Example code that demonstrates how to listen to journald using Python 3 + asyncio.

journal-reactor.py

#!/usr/bin/python3 -u

import asyncio
import sh

from systemd import journal
from systemd.daemon import notify


GATEWAY_IP = "192.168.10.1"
GATEWAY_MAC = "xx:xx:xx:xx:xx:xx"


loop = asyncio.get_event_loop()

j = journal.Reader()
j.log_level(journal.LOG_INFO)
j.add_match(SYSLOG_IDENTIFIER="kernel")
j.seek_tail()
j.get_previous()


async def process(event):
    if event["MESSAGE"].startswith("thinkpad_acpi: docked"):
        if sh.arp('-n', GATEWAY_IP).stdout.decode().strip().split('\n')[-1].split()[2]  == GATEWAY_MAC:
            print("Notified docking")
            await asyncio.create_subprocess_exec("/usr/bin/sudo", "/usr/bin/dash", "/root/scripts/unlock_usb.sh")
            await asyncio.create_subprocess_exec("/usr/bin/sudo", "/usr/bin/ifconfig", "eth0", "up")
        else:
            print("Nope")

    elif event["MESSAGE"].startswith("thinkpad_acpi: undocked"):
        print("Undocked!")
        await asyncio.create_subprocess_exec("/usr/bin/sudo", "/usr/bin/ifconfig", "eth0", "down")


def callback():
    notify("WATCHDOG=1")
    j.process()
    for entry in j:
        # FIXME: eats exceptions
        asyncio.ensure_future(process(entry))


loop.add_reader(j.fileno(), callback)
loop.run_forever()

journal-reactor.service

[Unit]
Description=Journal Reactor

[Service]
ExecStart=/usr/local/bin/journal-reactor.py
Restart=always
RestartSec=0
WatchdogSec=60s

[Install]
WantedBy=multi-user.target