mmap_reader.py

import mmap
from time import sleep

with open('cmd.txt', 'r+b') as f:
    m = mmap.mmap(f.fileno(), 0)
    while True:
        data = m.readline()
        print data
        sleep(0.5)
        m.seek(0)

mmap_writer.py

import mmap
from time import sleep

with open('cmd.txt', 'r+b') as f:
    m = mmap.mmap(f.fileno(), 0)
    m.seek(0)
    m.write("hihihi\n")