Azure policy to inherit tags from resource groups

az-policy-inherit-tags.json

{
    "policyRule": {
        "if": {
            "allOf": [
                {
                    "field": "[concat('tags[', parameters('tagName'), ']')]",
                    "exists": "false"
                },
                {
                    "not": {
                        "field": "type",
                        "equals": "Microsoft.Resources/subscriptions/resourceGroups"
                    }
                }
            ]
        },
        "then": {
            "effect": "append",
            "details": [
                {
                    "field": "[concat('tags[', parameters('tagName'), ']')]",
                    "value": "[resourcegroup().tags[parameters('tagName')]]"
                }
            ]
        }
    },
    "parameters": {
        "tagName": {
            "type": "String",
            "metadata": {
                "description": "The name of the tag to inherit",
                "displayName": "Tag name"
            }
        }
    }
}