The signalk_client command must be run from a console.

pi@raspberrypi:~ $ signalk_client -h
usage /usr/local/bin/signalk_client [host] -i -c -h [NAME]...
-i print info about each value type
-c continuous watch
-h show this message

You can run with no arguments to retrieve all available values.

To receive only the voltage:

pi@raspberrypi:~ $ signalk_client servo.voltage

To continuously receive the imu heading:

pi@raspberrypi:~ $ signalk_client -c imu.heading