CODEWITHSUNDEEP
kubernetesenvironment-variables
simohe
simohe

Reputation: 671

where in the pod config can I use the defined environment variable with $(x)

According to https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/#using-environment-variables-inside-of-your-config environment variables (defined in the config) can be used in the config like this:

...
spec:
  containers:
    env:
    - name: GREETING
      value: "Warm greetings to"
    ...
    args: ["$(GREETING) $(HONORIFIC) $(NAME)"]

Can I use the variables everywhere in the config? (Probably only in side container.)

This seems not to work:

    env:
    - name: DBHOST
      valueFrom:
        configMapKeyRef:
          key: db-host
          name: db-config
    - name: DB_URL
      value: mysql://$(DBHOST)/my_database

In the container, echo $DB_URL showed mysql://$(DBHOST)/my_database, but I expected DBHOST to be replaced.

So does it work in all other places except in env?

Maybe I have to look for another possibility to concatenate the values from the config map.

Upvotes: 1

Views: 424

Answers (1)

David Maze
David Maze

Reputation: 159998

In the detailed API documentation for a Container object, some of the fields specifically call out

Variable references $(VAR_NAME) are expanded using the container's environment.

This is specifically documented for args and command. Similar text is also in the documentation for EnvVar's value, with the caveat there that "references $(VAR_NAME) are expanded using the previous defined environment variables" (order of the env: block matters).

In a couple of places you can also exec a command inside a container. This includes postStart and preStop hooks and probes. These ultimately become an ExecAction API object. The command there is documented to not run inside a shell and can't directly use the $(VAR_NAME) syntax, but if you do run a shell explicitly, it should be able to use container-level environment variables. (If your command: or Dockerfile ENTRYPOINT sets environment variables, these will not be visible, similar to docker exec.)

apiVersion: v1
kind: Pod
metadata: {name: for-example}
spec:
  containers:
    - name: for-example
      env:
        - name: VAR_NAME
          value: a concrete string
        - name: COPY_OF_VAR_NAME
          value: '$(VAR_NAME)'
      command:
        - /bin/echo
        - '$(VAR_NAME)'
      args:
        - '$(VAR_NAME), again'
      livenessProbe:
        exec:
          command:
            - /bin/sh
            - -c
            - echo $VAR_NAME gets expanded by the shell
      lifecycle:
        postStart:
          exec:
            command:
              - /bin/sh
              - -c
              - echo $VAR_NAME gets expanded by the shell

Upvotes: 1

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