python执行cmd命令

import subprocess

p = subprocess.Popen("ls",
        stdout=subprocess.PIPE, universal_newlines=True)
p.wait()
result_lines = p.stdout.readlines()

for line in result_lines:
    print(line)

import subprocess

cmd = "ffmpeg -i bb_short.mp4 -vf \"select=\'eq(pict_type,  PICT_TYPE_I)\'\" -vsync vfr out%d.png"
p = subprocess.Popen(cmd, shell=True)
p.wait()