Kubernetes: How to refer to one environment variable from another?

Question

I've a Deployment object where I expose the POD ID using the Downward API. That works fine. However, I want to set up another env variable, log path, with reference to the POD ID. But, setting that variable value to /var/log/mycompany/${POD_ID}/logs isn't working, no logs are created in the container. I can make the entrypoint script or the app aware of the POD ID, and build up the log path, but I'd rather not do that.

Answer 1

The correct syntax is to use $(FOO), as is described in the v1.EnvVar value: documentation; the syntax you have used is "shell" syntax, which isn't the way kubernetes interpolates variables. So:

containers: - env:   - name: POD_ID     valueFrom: # etc etc   - name: LOG_PATH     value: /var/log/mycompany/$(POD_ID)/logs 

Also please note that, as mentioned in the Docs, the variable to expand must be defined before the variable referencing it.

Answer 2

I'd just like to add to this question, a caveat we ran into the other day. According to the documentation:

Variable references $(VAR_NAME) are expanded using the previous defined environment variables in the container and any service environment variables. If a variable cannot be resolved, the reference in the input string will be unchanged.

Emphasis mine. If you have

  - name: POD_ID     valueFrom: # etc etc   - name: LOG_PATH     value: /var/log/mycompany/$(POD_ID)/logs 

it will work, but if you have

  - name: LOG_PATH     value: /var/log/mycompany/$(POD_ID)/logs   - name: POD_ID     valueFrom: # etc etc 

it will not. If you're using a templating engine to generate your specs, beware.