python 通过自带的subprocess执行shell命令

def sh(cmd, **kwds):
    """run cmd in a subprocess and return its output.
    raises RuntimeError on error.
    """
    shell = True if isinstance(cmd, (str, unicode)) else False
    # Prevents subprocess to open error dialogs in case of error.
    flags = 0x8000000 if WINDOWS and shell else 0
    kwds.setdefault("shell", shell)
    kwds.setdefault("stdout", subprocess.PIPE)
    kwds.setdefault("stderr", subprocess.PIPE)
    kwds.setdefault("universal_newlines", True)
    kwds.setdefault("creationflags", flags)
    p = subprocess.Popen(cmd, **kwds)
    _subprocesses_started.add(p)
    stdout, stderr = p.communicate()
    if p.returncode != 0:
        raise RuntimeError(stderr)
    if stderr:
        warn(stderr)
    if stdout.endswith('\n'):
        stdout = stdout[:-1]
    return stdout