How to have netcat server close after n seconds

Question

I need to use nc in a weird way, where I want the server to first send a file, and then receive a file, and this repeats. I would like to have every instance of the server server close after some seconds. How can I do this in a script? I already have a client that can interact with the server.

Answer 1

Some netcat implementations have a timeout option. For the one on my system, it is -w. Thus

$ nc -l -p 2000 -w 5

will listen on port 2000 and exit in 5 seconds unless a connection is attempted on that port.

Answer 2

Using bash timeout:

$ timeout 3s nc -l -p 2000

Answer 3

Because of the documentation :The -w flag has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag

I tried nc ncat socat, no one can set timeout for server mode.

As far as I know, only busybox nc can follow -w option in server mode.

So you have to download busybox which compiled with CONFIG_NC_SERVER=y option, or compile it by your self.

So that you can

$busybox nc -w 10 -l -p 9999

But my system's busybox didn't compile with CONFIG_NC_SERVER=y option, and I don't want compile it. So I use this solution:

portnum=9999
(sleep 10 ;echo "T" | nc -w 1 127.0.0.1 $portnum) | nc -N -l -p $portnum 

After 10 seconds, send "T" to localhost:9999

The full code is

#!/bin/sh
portnum=9999
testmsg="Hello_World"
if [ "$( (sleep 10 ;echo "T" | nc -w 1 127.0.0.1 $portnum) | nc -N -l -p $portnum )" = $testmsg ]; then
  echo "Test pass"
else
  echo "Test not pass"
fi

If server received Hello_World in 10 seconds, it will print Test pass. Otherwise print Test not pass

You can try open another console, and type

echo "Hello_World" | nc -w 1 127.0.0.1 9999

to test it.

Answer 4

As stated by peterph the timeout (-w) while listening (-l) for connections works (only) with traditional nc implementations.

On Ubuntu such an implementation can be installed with apt install netcat-traditional and called with nc.traditional.