who@ — prints the list of active users on a remote host.
who@ uses tcpclient(1) to connect to TCP port 11 on
host and prints any data that it receives.
It uses delcr(1) and cat(1) to remove CR (\015) and convert unprintable characters to a visible format.
host is not supplied, who@ connects to the local host.
TCP port 11 is the well-known port for the Systat service.
Some computers respond to connections on this port with a list of active users.
For example, they may be running
tcpserver -R -H -l0 0 11 who using tcpserver(1).