dummy_file_generator.rb

#!/usr/bin/env ruby

print 'Enter file size (MB): '
the_size = gets.chomp

unless the_size =~ /^\d+$/
  puts "Error: bad file size"
  exit
end

puts the_size + 'MB file coming right up!'

# Generate the file
file_size = 0
string    = "abcdefghijklmnopqrstuvwxyz123456"

f = File.new(the_size + 'MB', 'w')

while file_size < the_size.to_i * 1048576 # bytes in 1MB
  f.print string
  file_size += string.size
end

f.close