我试图找出导致每次启动时短暂崩溃的原因，如last命令输出所示：
user pts/0 :0 Wed Jul 15 21:07 still logged in
user :0 :0 Wed Jul 15 20:29 still logged in
reboot system boot 3.15.3-200.fc20. Wed Jul 15 20:29 still running
(unknown :0 :0 Wed Jul 15 20:29 - crash (00:00)
reboot system boot 3.15.3-200.fc20. Wed Jul 15 20:29 still running
user pts/2 :0 Wed Jul 15 17:51 - 19:17 (01:26)
...
user :0 :0 Wed Jul 15 14:15 - 19:27 (05:11)
reboot system boot 3.15.3-200.fc20. Wed Jul 15 14:12 - 19:27 (05:14)
(unknown :0 :0 Wed Jul 16 14:12 - crash (00:00)
reboot system boot 3.15.3-200.fc20. Wed Jul 16 14:12 - 19:27 (05:14)
user pts/0 :0 Wed Jul 16 09:13 - 10:29 (01:15)
我试图通过查询日志日志来找出问题所在，但除了一些专用总线连接错误以及 sendmail 再次重新启动之外，我没有发现任何问题。
Jul 15 20:29:44 systemd[1]: Started Sendmail Mail Transport Agent.
Jul 15 20:29:44 systemd[1]: Starting Sendmail Mail Transport Client...
...
Jul 15 20:29:46 systemd[875]: Failed to open private bus connection: Failed to connect to socket /run/user/42/dbus/user_bus_socket: No such file or directory
...
Jul 15 20:29:51 systemd[1]: Stopping Sendmail Mail Transport Client...
Jul 15 20:29:51 systemd[1]: Stopping Sendmail Mail Transport Agent...
Jul 15 20:29:51 systemd[1]: Starting Sendmail Mail Transport Agent...
...
Jul 15 20:29:57 systemd[1862]: Failed to open private bus connection: Failed to connect to socket /run/user/1000/dbus/user_bus_socket: No such file or directory
谁能告诉我如何找出导致崩溃的原因？