Best way to convert text files between character sets?

Question

What is the fastest, easiest tool or method to convert text files between character sets?

Specifically, I need to convert from UTF-8 to ISO-8859-15 and vice versa.

Answer 1

With ruby:

ruby -e "File.write('output.txt', File.read('input.txt').encode('UTF-8', 'binary', invalid: :replace, undef: :replace, replace: ''))"

Source: https://robots.thoughtbot.com/fight-back-utf-8-invalid-byte-sequences