Restarting a systemd service with new command line options from systemctl?

[mike@xxxxxxxxxxxxxxxxx (Michael Chapman)]

On Sat, 7 Jul 2018, Rick Beldin wrote:
[...]
> # systemctl restart systemd-udevd --debug
> systemctl: unrecognized option '--debug'

You would need to override the service's ExecStart= setting if you wanted 
to do it that way.

> Is there a more supported way of doing this with systemctl for systemd-udevd,
> perhaps in general for all systemd services?   I tend to think that there
> isn't, but thought I would I ask.

For udev in particular you can use:

    udevadm control --log-priority=debug

to change its log level without restarting it.

All of systemd's components will default to a debug log level if "debug" 
or "systemd.log_level=debug" appears on the kernel command-line, or if 
their environment contains "SYSTEMD_LOG_LEVEL=debug". You could implement 
the latter for specific services with drop-in files.

For the the systemd manager itself, you can also bump its log level by 
sending it SIGRTMIN+22 (documented in the systemd(1) manpage).