$ sudo apt-get update
$ sudo apt-get install ruby-directory-watcher 

ruby-directory-watcher is installed in your system.

Make ensure the ruby-directory-watcher package were installed using the commands given below,

$ sudo dpkg-query -l | grep ruby-directory-watcher *

You will get with ruby-directory-watcher package name, version, architecture and description in a table

ruby-directory-watcher package basic information: