisquel

Then you can work with the bash shell:

As default, docker will download the latest version available, but you can additionally specify a version (called “tag”) you want to work with:

$ docker run redis:4.0

If you want to access a webapp or, for example, a database, you generally get an assigned url with a port number. But you can change the ports, by adding them as parameters.

For example, if you want to run mysql and access it at a different port than 3306, use the “-p” option to give <host>:<container-port>:

$ docker run  -p 52000:3307 mysql

To persist your data, even when the container is killed, you can mount a persistent volume with “-v”:

$ docker run -v /opt/data/mysql mysql

You can pass an environment variable to you container, which might help you to avoid modifying the image and achieve some strategic solution:

$ docker run -e BACKGROUND_COLOR simple-web-app

To see which containers are running:

$ docker ps

To see the stopped containers to

$ docker ps -a

If you want to get the details of a specific container:

$ docker inspect vibrant_chatelet

You can check the log by running:

$ docker logs vibrant_chatelet

You can start a container in background mode with the “-d” option:

$ docker -d kpengboy/trisquel

To remove a container (even if it´s running)

$ docker rm vibrant_chatelet

Or by containerID

$ docker rm 0d6d64f9053c

To remove the image you need to make sure that no container is running it first. You migh use “docker stop” or just remove it directly.

To remove an image you need:

$ docker rmi kpengboy/trisquel

Kubernetes – Running from a custom Docker image

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Posted inCONTAINERS , DOCKER , Kubernetes

As I am getting ready for CKA exam, I will show you how to run a pod on Kubernetes starting from the a Dockerfile.

Let´s say we want to create a Node.Js simple server, a simpe app.js file.

const http = require('http');
const os = require('os');

console.log("My node js server is starting...");

var handler = function(request, response) {
  console.log("Received request from " + request.connection.remoteAddress);
  response.writeHead(200);
  response.end("You've hit " + os.hostname() + "\n");
};

var www = http.createServer(handler);
www.listen(8080);

Once we have the app.js file, we can create a Dockerfile too:

FROM node:7
ADD app.js /app.js
ENTRYPOINT ["node", "app.js"]

Then we can build the image:

docker build -t node-js-server-image .

Once we have created the image, we need to login to Docker Hub with the “docker login” command, then tag and push our image:

$ docker login

$ docker tag node-js-server-image lauraliparulo/node-js-server-image

$ docker push lauraliparulo/node-js-server-image

Then you can use the image to create a kubernetes pod

$ kubectl run nodejs --image=lauraliparulo/node-js-server-image --port=8080

With “kubectl describe pod node-js” we can find the IP of the exposed pod:

Then we can check the content with “curl”:

Docker – hello world example

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Posted inCONTAINERS , DOCKER

To run your first docker container, you can simply use a busybox image. With a single command, docker will download and execute the application inside a n isolated container:

> docker run busybox echo "Hello world"

As the image was not present on the machine, Docker pulled it from the Docker Hub Registry.

Another way is to create a simple two lines Dockerfile, like:

FROM alpine
CMD ["/bin/sh", "-c", "echo 'hello world'"]

As you can see, the starting point (with “FROM”) is an alpine base image. Then it´s just a matter of running an echo command in it.
From the folder where the Dockerfile is stored, invoke:

docker build -t helloworld .

And you get in the console:

Run:  
docker run hello-world
And you will see again:
hello world
As you can see, docker pulls the base alpine image  (6dbb9cc54074)
You can check the history of the container:
docker history hello-world

Or inspect it:

docker inspect f5d273aa2dcb

And finally remove it:

docker rmi -f f5d237aa2dcb

Kubernetes – Imperative Job creation

Posted on
Posted inCONTAINERS , Kubernetes , Programming

Let´s create a job the imperative way, by using the docker whalesay image:

kubectl create job whalesay --image=docker/whalesay --dry-run=client -o yaml > job.yaml -- cowsay I am going to ace CKAD!

We are using the “dry-run” option to create a yaml manifest without creating the job.

In the last part of the command with add a command for the container, that will be put directly in the manifest.

Once the file is created, we can add parameters like completions, parallelism and backoffLimit under the spec.template section, like this:

Then we need to create the job, by running:

kubectl create -f job.yaml -n <your-namespace>

After a while we can see the pods have been run and completed:

If you inspect the log of one of the pod, you can see the funny whale comics:

kubectl logs whalesay-7h27f

MORE ABOUT JOBS

Specifying the restartPolicy is mandatory for a Job.

Notice that:

  • a job is persisted and survives cluster restarts
  • a completed job is kept for tracking purposes
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