code: function copyOne(file, dir, options, cb) {
    if (arguments.length < 3) {
      return invalid.apply(null, arguments);
    }
  
    if (typeof options === 'function') {
      cb = options;
      options = {};
    }
  
    var opts = utils.extend({}, options);
    if (typeof opts.cwd === 'undefined') {
      opts.cwd = process.cwd();
    }
    if (typeof file === 'string') {
      file = path.resolve(opts.cwd, file);
    }
  
    if (!opts.srcBase && opts.patterns) {
      opts.srcBase = path.resolve(opts.cwd, utils.parent(opts.patterns));
    }
  
    toDest(dir, file, opts, function(err, out) {
      if (err) {
        cb(err);
        return;
      }
  
      base(file, out.path, opts, function(err) {
        if (err) {
          cb(err);
          return;
        }
  
        cb(null, out);
      });
    });
  }
docs: Copy a single `file` to the given `dest` directory, using
  the specified options and callback function.
  
  ```js
  copy.one('foo.txt', 'dist', function(err, file) {
  if (err) throw err;
  // exposes the vinyl `file` that is created when the file is copied
  });
  ```
  @name .copy.one
  @param {String|Object} `file` Filepath or vinyl file
  @param {String} `dir` Destination directory
  @param {Object|Function} `options` or callback function
  @param {Function} `cb` Callback function if no options are specified
  @api public