Dir.glob to get all csv and xls files in folder

Well, yielding is simple. Just yield.

def get_csv_file_paths(path)
  Dir.glob(path + '/**/*.csv').each do |f|
    yield f
  end
end

def get_xlsx_file_path(path)
  Dir.glob(path + '/**/*.xls').each do |f|
    yield f
  end
end

files_to_process = []
get_csv_file_paths(folder_path) {|f| files_to_process << f }
get_xlsx_file_path(folder_path) {|f| files_to_process << f }

puts files_to_process.length

Every method in ruby can be passed a block. And yield keyword sends data to that block. If the block may or may not be provided, yield is usually used with block_given?.

yield f if block_given?

Update

The code can be further simplified by passing your block directly to glob.each:

def get_csv_file_paths(path, &block)
  Dir.glob(path + '/**/*.txt').each(&block)
end

def get_xlsx_file_path(path, &block)
  Dir.glob(path + '/**/*.xls').each(&block)
end

Although this block/proc conversion is a little bit of advanced topic.

def get_folder_paths(root_path)
  Dir.glob('**/*.csv') + Dir.glob('**/*.xls')
end

folder_path = File.join(Dir.pwd, ARGV.first || '')
raise "#{folder_path} is not a valid folder" unless File.directory?(folder_path)

puts get_folder_paths(folder_path).length

The get_folder_paths method returns an array of CSV and XLS files. Building an array of file names may not be what you really want, especially if there are a lot of them. An approach using the Enumerator returned by Dir.glob would be more appropriate in that case if you did not need the file count first.

Just stick the file extensions together into one group:

Dir[path + "/**/*.{csv,xls}"]