Python script does not continue after os.system command

Question

First post and also first time experimenting with a PI & Python.

I have a pi-3 model B with the 7" touchscreen and PIR sensor. My goal is to have it set up so that it constantly displays the stream from my IP camera. To save power consumption I also want to include a motion detection to toggle the display on/off when motion is detected.

My startup.py script is as follows:

import os, time, logging
logging.basicConfig(filename='Startup.log',level=logging.DEBUG)
logging.info('Logging started')
from gpiozero import MotionSensor
#Start stream
os.system("omxplayer --aspect-mode stretch rtsp://Username:Password@IPaddress:port/videoMain")
#Screen OFF
os.system("bash -c 'echo 1 > /sys/class/backlight/rpi_backlight/bl_power'")
#PIR sensor function, motion detected -> screen ON, wait x seconds, screen off
pir = MotionSensor(4)
while True:
 if pir.motion_detected:
 #Screen ON
 logging.info('Motion detected')
 os.system("bash -c 'echo 0 > /sys/class/backlight/rpi_backlight/bl_power'")
 #Wait x seconds
 time.sleep(180)
 logging.info('timer reached')
 #Screen OFF
 os.system("bash -c 'echo 1 > /sys/class/backlight/rpi_backlight/bl_power'")

When running this script, omxplayer plays the stream, but it seems the python script does not continue. Only when killing omxplayer, the script continues, turning the screen on/off when motion is detected. How can I get the stream to play combined with the motion detection functionality?

Answer 1

for what it's worth or for others reading the question, you can solve this easily by using "&" in the call:

replace

os.system("omxplayer --aspect-mode stretch rtsp://Username:Password@IPaddress:port/videoMain")

by

os.system("omxplayer --aspect-mode stretch rtsp://Username:Password@IPaddress:port/videoMain &")

Answer 2

When you call os.system(abc123) python will wait until that process has completed before continuing. You may want to consider using something that simply spawns a subprocess and continues like

import os, time, logging
from gpiozero import MotionSensor
import subprocess
logging.basicConfig(filename='Startup.log', level=logging.DEBUG)
logging.info('Logging started')
ON, OFF = 0, 1 # map these to be explicit (are these backwards?)
''' Simple func to switch screen state '''
def switch_screen(state=ON):
 logging.info('setting screen state to {}'.format(state))
 os.system("bash -c 'echo {} > /sys/class/backlight/rpi_backlight/bl_power'".format(state)) # let this block
process = subprocess.Popen("omxplayer --aspect-mode stretch rtsp://184.72.239.149/vod/mp4:BigBuckBunny_115k.mov", shell=True) #Start stream
switch_screen(OFF)
pir = MotionSensor(4) #PIR sensor function, motion detected -> screen ON, wait x seconds, screen off
while True:
 if pir.motion_detected:
 #Screen ON
 logging.info('Motion detected')
 switch_screen(ON)
 time.sleep(180) #Wait x seconds
 logging.info('timer reached')
 switch_screen(OFF) #Screen OFF
# clean up, maybe GPIO too?
logging.info('stopping')
process.kill() # stop OMX player
switch_screen(ON) # turn this back on at the end?
logging.info('stopped')

• KawaGreen: Unfortunately this doesn't seem to work. I get the same result as before. However I currently use a workaround with the crontab file. As it was my intention to launch the script at startup, I lauch the omxplayer line and also the python script. This seems to work well so far but I don't know if this is best-practice.
• user29019: I've updated my example to better show what I meant.
• KawaGreen: Same result, script won't continue to switch_screen(OFF). I wish to accomplish a permanent 24/7 streaming of the IP camera, I wish to turn on the screen when I walk towards it and let it turn off again after x seconds. That way I'll save power and won't burn in the screen. But it seems to be running well with my workaround.
• user29019: I tried this script with the Big Buck Bunny demo stuff and the only issue i had was having to run the script as sudo and adding a shell=True argument to Popen to satisfy omxplayer. I was getting an error on this call which might be what stopped you. Regardless, if you've found a work around then great, apologies for the dead end.