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.
I need to use
Some netcat implementations have a timeout option. For the one on my system, it is
$ nc -l -p 2000 -w 5
will listen on port 2000 and exit in 5 seconds unless a connection is attempted on that port.
nc's suck these days, though, because they are not kept.
ncat- by the nmap people - is the exception, i think. and, of course, there is
socatif you have the determination to learn a new programming language.– mikeservJan 5, 2016 at 15:06
1This is incorrect. According to man nc(1): "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" Mar 17, 2019 at 11:38
Using bash timeout:
$ timeout 3s nc -l -p 2000
That will kill the
ncafter 3s even if it had accepted a connection.– user313992Dec 25, 2019 at 7:47
... which may actually be what the OP was asking, but it's not what the other answers do.– user313992Dec 25, 2019 at 7:58
actually I didn't try to accept connection... it was perfect for my use case (I need the port to accept connection just to make remote script happy).– chenchukDec 25, 2019 at 19:04
You should clearly explain what it's doing. The question is unclear and people come here with different ideas -- in this case the other two answers do something completely different than yours: they will wait up to N seconds for a connection, and if a connection happens within the N seconds, they will accept it and will continue servicing it even after the N seconds have passed.– user313992Dec 25, 2019 at 22:39
in my use case, I got remote ansible script which check 9092 port (kafka) and fails because kafka fails. since I don't care about this, I need a mechanism to make the port open. using nc in a loop did the job for me. ansinle tries to reach the port for 5 min, I made it to make ansible happy and continue execution. (after this I fix Kafka which is not relevant here)– chenchukDec 26, 2019 at 8:21
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
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.
As stated by peterph the timeout (
-w) while listening (
-l) for connections works (only) with traditional
On Ubuntu such an implementation can be installed with
apt install netcat-traditional and called with