How do I write to the device Console log from the command line? How do I write to the log for my "Device" in the Console app from the command line. I've tried 

syslog -s -k Facility com.apple.console \
             Level Alert \
             Sender SomeSender \
             Message "some message"


and 

syslog -s -l error "message to send"


but don't see anything.
Apple has a new logging system so the old tail and syslog commands are deprecated. To start down the new unified log path, open two windows the first will just stream the logs (and they will fly by quickly normally)

log stream


Then to emit a log:

logger -p user.error "my new alert"


More good links:


https://developer.apple.com/documentation/os/logging
https://eclecticlight.co/2017/10/10/inside-the-macos-log-logd-and-the-files-that-it-manages/
http://krypted.com/mac-os-x/logs-logging-logger-oh/