running shell command in python

Question

When I run a terminal command in python, it throws an error. Below is the code I run from a .py-file in terminal:

import time
import os
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.IN, pull_up_down = GPIO.PUD_UP)
GPIO.setup(16, GPIO.IN, pull_up_down = GPIO.PUD_UP)
GPIO.setup(18, GPIO.IN, pull_up_down = GPIO.PUD_UP)
while True:
   if(GPIO.input(12) == 0):
      os.system ('omxplayer /home/pi/Downloads/bell.mp3')
      print("DOOR BELL RINGING")
      os.system ("cd /home/pi/picam")
      os.system ("./picam --alsadev hw:1,0 -w 640 -h 360 -v 800000 -f 24 --tcpout $")
      time.sleep(1)

This code is for video streaming, but it doesn't work. The error message is:

sh: 1: ./picam: not found

Answer 1

Note that the two separate calls of os.system are exactly that - completely separate from each other. Whatever happens in the first call is lost and forgotten in the second (like tears in rain).

os.system ("cd /home/pi/picam")
os.system ("./picam --alsadev ...")

Changing to /home/pi/picam is of no relevance to the next call which leads to the expected result - the error that is thrown - not found. Simplest solution here: use the full path /home/pi/picam/picam --alsadev ... instead of ./picam --alsadev ....

From https://docs.python.org/3/library/os.html#os.system it is recommended to use subprocess over os.system:

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.