I want to open a netcat connection and keep sending some text forever

Question

echo " The NC send commands  on port 2612"
while :
do
echo "hello " | nc -q -1  <some IP> 2612 
done

I want open a netcat session forever, and that is been achived by -q -1.

How can I send "hello" on the same channed in every 20 second.

my earlier script was as following, but that opens nc connection every time. What I really want is to open connection once and send "echo hello" evey 20 sec.

while :
do
echo "hello " | nc  192.168.100.161 2612 
sleep 20 
done

Answer 1

while echo "hello"; do
  sleep 20
done | nc -q 192.168.100.161 2612

Note that we moved the echo into the condition of the loop, so that we stop trying to run it if nc exits (causing the echos to fail).

---

If you want to be able to retain state (for instance, a counter) from inside the loop and access it after the pipeline exited, then things need to change a bit further:

#!/bin/bash
#      ^^^^- process substitutions aren't part of POSIX sh

count=0
while echo "hello"; do
  (( ++count ))
  sleep 20
done > >(nc -q 192.168.100.161 2612)

echo "Ran $count loops before exiting" >&2

There, the redirection is done as a process substitution, and the loop takes place inside the same shell instance that continues after it exits. See BashFAQ #24 for more details on this problem and solution.