Simple logwatcher in python3. Watchs a log file for a line with a stiing, then runs an executable with the line as arguments.

logwatcher.py

#!/usr/bin/env python3
# usage
#
# 1. Make the above script executable:
#    chmod +x logwatcher.py
#
# 2. Add a cronjob to start it after reboot like:
#    @reboot /home/YOURUSERNAME/logwatcher.py "/var/log/auth.log" "session opened for user" "/sbin/myscript.sh"
#
# arguments are:
#    ./logwatcher.py <log_file_to_watch> <string_to_look_for> <script_to_execute_then>

import sys
import os
import time
from subprocess import Popen

def follow(thefile):
    thefile.seek(0,2)
    while True:
        line = thefile.readline()
        if not line:
            time.sleep(0.5)
            continue
        yield line

if __name__ == '__main__':
    logfilename = sys.argv[1]
    pattern_string = sys.argv[2]
    command_to_execute = sys.argv[3]

    print("Log filename is: {}".format(logfilename))

    logfile = open(logfilename, "r")
    loglines = follow(logfile)

    for line in loglines:
        if pattern_string in line:
            Popen(args=line.split(' '), executable=command_to_execute)