How to make an Echo server with Bash?

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"

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.

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.

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.