How to make an Echo server with Bash?

Question

How to write a echo server bash script using tools like nc, echo, xargs, etc capable of simultaneously processing requests from multiple clients each with durable connection?

The best that I've came up so far is

nc -l -p 2000 -c 'xargs -n1 echo'

but it only allows a single connection.

Swanhildas

Ubuntu 12.04's nc has no -c option =( (netcat-openbsd package). – Bordelaise

Answer 1

If you use ncat instead of nc your command line works fine with multiple connections but (as you pointed out) without -p.

ncat -l 2000 -k -c 'xargs -n1 echo'

ncat is available at http://nmap.org/ncat/.

P.S. with the original the Hobbit's netcat (nc) the -c flag is not supported.

Update: -k (--keep-open) is now required to handle multiple connections.

Proxy

Thanks! It works but with a minor change - we should use it without the "-p" option: ncat -l 2000 -c 'xargs -n1 echo' – Swanhildas

@BinWang updated. Follow douyw link below for further explanations – Proxy

xargs -n1 echo splits up lines by whitespace, but xargs -l1 echo keeps lines intact. – Haver

Answer 2

Here are some examples. ncat simple services

TCP echo server

ncat -l 2000 --keep-open --exec "/bin/cat"

UDP echo server

ncat -l 2000 --keep-open --udp --exec "/bin/cat"

Bilection

Beware though, that with --udp, ncat will keep the "connections" open and if you don't want to hit the default limit of 100 connections, then you should either increase the maximum with the --max-conns option or let the "connections" die after an idle timeout that you can set with --idle-timeout. – Buckram

Answer 3

In case ncat is not an option, socat will also work:

socat TCP4-LISTEN:2000,fork EXEC:cat

The fork is necessary so multiple connections can be accepted. Adding reuseaddr to TCP4-LISTEN may be convenient.

Drue

Answer 4

netcat solution pre-installed in Ubunutu

The netcat pre-installed in Ubuntu 16.04 comes from netcat-openbsd, and has no -c option, but the manual gives a solution:

sudo mknod -m 777 fifo p
cat fifo | netcat -l -k localhost 8000 > fifo

Then client example:

echo abc | netcat localhost 8000

TODO: how to modify the input string value? The following does not return any reply:

cat fifo | tr 'a' 'b' | netcat -l -k localhost 8000 > fifo

The remote shell example however works:

cat fifo | /bin/sh -i 2>&1 | netcat -l -k localhost 8000 > fifo

I don't know how to deal with concurrent requests simply however.

Bordelaise

Answer 5

what about...

#! /bin/sh

while :; do
/bin/nc.traditional -k -l -p 3342 -c 'xargs -n1 echo'
done

Chatter

while :; do /bin/nc.traditional -k -l -p 3342 -c xargs -n1 echo ; done – Chatter