bt.py

#!/usr/bin/python2
import subprocess
import time

exec_path = "/home/tsu-nera/tmp/a.out"
core_path = "/home/tsu-nera/tmp/core"
bt_path = "/home/tsu-nera/tmp/backtrace"

core_time = subprocess.check_output(['ls', '-l', core_path]).split()
bt_time = subprocess.check_output(['ls', '-l', bt_path]).split()

core_time_p = time.strptime(core_time[7], "%H:%M")
bt_time_p = time.strptime(bt_time[7], "%H:%M")

if core_time_p > bt_time_p:
    with open(bt_path, 'w') as f:
        subprocess.call(['gdb', '-q', '-n', '-ex', 'bt', '-batch',
                                  exec_path, core_path], stdout=f)

[Unit]
Description=get backtrace
After=syslog.target
[Service]
Type=oneshot
ExecStart=/usr/bin/bt.py
[Install]
WantedBy=multi-user.target