Changing file extension using ruby

Simpler

'abc . . def.mp3'.sub /\.[^\.]+$/, '.opus'

Improving the previous answer a little:

require 'fileutils'
Dir.glob('/path_to_file_directory/*.eml').each do |f|
    FileUtils.mv f, "#{File.dirname(f)}/#{File.basename(f,'.*')}.html"
end

The File.basename(f,'.*') will give you the name without the extension otherwise the files will endup being file_name.eml.html instead of file_name.html

Rake offers a simple command to change the extension:

require 'rake'
p 'xyz.eml'.ext('html') # -> xyz.html

Improving the previous answers again a little:

require 'rake'
require 'fileutils'
Dir.glob('/path_to_file_directory/*.eml').each do |filename|
    FileUtils.mv( filename, filename.ext("html"))
end

Pathname has the sub_ext() method to replace the extension, as well as glob() and rename(), allowing the accepted answer to be rewritten a little more compactly:

require 'pathname'
Pathname.glob('/path_to_file_directory/*.eml').each do |p|
    p.rename p.sub_ext(".html")
end