Copy pod.yaml to pod-sidecar.yaml and modify the file as here:

File: /home/student/pod-sidecar.yaml

apiVersion: v1
kind: Pod
metadata:
  name: website
  labels:
    app: application01
spec:
  containers:
  - name: nginx
    image: nginx
    ports:
    - containerPort: 80
  - name: centos
    image: centos:7
    command: [ "/bin/bash", "-c", "--" ]
    args: [ "while true; do sleep 30; done;" ]

Better to delete the POD create few minutes ago.

kubectl delete pod website

pod "website" deleted

Now you can apply using Declarative approach.

kubectl apply -f pod-sidecar.yaml

pod/website created

List POD and check if 2/2 are ready. 2 containers (nginx and CentOS) of 2 are correctly ready.

kubectl get pod

NAME      READY   STATUS    RESTARTS   AGE
website   2/2     Running   0          94s

Try to execute the command env inside the POD

student@master:~$ kubectl exec website env

Defaulting container name to nginx.
Use 'kubectl describe pod/website -n default' to see all of the containers in this pod.
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=website
KUBERNETES_SERVICE_HOST=10.96.0.1
KUBERNETES_SERVICE_PORT=443
KUBERNETES_SERVICE_PORT_HTTPS=443
KUBERNETES_PORT=tcp://10.96.0.1:443
KUBERNETES_PORT_443_TCP=tcp://10.96.0.1:443
KUBERNETES_PORT_443_TCP_PROTO=tcp
KUBERNETES_PORT_443_TCP_PORT=443
KUBERNETES_PORT_443_TCP_ADDR=10.96.0.1
NGINX_VERSION=1.17.1
NJS_VERSION=0.3.3
PKG_RELEASE=1~stretch
HOME=/root

As you can see the hostname of this POD is website. You can try to execute command directly in the container passing the parameter -c name_of_container

student@master:~$ kubectl exec website -c centos env

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=website
KUBERNETES_PORT_443_TCP=tcp://10.96.0.1:443
KUBERNETES_PORT_443_TCP_PROTO=tcp
KUBERNETES_PORT_443_TCP_PORT=443
KUBERNETES_PORT_443_TCP_ADDR=10.96.0.1
KUBERNETES_SERVICE_HOST=10.96.0.1
KUBERNETES_SERVICE_PORT=443
KUBERNETES_SERVICE_PORT_HTTPS=443
KUBERNETES_PORT=tcp://10.96.0.1:443
HOME=/root

You can use the same command for connect in interactive mode and in tty on the container CentOS.

kubectl exec website -ti -c centos -- /bin/bash

[root@website /]#

You are inside the container CentOS.

Try now to use curl for connect on localhost website.

[root@website /]# curl localhost

<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
    body {
        width: 35em;
        margin: 0 auto;
        font-family: Tahoma, Verdana, Arial, sans-serif;
    }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>

<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>

<p><em>Thank you for using nginx.</em></p>
</body>
</html>

From CentOS you can contact other containers using localhost. Try to create a folder inside your containers.

[root@website /]# mkdir container-centos

[root@website /]# ls

anaconda-post.log  container-centos  etc   lib    media  opt   root  sbin  sys  usr
bin                dev               home  lib64  mnt    proc  run   srv   tmp  var

The folder container-centos are here. Check the process of the container CentOS:

[root@website /]# ps aux

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  11692  2736 ?        Ss   10:34   0:00 /bin/bash -c -- while true; do sleep 30; done;
root        74  0.0  0.0   4372   688 ?        S    10:57   0:00 sleep 30
root        75  2.0  0.0  11832  3036 pts/0    Ss   10:57   0:00 /bin/bash
root        88  0.0  0.0  51752  3520 pts/0    R+   10:57   0:00 ps aux

As you can see, you can read only the process of this container, and not of nginx.

You can exit from CentOS container

[root@website /]# exit

exit

Now enter in nginx container

kubectl exec website -ti -c nginx -- /bin/bash

root@website:/#

root@website:/# ls

bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

The folder container-centos are not here. The image of nginx do not container the command ps, so you should install it.

root@website:/# apt-get update && apt-get install procps

<output_omitted>

Check the processes of the container nginx

root@website:/# ps aux

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  32656  5160 ?        Ss   10:34   0:00 nginx: master process nginx -g daemon off;
nginx        6  0.0  0.0  33112  3016 ?        S    10:34   0:00 nginx: worker process
root        17  0.0  0.0  18188  3352 pts/0    Ss   10:48   0:00 /bin/bash
root       284  0.0  0.0  36636  2872 pts/0    R+   10:56   0:00 ps aux

The namespace pids and mounts are private for container, while net, IPC e UTS are shared namespace of the POD.

[root@website /]# exit

exit

Now you can delete the pod.

kubectl delete pod website

pod "website" deleted