我有一个名为user的系统用户帐户，如下所示：
useradd -r user
我有一个非常简单的Python程序，它只记录一行：
#!/usr/bin/env python
import logging
import os
# Set logging options
logging.basicConfig(level=os.environ.get("LOGLEVEL", "INFO"))
logging.info('LOG SOMETHING')
我有一个systemd服务来运行Python程序：
[Unit]
Description=Run simple.py
[Service]
ExecStart=/usr/local/bin/simple.py
User=user
如果我从命令行运行Python程序，同时使用root和user个用户，它的工作正常：
# /usr/local/bin/simple.py
INFO:root:LOG SOMETHING
# runuser -l user -c '/usr/local/bin/simple.py'
runuser: warning: cannot change directory to /home/user: No such file or directory
INFO:root:LOG SOMETHING
忽略该警告。那是因为系统用户帐户user没有主目录。
现在，如果我以systemd用户身份运行user服务（请注意服务文件末尾的User=user），则日志记录有时会失败，有时不会失败：
# systemctl start simple.service
# systemctl start simple.service
# systemctl start simple.service
# systemctl start simple.service
# systemctl start simple.service
# systemctl start simple.service
# journalctl -u simple.service | tail -18
Dec 08 17:57:23 localhost systemd[1]: Starting Run simple.py...
Dec 08 17:57:23 localhost systemd[1]: Started Run simple.py.
Dec 08 17:57:24 localhost systemd[1]: Starting Run simple.py...
Dec 08 17:57:24 localhost systemd[1]: Started Run simple.py.
Dec 08 17:57:25 localhost systemd[1]: Starting Run simple.py...
Dec 08 17:57:25 localhost systemd[1]: Started Run simple.py.
Dec 08 17:57:25 localhost systemd[1]: Starting Run simple.py...
Dec 08 17:57:25 localhost systemd[1]: Started Run simple.py.
Dec 08 17:57:26 localhost systemd[1]: Starting Run simple.py...
Dec 08 17:57:26 localhost simple.py[12774]: INFO:root:LOG SOMETHING
Dec 08 17:57:26 localhost systemd[1]: Started Run simple.py.
Dec 08 17:57:27 localhost systemd[1]: Starting Run simple.py...
Dec 08 17:57:27 localhost systemd[1]: Started Run simple.py.
现在，如果我以systemd用户身份运行root服务（只删除服务文件末尾的User=user，当然还有systemctl daemon-reload} ，记录始终有效：
# systemctl start simple.service
# journalctl -u simple.service | tail -18
Dec 08 18:04:03 localhost systemd[1]: Starting Run simple.py...
Dec 08 18:04:03 localhost simple.py[12827]: INFO:root:LOG SOMETHING
Dec 08 18:04:03 localhost systemd[1]: Started Run simple.py.
Dec 08 18:04:03 localhost systemd[1]: Starting Run simple.py...
Dec 08 18:04:03 localhost simple.py[12835]: INFO:root:LOG SOMETHING
Dec 08 18:04:03 localhost systemd[1]: Started Run simple.py.
Dec 08 18:04:04 localhost systemd[1]: Starting Run simple.py...
Dec 08 18:04:04 localhost simple.py[12843]: INFO:root:LOG SOMETHING
Dec 08 18:04:04 localhost systemd[1]: Started Run simple.py.
Dec 08 18:04:05 localhost systemd[1]: Starting Run simple.py...
Dec 08 18:04:05 localhost simple.py[12851]: INFO:root:LOG SOMETHING
Dec 08 18:04:05 localhost systemd[1]: Started Run simple.py.
Dec 08 18:04:05 localhost systemd[1]: Starting Run simple.py...
Dec 08 18:04:05 localhost simple.py[12859]: INFO:root:LOG SOMETHING
Dec 08 18:04:05 localhost systemd[1]: Started Run simple.py.
Dec 08 18:04:06 localhost systemd[1]: Starting Run simple.py...
Dec 08 18:04:06 localhost simple.py[12867]: INFO:root:LOG SOMETHING
Dec 08 18:04:06 localhost systemd[1]: Started Run simple.py.
为什么？