Setting requirepass with args while deploying Redis in k8s works with $() but not with ${}

Question

I am trying to deploy Redis (by creating a Helm chart) as a StatefulSet in Kubernetes cluster. I am not creating another Redis image on top of Official Redis Docker image, rather I am just trying to use the defaults available in Official Redis Docker image and just provide my redis.conf and requirepass at runtime.

To provide redis.conf, I am using a ConfigMap and mounting it in /config/redis.conf in the container.

Now, I want to pass --requirepass option as args in Kubernetes as below:

...
containers: [
  {
    name: redis,
    image: {
      repository: redis,
      tag: 5.0
    },
    imagePullPolicy: Always,
    workingDir: /data/,
    args: [ "/config/redis.conf", "--requirepass", "<password>" ],  # line of concern
    ports: [
      containerPort: 6379
    ],
    env: [
      {
        name: REDIS_AUTH,
        valueFrom: {
          secretKeyRef: {
            name: redis,
            key: password
          }
        }
      }
    ],
...

The following line fails:

args: [ "/config/redis.conf", "--requirepass", "${REDIS_AUTH}" ]

and on the contrary, this works:

args: [ "/config/redis.conf", "--requirepass", "$(REDIS_AUTH)" ]

Even though, $() syntax is for command substitution and REDIS_AUTH is an environment variable rather than an executable, how does it work and ${REDIS_AUTH} does not?

Answer 1

This is a Kubernetes specific feature that if you want to expand an environment variable in command or args field then you've to use the $() syntax instead of ${} syntax.

Check this link: https://kubernetes.io/docs/tasks/inject-data-application/define-command-argument-container/#use-environment-variables-to-define-arguments