I'm now trying to run a simple container with shell (/bin/bash) on a Kubernetes cluster.
I thought that there was a way to keep a container running on a Docker container by using pseudo-tty
and detach option (-td
option on docker run
command).
For example,
$ sudo docker run -td ubuntu:latest
Is there an option like this in Kubernetes?
I've tried running a container by using a kubectl run-container
command like:
kubectl run-container test_container ubuntu:latest --replicas=1
But the container exits for a few seconds (just like launching with the docker run
command without options I mentioned above). And ReplicationController launches it again repeatedly.
Is there a way to keep a container running on Kubernetes like the -td
options in the docker run
command?
This question is related to
docker
containers
kubernetes
google-kubernetes-engine
My few cents on the subject. Assuming that kubectl
is working then the closest command that would be equivalent to the docker command that you mentioned in your question, would be something like this.
$ kubectl run ubuntu --image=ubuntu --restart=Never --command sleep infinity
Above command will create a single Pod
in default
namespace and, it will execute sleep
command with infinity
argument -this way you will have a process that runs in foreground keeping container alive.
Afterwords, you can interact with Pod
by running kubectl exec
command.
$ kubectl exec ubuntu -it -- bash
This technique is very useful for creating a Pod resource and ad-hoc debugging.
The simplest command as it can be for k8s pod manifest to run container forever:
apiVersion: v1
kind: Pod
metadata:
name: ubuntu
spec:
containers:
- name: ubuntu
image: ubuntu:latest
# Just sleep forever
command: [ "sleep" ]
args: [ "infinity" ]
Containers are meant to run to completion. You need to provide your container with a task that will never finish. Something like this should work:
apiVersion: v1
kind: Pod
metadata:
name: ubuntu
spec:
containers:
- name: ubuntu
image: ubuntu:latest
# Just spin & wait forever
command: [ "/bin/bash", "-c", "--" ]
args: [ "while true; do sleep 30; done;" ]
There are many different ways for accomplishing this, but one of the most elegant one is:
kubectl run -i --tty --image ubuntu:latest ubuntu-test --restart=Never --rm /bin/sh
Use Kubernetes Deployment and services.
In my case, a pod with an initContainer failed to initialize. Running docker ps -a
and then docker logs exited-container-id-here
gave me a log message which kubectl logs podname
didn't display. Mystery solved :-)
You could use this CMD in your Dockerfile
:
CMD exec /bin/bash -c "trap : TERM INT; sleep infinity & wait"
This will keep your container alive until it is told to stop. Using trap and wait will make your container react immediately to a stop request. Without trap/wait stopping will take a few seconds.
For busybox based images (used in alpine based images) sleep does not know about the infinity argument. This workaround gives you the same immediate response to a docker stop
like in the above example:
CMD exec /bin/sh -c "trap : TERM INT; sleep 9999999999d & wait"
I did a hack by putting it in background:
[root@localhost ~]# kubectl run hello -it --image ubuntu -- bash &
[2] 128461
Exec on pod hello
[root@localhost ~]# kubectl exec -it hello -- whoami
root
[root@localhost ~]# kubectl exec -it hello -- hostname
hello
Getting a shell
[root@localhost ~]# kubectl exec -it hello -- bash
root@hello:/# ls
bin boot dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
I was able to get this to work with the command sleep infinity
in Kubernetes, which will keep the container open. See this answer for alternatives when that doesn't work.
In order to keep a POD running it should to be performing certain task, otherwise Kubernetes will find it unnecessary, therefore it stops. There are many ways to keep a POD running.
I have faced similar problems when I needed a POD just to run continuously without doing any useful operation. The following are the two ways those worked for me:
Although the first option is easier than the second one and may suffice the requirement, it is not the best option. As, there is a limit as far as the number of seconds you are going to assign in the sleep command. But a container with infinite loop running inside it never exits.
However, I will describe both the ways(Considering you are running busybox container):
1. Sleep Command
apiVersion: v1
kind: Pod
metadata:
name: busybox
labels:
app: busybox
spec:
containers:
- name: busybox
image: busybox
ports:
- containerPort: 80
command: ["/bin/sh", "-ec", "sleep 1000"]
nodeSelector:
beta.kubernetes.io/os: linux
2. Infinite Loop
apiVersion: v1
kind: Pod
metadata:
name: busybox
labels:
app: busybox
spec:
containers:
- name: busybox
image: busybox
ports:
- containerPort: 80
command: ["/bin/sh", "-ec", "while :; do echo '.'; sleep 5 ; done"]
nodeSelector:
beta.kubernetes.io/os: linux
Run the following command to run the pod:
kubectl apply -f <pod-yaml-file-name>.yaml
Hope it helps!
Use this command inside you Dockerfile to keep the container running in your K8s cluster:
In your Dockerfile use this command:
CMD ["sh", "-c", "tail -f /dev/null"]
Build your docker image.
kubectl run debug-container -it --image=<your-image>
Source: Stackoverflow.com