code: function executeCommand(parts, callback) {
      var name,
          shouldLoad = true,
          command,
          usage;
  
      if (typeof parts === 'undefined' || typeof parts === 'function') {
        throw(new Error('parts is a required argument of type Array'));
      }
  
      name = parts.shift();
  
      if (app.cli.source || app.commands[name]) {
        if (app.commands[name]) {
          shouldLoad = false;
          if (typeof app.commands[name] != 'function' && !app.commands[name][parts[0]]) {
            shouldLoad = true;
          }
        }
  
        if (shouldLoad && !loadCommand(name, parts[0])) {
          return callback();
        }
  
        command = app.commands[name];
        while (command) {
          usage = command.usage;
  
          if (!app.argv.h && !app.argv.help && typeof command === 'function') {
            while (parts.length + 1 < command.length) {
              parts.push(null);
            }
  
            if (command.destructive) {
              return ensureDestroy(function (err) {
                return err ? callback() : command.apply(app, parts.concat(callback));
              })
            }
  
            command.apply(app, parts.concat(callback));
            return;
          }
  
          command = command[parts.shift()];
        }
  
        //
        // Since we have not resolved a needle, try and print out a usage message
        //
        if (usage || app.cli.usage) {
          showUsage(usage || app.cli.usage);
          callback(false);
        }
      }
      else if (app.usage) {
        //
        // If there's no directory we're supposed to search for modules, simply
        // print out usage notice if it's provided.
        //
        showUsage(app.cli.usage);
        callback(true);
      }
    }
docs: Helper function which executes the command represented by the Array of `parts` passing control to the `callback`.