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

Answer 1

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.