[SOLVED] Searching for an expression returned from a shell process that never finishes using python 2.7 on raspberry PI

Issue

I am having a raspberry pi 4 (RPI) with python 2.7 installed. Within a python script, I am executing a shell script, which flashes a µController (µC) connected to the pi. The shell script prints some stuff and reaches idle after printing "Connecting". Note that the script does not finish at this point!

Now I want to use subprocess (or any other function) to forward me all the prints from the shell script. I do then want to check if the keyphrase "Connecting" has been printed. Besides, I need a timeout if the shell script gets stuck before printing "Connecting". However, I am quite new to python, thus I dont know how to use the subprocess correctly to be able to retrieve the prints from the shell script and set a timeout for the script as well.

Here is some sort of pseudo code:

output = subprocess.Popen(["./prebuilt/bin/bbb_cc13xx-sbl /dev/ttyACM0 {hexfileName} cc13x2 -e -p -v"], \
        stdout=subprocess.PIPE, stderr = subprocess.PIPE, shell = True)

expression_found = False

for i in range(5)
   if(output.stdout.find('Expression') != -1):
      expression_found = True
      break
   time.sleep(1)

if(expression_found):
   do that..
else:
   do this...

Is there an easy way to implement my two needs?

EDIT: Adding the prints to the terminal like os.system() does would be great, too.

Best wishes Slev1n

Solution

I actually found a simple solution, the mistake is to pipe stderr instead of stdout. The first one being empty all/most of the time.

Here is a solution where the prints from the child process where displayed on the terminal in real time and where I was able to search for a keyword within the stdout pipe. I was also able to terminate the child process without errors. I could also add a timeout as well for terminating the child process. The codes was also validated on raspberry pi 4B with python 2.7.

Here the main process:

import subprocess, sys
import time

cmd = "contprint.py"
p = subprocess.Popen( cmd , shell=True, 
                    stdout=subprocess.PIPE,
                    universal_newlines=True)

startTime = time.time()
maxTime = 8
while True:
    currentTime = time.time()
    if (currentTime - startTime) > maxTime:
        p.terminate()
        break
    else:
        output = p.stdout.readline()
        print(output)
        keyWord = "EOF"
        if keyWord in output:
            print("Keyword {} has be found".format(keyWord))
            p.terminate()
            break
        if len(output) == 0:
            print("Output is empty")
            p.terminate()
            break
        if p.poll() is not None:
            print("p.poll is not none")
            p.terminate()
            break

And here the child process:

import time, sys
count = 0

while(1):
    count += 1
    print(count)
    try:
        sys.stdout.flush()
    except:
        print("Escape Error!")

    time.sleep(0.5)
    if(count == 10):
        print("EOF")
    if(count == 20):
        pass`enter code here`

Any comments are welcome.

Answered By - Slev1n