How to use bash to create a socket server and allow multiple clients in the same port?

Question

I have 4 programs, will be increased in the future, these programs have to connect to the same ip:port to send and receive messages at the same time.

Until now I have the socket opened, I also would like to keep the connection alive between the programs and the server.

#!/bin/sh
nc -lvk 88.109.110.161 100 > port100.txt 2>&1

Answer 1

nc does not handle multiple connected clients in parallel and is the wrong tool for this job. There are quite a few right tools for this job, including:

1. Bernstein tcpserver (original or djbwares) or Hoffman tcpserver:

   tcpserver -v -R -H -l 0 88.109.110.161 100 sh -c 'exec cat 1>&2' 2>&1 |
   cyclog port100/

2. my tcpserver shim:

   tcpserver -v 88.109.110.161 100 sh -c 'exec cat 1>&2' 2>&1 |
   cyclog port100/

3. my UCSPI-TCP tools:

   tcp-socket-listen 88.109.110.161 100 tcp-socket-accept --verbose sh -c 'exec cat 1>&2' 2>&1 |
   cyclog port100/

4. Bercot s6-tcpserver4:

   s6-tcpserver4 -v 2 88.109.110.161 100 sh -c 'exec cat 1>&2' 2>&1 |
   cyclog port100/

5. Bercot s6-networking tools:

   s6-tcpserver4-socketbinder 88.109.110.161 100 s6-tcpserver4d -v 2 sh -c 'exec cat 1>&2' 2>&1 |
   cyclog port100/

6. Pape tcpsvd:

   tcpsvd -v 88.109.110.161 100 sh -c 'exec cat 1>&2' 2>&1 |
   cyclog port100/

7. Sampson onenetd:

   onenetd -v 88.109.110.161 100 sh -c 'exec cat 1>&2' 2>&1 |
   cyclog port100/

And one can substitute multilog, s6-log, svlogd, or tinylog for cyclog.

Further reading

• Protocol:
  • Jonathan de Boyne Pollard (2016). The gen on the UNIX Client-Server Program Interface. Frequently Given Answers.
  • Daniel J. Bernstein (1996). UNIX Client-Server Program Interface. cr.yp.to.
• toolsets:
  • Daniel J. Bernstein. ucspi-tcp. cr.yp.to.
  • Erwin Hoffmann. ucspi-tcp6. fehcom.de.
  • s6-networking. Laurent Bercot. skarnet.org.
  • Jonathan de Boyne Pollard (2019). nosh. Softwares.
  • Jonathan de Boyne Pollard (2019). djbwares. Softwares.
  • ipsvd. Gerrit Pape. smarden.org.
  • onenetd. Adam Sampson. offog.org.
• reference manuals:
  • Daniel J. Bernstein. The tcpserver program. ucspi-tcp.
  • Erwin Hoffmann. tcpserver. ucspi-tcp6. fehcom.de.
  • s6-tcpserver4. Laurent Bercot. s6-networking. skarnet.org.
  • tcpsvd. ipsvd. Gerrit Pape. smarden.org.
  • Jonathan de Boyne Pollard (2019). tcpserver. djbwares. Softwares.
  • Jonathan de Boyne Pollard (2019). tcp-socket-listen. nosh Guide. Softwares.
  • Jonathan de Boyne Pollard (2019). tcp-socket-accept. nosh Guide. Softwares.
  • Jonathan de Boyne Pollard (2019). tcpserver. nosh Guide. Softwares.
• Logging:
  • https://unix.stackexchange.com/a/340631/5132
  • https://unix.stackexchange.com/a/505854/5132

Answer 2

It's a real pity that there's no standard/readily available tool for such basic task, but if you're not on some embedded system and have some scripting language like perl or python available, you can quickly put something together:

tcpsrv:

#! /usr/bin/perl
use strict;
use IO::Socket::INET6;
die "usage: $0 host:port { shell_cmd | cmd args ... }\n" unless @ARGV >= 2;
my $h = shift;
my $s=new IO::Socket::INET6(ReusePort=>1, Listen=>6, LocalAddr=>$h)
        or die "IO::Socket::INET($h): $!";
warn "listening on ", $s->sockhost, "/", $s->sockport, "\n";
$SIG{CHLD} = sub { use POSIX qw(WNOHANG); 1 while waitpid(-1, WNOHANG) > 0 };
while(1){
        my $a = $s->accept or do { die "accept: $!" unless $!{EINTR}; next };
        warn "connection from ", $a->peerhost, "/", $a->peerport, "\n";
        die unless defined(my $p = fork);
        close($a), next if $p;
        open STDIN, "<&", $a and open STDOUT, ">&", $a or die "dup: $!";
        close $s and close $a or die "close: $!";
        exec(@ARGV); die "exec @ARGV: $!";
}

Usage: tcpsrv host:port cmd

This will listen on host:port, and anytime a client connects to host:host, it will fork & exec cmd with its stdin and stdout redirected from/to the connection:

tcpsrv :9999 ls .
tcpsrv 127.0.0.1:7000 uptime
tcpsrv [::]:7000 uptime
tcpsrv 88.109.110.161:2000 'cat > port2000.txt'