Large projects containing hundreds or even thousands of files can take a really long time to inspect, but RuboCop has functionality to mitigate this problem. There's a caching mechanism that stores information about offenses found in inspected files.

Cache Validity

Later runs will be able to retrieve this information and present the stored information instead of inspecting the file again. This will be done if the cache for the file is still valid, which it is if there are no changes in: the contents of the inspected file RuboCop configuration for the file the options given to rubocop, with some exceptions that have no bearing on which offenses are reported the Ruby version used to invoke rubocop * version of the rubocop program (or to be precise, anything in the source code of the invoked rubocop program)

Enabling and Disabling the Cache

The caching functionality is enabled if the configuration parameter AllCops: UseCache is true, which it is by default. The command line option --cache false can be used to turn off caching, thus overriding the configuration parameter. If AllCops: UseCache is set to false in the local .rubocop.yml, then it's --cache true that overrides the setting.

Cache Path

By default, the cache is stored in a subdirectory of the temporary directory, /tmp/rubocop_cache/ on Unix-like systems. The configuration parameter AllCops: CacheRootDirectory can be used to set it to a different path. One reason to use this option could be that there's a network disk where users on different machines want to have a common RuboCop cache. Another could be that a Continuous Integration system allows directories, but not a temporary directory, to be saved between runs.

Cache Pruning

Each time a file has changed, its offenses will be stored under a new key in the cache. This means that the cache will continue to grow until we do something to stop it. The configuration parameter AllCops: MaxFilesInCache sets a limit, and when the number of files in the cache exceeds that limit, the oldest files will be automatically removed from the cache.