Restrictions on setting environment variables in Compute Policies

[tcmx]

As recommended by Databricks, we are trying to use Compute Policies to set environment variables, which are used by our notebooks, across clusters.

However, when specifying a JSON string as env var, we are getting this error upon applying the policy to a shared cluster: 

Cannot update compute configuration:
$$, \, ", ', | and backticks are not allowed in the environment variable value for Unity Catalog shared clusters. Please use single user clusters instead.

Where is this error coming from? It's not documented anywhere, and seems arbitrary. 
What are possible ways around this?  

[-werners- · Accepted solution]

This is because you use Shared access mode.
This enables multiple users to use the cluster simultaneously.
However, there are features that do not work on these Shared access mode clusters:
https://docs.databricks.com/en/compute/access-mode-limitations.html#shared-access-mode-limitations-o...
The limitation about backticks in env vars is not mentioned but the error message is clear: it is not supported.
To overcome this limitation, use Single user as access mode.