Define a Command and Arguments for a Container

This page shows how to define commands and arguments when you run a container in a Pod.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

• Katacoda
• Play with Kubernetes

Define a command and arguments when you create a Pod

When you create a Pod, you can define a command and arguments for the containers that run in the Pod. To define a command, include the command field in the configuration file. To define arguments for the command, include the args field in the configuration file. The command and arguments that you define cannot be changed after the Pod is created.

The command and arguments that you define in the configuration file override the default command and arguments provided by the container image. If you define args, but do not define a command, the default command is used with your new arguments.

Note: The command field corresponds to entrypoint in some container runtimes. Refer to the Notes below.

In this exercise, you create a Pod that runs one container. The configuration file for the Pod defines a command and two arguments:

pods/commands.yaml

apiVersion: v1
kind: Pod
metadata:
  name: command-demo
  labels:
    purpose: demonstrate-command
spec:
  containers:
  - name: command-demo-container
    image: debian
    command: ["printenv"]
    args: ["HOSTNAME", "KUBERNETES_PORT"]
  restartPolicy: OnFailure

1. Create a Pod based on the YAML configuration file:

   kubectl apply -f https://k8s.io/examples/pods/commands.yaml
   

2. List the running Pods:

   kubectl get pods
   

   The output shows that the container that ran in the command-demo Pod has completed.

3. To see the output of the command that ran in the container, view the logs from the Pod:

   kubectl logs command-demo
   

   The output shows the values of the HOSTNAME and KUBERNETES_PORT environment variables:

   command-demo
   tcp://10.3.240.1:443
   

Use environment variables to define arguments

In the preceding example, you defined the arguments directly by providing strings. As an alternative to providing strings directly, you can define arguments by using environment variables:

env:
- name: MESSAGE
  value: "hello world"
command: ["/bin/echo"]
args: ["$(MESSAGE)"]

This means you can define an argument for a Pod using any of the techniques available for defining environment variables, including ConfigMaps and Secrets.

Note: The environment variable appears in parentheses, "$(VAR)". This is required for the variable to be expanded in the command or args field.

Run a command in a shell

In some cases, you need your command to run in a shell. For example, your command might consist of several commands piped together, or it might be a shell script. To run your command in a shell, wrap it like this:

command: ["/bin/sh"]
args: ["-c", "while true; do echo hello; sleep 10;done"]

Notes

This table summarizes the field names used by Docker and Kubernetes.

When you override the default Entrypoint and Cmd, these rules apply:

• If you do not supply command or args for a Container, the defaults defined in the Docker image are used.

• If you supply a command but no args for a Container, only the supplied command is used. The default EntryPoint and the default Cmd defined in the Docker image are ignored.

• If you supply only args for a Container, the default Entrypoint defined in the Docker image is run with the args that you supplied.

• If you supply a command and args, the default Entrypoint and the default Cmd defined in the Docker image are ignored. Your command is run with your args.

Here are some examples:

What's next

• Learn more about configuring pods and containers.
• Learn more about running commands in a container.
• See Container.