Send message to a Python Script

Question

I'm trying to write a little python program for shutdown or Reboot my Raspberry PI, drived by a button connected to an GPIO. The program can show the current status of the raspberry PI (Booting,Running,Halting,Rebooting) via two leds. The python program is executed as daemon, started by a init.d bash script (written using the /etc/init.d/skeleton).

Now I can start/stop/verify the status of the daemon, and the daemon can check the input where the button is connected, to perform the command "shutdown -h now" or "shutdown -r now" .

For show the current status of the raspberry PI, I had thought of send messages to the daemon, using some script in the runlevels directorys, for change the status of the leds. But I don't know how receive message in the python program.

Someone can help me?

Thanks.

Answer 1

There are several methods to send a message from one script/app to another:

For you application a valid method is to use a named pipe. Create it using os.mkfifo, open it read-only in your python app and then wait for messages on it.

If you want your app to do another things while waiting, I reccomend you open the pipe in non-blocking mode to look for data availability without blocking your script as in following example:

import os, time

pipe_path = "/tmp/mypipe"
if not os.path.exists(pipe_path):
    os.mkfifo(pipe_path)
# Open the fifo. We need to open in non-blocking mode or it will stalls until
# someone opens it for writting
pipe_fd = os.open(pipe_path, os.O_RDONLY | os.O_NONBLOCK)
with os.fdopen(pipe_fd) as pipe:
    while True:
        message = pipe.read()
        if message:
            print("Received: '%s'" % message)
        print("Doing other stuff")
        time.sleep(0.5)

Then you can send messages from bash scripts using the command

echo "your message" > /tmp/mypipe

EDIT: I can not get select.select working correctly (I used it only in C programs) so I changed my recommendation to a non-bloking mode.

Answer 2

Is not more convenient this version? With the with costruct inside the while true: loop? In this way, all other code inside the loop is executable even in case of error in the pipe file management. Eventually I can use the try: costuct for catching the error.

import os, time

pipe_path = "/tmp/mypipe"
if not os.path.exists(pipe_path):
    os.mkfifo(pipe_path)
# Open the fifo. We need to open in non-blocking mode or it will stalls until
# someone opens it for writting
pipe_fd = os.open(pipe_path, os.O_RDONLY | os.O_NONBLOCK)

while True:
    with os.fdopen(pipe_fd) as pipe:
        message = pipe.read()
        if message:
            print("Received: '%s'" % message)

    print("Doing other stuff")
    time.sleep(0.5)