Two way serial communication with raspberry Pi (radio project)

Projects Interfacing w/ Software on the Computer
18

This is how I read on the pi what arduino is sending:

#!/bin/bash
set -x
ALTWERT=1
while true
do
WERT=$(head -n 1 < /dev/ttyACM0)
WERT=$(echo  $WERT | sed "s:\r::")
NUMBSTATIONS=$(mpc playlist | wc -l)

if [ $ALTWERT -ne $WERT ]; then
mpc play "$WERT"
ALTWERT=$WERT
#sleep 1
fi

#echo  "$NUMBSTATIONS\n"  > /dev/ttyACM0
done

Variable WERT is filled with what is coming from arduino. So by turning the pot I can "tune" through my playlist.

Variable NUMBSTATIONS hold the number of radio stations in my playlist. This is the value that I want to echo to the pot.

Echo is commented # in the code above;

But as I leared in this thread in the last two hours "echo" does not work in this case.