promtool has recently gained the ability to run unit tests against defined recording and alerting rules. This feature is very useful in situations where you might need to check whether an expression matches certain conditions that have never happened before, thus making it difficult to be sure they will work when the time comes. This subcommand is called test rules and takes one or more test files as arguments. We're going to provide a deep dive of this feature later on, when we tackle how to best take advantage of rules in Chapter 9, Defining Alerting and Recording Rules.