Run Interactive Shell Command in Erlang

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The following simple Erlang netcat client demonstrates the communication between Erlang program and external OS process. The rc.erl start a netcat client to connect to a netcat server, and send some message to the server at a given time.

1. Create Erlang server;

   -module(rc). -export([cmd/1]). cmd(Cmd) -> Opt = [stream, exit_status, stderr_to_stdout, eof], P = open_port({spawn, Cmd}, Opt), get_data(P, [], 0). get_data(P, Sofar, 3) -> port_command(P, "abcd"), get_data(P, Sofar, 4); get_data(P, Sofar, Cnt) -> receive {P, {data, Bin}} -> io:format("rec Bin: ~p, Cnt=~p~n", [Bin, Cnt]), get_data(P, [Bin|Sofar], Cnt+1); {P, eof} -> port_close(P), receive {P, {exit_status, N}} -> {N, lists:reverse(Sofar)} end end.

2. Start netcat server: run command nc -l 3333 at host 10.21.3.31;

3. Start netcat client:

   $ erl ...

   1> c(rc).

   {ok,rc} 2> rc:cmd("nc 10.21.3.31 3333").

4. Send message from server to client: press "1234^C", client output as follows:

   rec Bin: "1\n", Cnt=0 rec Bin: "2\n", Cnt=1 rec Bin: "3\n", Cnt=2 rec Bin: "4\n", Cnt=4 {0,["1\n","2\n","3\n","4\n"]} 3>

When you press after 3, you can receive "abcd" from netcat client.

The pattern of Erlang-Shell communication is:

• Use "open_port" to start a external shell process;

• Use "receive" clause to receive output of the process;

• Use "port_command" to send message to process;

• Use "port_close" to close the shell;

Notes:

1. For non-interactive shell command, os:cmd/1 is a better choice;

2. If you want to catch the event that the peer close actively, add "eof" into option list of open_port; If you want to catch the exit value of the shell process, add "exit_status" to option list of open_port;

3. Not specify "in" in option list of open_port when you want to send the process some message, or a "ebadf" exception arises, because "in" means this process only used for input, and "ebadf" means the file is not open for reading or writing.

4. Erlang port communicate with python is a good demonstration of erlang calling a python script; Chapter 12 of "Programming Erlang" also concentrate Erlang interfacing techniques.

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