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