famd and similar services allow applications to tell (without polling) when
files and directories have been changed; for example, they allow a file manager
to automatically update its display when the contents of a directory changes.
However, there's no good command-line interface to this.
I'd like a daemon that reads a configuration file containing a list of paths to
watch and commands to execute when they change. It should use famd or an
appropriate other mechanism. This could be used, for example, to automatically
update a web page when an underlying data file changes, or to do automatic
off-site backups of important files, or to update fontconfig or ld.so caches
on software installation.