mix new echo --bare  

There is a Erlang module called gen_tcp that we'll use to for communicating with TCP sockets.

In lib/server.ex we have the module responsible for that. It starts a server

def start(port) do
  tcp_options = [:binary, {:packet, 0}, {:active, false}]
  {:ok, socket} = :gen_tcp.listen(port, tcp_options)
  listen(socket)
end

then loop forever accepting income connections

defp listen(socket) do
  {:ok, conn} = :gen_tcp.accept(socket)
  spawn(fn -> recv(conn) end)
  listen(socket)
end

when a client connects it spawns a new process and start receiving data from the new made connection

defp recv(conn) do
  case :gen_tcp.recv(conn, 0) do
    {:ok, data} ->
      :gen_tcp.send(conn, data)
      recv(conn)
    {:error, :closed} ->
      :ok
  end
end

To run this open a console and start the server.

$ iex -S mix
iex> Echo.Server.start(6000)

The -S mix options will load your project into the current session.

Connect using telnet or netcat and try it out.

Create a lib/tasks.ex file and a module called Mix.Tasks.Start. The run function will be called by mix when we invoke the task.

def run(_) do
  Echo.Server.start(6000)
end

Compile your app and start the server

$ mix compile
$ mix start

or

$ mix do compile, start