man pdsend (Commandes) - send messages to pd on this or a remote machine
NAME
pdsend - send messages to pd on this or a remote machine
SYNOPSIS
pdsend port-number [hostname] [udp|tcp]
DESCRIPTION
Pdsend sends messages to pd(1), via a socket conection, from pdsend's standard input. This input can be any stream of Pd messages separated by semicolons. This is probably the easiest way to control pd from another application. The protocol used is easy to implement and is called FUDI.
The port number should agree with the port number of a "netreceive" object within pd. The hostname is "localhost" by default and can be a domain name or an IP address. The protocol is "tcp" by default; this does a handshake to guarantee that all messages arrive complete and in their correct order; if you are sending messages locally or point-to-point you can often get away with the faster udp protocol instead.
You can also use this to talk to a Max "pdnetreceive" object or even just a "pdreceive" in another shell. If you're writing another program you're welcome to just grab the sources for pdsend/pdreceive and adapt them to your own ends; they're part of the Pd distribution.