This page shows how to define commands and arguments when you run a container in a Kubernetes Pod.
- Before you begin
- Defining a command and arguments when you create a Pod
- Using environment variables to define arguments
- Running a command in a shell
- What’s next
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.
Defining 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. For more information, see Commands and Capabilities.
In this exercise, you create a Pod that runs one container. The configuration file for the Pod defines a command and two arguments:
commands.yaml |
|---|
|
-
Create a Pod based on the YAML configuration file:
kubectl create -f http://k8s.io/docs/tasks/configure-pod-container/commands.yaml -
List the running Pods:
kubectl get podsThe output shows that the container that ran in the command-demo Pod has completed.
-
To see the output of the command that ran in the container, view the logs from the Pod:
kubectl logs command-demoThe output shows the values of the HOSTNAME and KUBERNETES_PORT environment variables:
command-demo tcp://10.3.240.1:443
Using 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.
Running 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"]
What’s next
- Learn more about containers and commands.
- Learn more about configuring containers.
- Learn more about running commands in a container.
- See Container.