FSEvents

The file system events API provides a way for your application to ask for notification when the contents of a directory hierarchy are modified. For example, your application can use this to quickly detect when the user modifies a file within a project bundle using another application. It also provides a lightweight way to determine whether the contents of a directory hierarchy have changed since your application last examined them. For example, a backup application can use this to determine what files have changed since a given time stamp or a given event ID.

The binary is a part of CoreServices public framework.