GNU Direvent is a daemon that monitors file system directories for events,
such as creating, deleting or modifying files.  It can be configured to
monitor different sets of directories for different events.  When an event
is detected, direvent(8) calls an external program associated with it,
supplying it with the information about the event and the location within
the file system where it occurred.

GNU Direvent provides an easy way to configure the system to react
immediately if certain files undergo changes.  This may be helpful, for
example, to track changes in important configuration files.

The program aims to provide a uniform and system-independent command-level
interface for file system events.  The current version is known to work on
GNU/Linux and BSD systems (FreeBSD, NetBSD, OpenBSD, Darwin).

WWW: https://www.gnu.org.ua/software/direvent/