--- File.expand_path(path[, default_dir])

    path ХѥŸʸ֤ޤ
    path ХѥǤ default_dir ˤޤ
    default_dir  nil ޤͿʤäˤϥ
    ȥǥ쥯ȥ꤬Ȥޤ

    Ƭ ~ ϥۡǥ쥯ȥ(Ķѿ HOME Ȥޤ)ˡ
    ~USER ϤΥ桼Υۡǥ쥯ȥŸޤ

      p File.expand_path("..")         #=> "/home/matz/work"
      p File.expand_path("..", "/tmp") #=> "/"
      p File.expand_path("~")          #=> "/home/matz"
      p File.expand_path("~foo")       #=> "/home/foo"

