Ruby: flatten directory [ENG]

We've organized files in subdirectories according to tags and according to creation date. What if you would like to flatten a tree, moving all files from the sub-directories into a single directory?

"Move all the files from the tree starting at source_dir"
"into dest_dir out of the tree"
def flatten(source_dir, dest_dir)
 Dir.foreach(source_dir) do |filename|
    # exclude current dir and parent dir
    if (!filename.eql?(".") and !filename.eql?("..")) 
     if File.directory?(filename) # subdirectory
       flatten(filename, dest_dir) # recursively flatten subdir
     else #file
       dirname = File.dirname(filename)
       filenameonly = filename[(dirname.length-1)..-1]
       old_name = source_dir + "/" + filename
       new_name = dest_dir + "/" + filenameonly
       puts("moving '#{old_name}' to '#{new_name}'")
       File.rename(old_name, new_name) 
     end
    end
  end
end

# read line parameters

source_dir = ARGV.shift
# default source dir is current dir
if (source_dir == nil) 
  source_dir = "."
end

dest_dir = ARGV.shift
# default destination dir current dir
if (dest_dir == nil)
  dest_dir = "."
end 

flatten(source_dir, dest_dir)