python的cmd命令行在哪里

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()