Gqrx is a Software-defined radio receiver software for Linux and Mac. It’s a simple and convenient GUI with an FFT of the spectrum, a waterfall graph, and a panel to set the frequency and demodulation type.
Gqrx has a setting called Remote control that allows an external program to connect to it and issue commands.
By default, the port is 7356, and only connections from 127.0.0.1 are allowed. This is a sensible default for giving local applications control without allowing external commands.
The protocol is a human-readable, line-delimited ASCII protocol. Commands and their arguments are delimited by a space character. The responses are also sent as lines.
f.F freq where freq is the frequency in Hz.The handy “remote control” feature that allows you to connect to Gqrx over a TCP socket, and send commands to it.
Aside from changing the frequency, it also allows you to read the current signal strength on the tuned frequency.
This combination of features allows you to write a simple scanner that will scan a range of frequencies, and turn on the squelch when it detects a signal.