Kubernetes 101 : kubectl - communication with pods and containers / running commands inside pods and containers -
Running a command inside a pod:
Below is a short explanation of the options used in the above command:
--command -- : after the "--" we can specify the command we want to run inside the container. It overrides the "ENTRYPOINT" command used in the dockerfile when building the image.
--rm : it instructs Kubernetes to delete the container image when the pod stops.
-it : "-i" runs the container in interactive mode, and "-t" displays the output on the terminal.
--restart=Never : it instructs Kubernetes to not restart a container
when it exits or crashes.
-it : "-i" runs the container in interactive mode, and "-t" displays the output on the terminal.
--restart=Never : it instructs Kubernetes to not restart a container
when it exits or crashes.
Running a command inside a running pod:
We start by getting the name of the running pod using the below the command:
We can then run our command inside the running pod:
Automatically generating a Yaml file:
We use the below command to generate a YAML file for a kubernetes object:
--dry-run : prevents kubectl from creating the pod, it only creates the Yaml file for the resource.
-o yaml : creates the configuration file for the resource in a Yaml format.
Remark:
Because of the "--dry-run" option, the resource was not created. If we want to create the pod, we need to use the below command:
The kubectl diff:
After making changes for example to a Yaml deployment file, we could run the below command to see what would happen if case we apply the changes to our cluster:
We can see than the number of replicas would change if we apply the changes.
Watching the pods:
We could use the below command to watch the pods "live":
The above command displays all the states of the pod "demo-95333875c-z9xh2" from its creation until it reaches the "running" state.
Attaching a container to the terminal:
We could attach a container to the terminal and view its output on our screen, using the below command:
Connecting directly to a pod:
To connect directly to an application running inside a pod, we can "publish" the pod's port on the host, and access the application through the "published" port:
Port 26017 on the host is forwarded to port 25013 on the pod where the application is running.
Running command in pods with multiple containers:
The command "kubectl exec" runs its command in the first container if a pod hosts more than one container.
If we want to run a command on a specific container, we use the "-c"
option as below:
option as below:
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