Underscore 1.1.0
Underscore is a command line program to replace spaces with underscores in all filenames in the directory it is called in.
Directions: Add Underscore to your PATH. Navigate to the directory containing the files you want to rename and run Underscore
Notes: This program was tested with Ruby version 1.8. Underscore is released under version 2 of the GPL (and only version 2).
Download Underscore 1.1.0 source