Hi, I'm trying to install a library using the init script option. But the $DB_CLUSTER_ID it is empty, why? Also I tried to do in a Notebook cell a %sh echo $DB_CLUSTER_ID and also is empty

curl -X POST https://dbc-myid.cloud.databricks.com/api/2.0/libraries/install -H 'Cache-Control:no-cache' -H 'Content-Type:application/json' -H 'Authorization: Bearer mytoken' -d '{"cluster_id": "$DB_CLUSTER_ID", "libraries": [{"egg": "dbfs:/FileStore/jars/library-0.1-py3.7.egg"}]}'