How to create port listeners in Linux machine?

Question

Below command creates a port listener:

nc –l 5500 &

as shown below:

[root@xyz ~]# netstat -anlp | grep 5500
tcp       0     0 0.0.0.0:5500               0.0.0.0:*                   LISTEN
   21085/nc

I would like to create 100 port listeners.

But below syntax:

 $ for i in {10000..10100}; do nc -l $i &; done

gives error:

 bash: syntax error near unexpected token `;'

Duplicate of unix.stackexchange.com/questions/505843/… or unix.stackexchange.com/questions/271714/… and/or unix.stackexchange.com/questions/110822/… — Jeff Schaller
– Jeff Schaller ♦, Commented Nov 5, 2020 at 19:57

Kusalananda · Accepted Answer · 2020-11-05 20:01:50Z

As mentioned in the chat, the & is a statement terminator, just like ;, so the ; before done shouldn't be there:

for i in {10000..10100}; do nc -l "$i" & done

or,

for i in {10000..10100}; do
    nc -l "$i" &
done

You may kill all your nc processes using pkill nc.

Alternatively, save the PIDs of each nc process in an array and kill them using this array:

pids=()
for i in {10000..10100}; do
    nc -l "$i" & pids+=("$!")
done

Later:

kill "${pids[@]}"

This obviously assumes that none of the nc commands' PIDs have been recycled for other processes.

As roaima points out in comments, the brace expansion {10000..10100} expands to 101 numbers, not 100.

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