code: def shell(cmd, output=None, mode='w', cwd=None, shell=False):
      
      if not output:
          output = os.devnull
      else:
          folder = os.path.dirname(output)
          if folder and not os.path.isdir(folder):
              os.makedirs(folder)
  
      if not isinstance(cmd, (list, tuple)) and not shell:
          cmd = shlex.split(cmd)
  
      def run_shell():
          try:
              p = Popen(cmd, stdin=PIPE, stdout=PIPE, stderr=PIPE, cwd=cwd,
                        shell=shell)
          except OSError as e:
              logger.error(e)
              if e.errno == os.errno.ENOENT:  # file (command) not found
                  logger.error("maybe you haven't installed %s", cmd[0])
              return e
          stdout, stderr = p.communicate()
          if stderr:
              logger.error(stderr)
              return stderr
          #: stdout is bytes, decode for python3
          if PY3:
              stdout = stdout.decode()
          with open(output, mode) as f:
              f.write(stdout)
  
      return run_shell
docs: Execute a shell command.
  
      You can add a shell command::
  
          server.watch(
              'style.less', shell('lessc style.less', output='style.css')
          )
  
      :param cmd: a shell command, string or list
      :param output: output stdout to the given file
      :param mode: only works with output, mode ``w`` means write,
                   mode ``a`` means append
      :param cwd: set working directory before command is executed.
      :param shell: if true, on Unix the executable argument specifies a
                    replacement shell for the default ``/bin/sh``.