variable-use.json

{
  "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "principalId": {
      "type": "string",
      "metadata": {
        "description": "The principal to assign the role to"
      }      
    },
    "builtInRoleType": {
      "type": "string",
      "allowedValues": [
        "Owner",
        "Contributor",
        "Reader"
      ],
      "metadata": {
        "description": "Built-in role to assign"
      }      
    },
    "roleNameGuid": {
      "type": "string",
      "metadata": {
        "description": "A new GUID used to identify the role"
      }      
    }
  },
  "variables": {
    "Owner": "[concat('/subscriptions/', subscription().subscriptionId, '/providers/Microsoft.Authorization/roleDefinitions/', '8e3af657-a8ff-443c-a75c-2fe8c4bcb635')]",
    "Contributor": "[concat('/subscriptions/', subscription().subscriptionId, '/providers/Microsoft.Authorization/roleDefinitions/', 'b24988ac-6180-42a0-ab88-20f7382dd24c')]",
    "Reader": "[concat('/subscriptions/', subscription().subscriptionId, '/providers/Microsoft.Authorization/roleDefinitions/', 'acdd72a7-3385-48ef-bd42-f606fba81ae7')]",
    "scope": "[resourceGroup().id]"
  },
  "resources": [
    {
      "type": "Microsoft.Authorization/roleAssignments",
      "apiVersion": "2017-05-01",
      "name": "[parameters('roleNameGuid')]",
      "properties": {
        "roleDefinitionId": "[variables(parameters('builtInRoleType'))]",
        "principalId": "[parameters('principalId')]",
        "scope": "[concat(subscription().id, '/resourceGroups/unifi')]"
      }
    }
  ]
}