How can I send a message to the systemd journal from the command line?

systemd-cat is the equivalent to logger:

echo 'hello' | systemd-cat

In another terminal, running journalctl -f:

Feb 07 13:38:33 localhost.localdomain cat[15162]: hello

Priorities are specified just by part of the string:

echo 'hello' | systemd-cat -p info
echo 'hello' | systemd-cat -p warning
echo 'hello' | systemd-cat -p emerg

Warnings are bold, emergencies are bold and red. Scary stuff.

You can also use an ‘identifier’ which is arbitrary, to specify the app name. These are like syslog’s old facilities, but you’re not stuck with ancient things like lpr uucp nntp or the ever-descriptive local0 through local7. 

echo 'hello' | systemd-cat -t someapp -p emerg

Is logged as:

Feb 07 13:48:56 localhost.localdomain someapp[15278]: hello

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