Netcat [nc] listen grep ip and disconnect - linux

Question

Is there a way to grep the IP address of the inbound connection and disconnect after a timeout?
If I do
nc -vv -l -p <portnum>
it's connected forever.
$nc -h
[v1.10]
connect to somewhere: nc [-options] hostname port[s] [ports] ...
listen for inbound: nc -l -p port [-options] [hostname] [port]
options:
-4 Use IPv4 (default)
-6 Use IPv6
-c shell commands as -e; use /bin/sh to exec [dangerous!!]
-e filename program to exec after connect [dangerous!!]
-A algorithm cast256, mars, saferp, twofish, or rijndael
-k password AES encrypt and ascii armor session
-b allow broadcasts
-g gateway source-routing hop point[s], up to 8
-G num source-routing pointer: 4, 8, 12, ...
-h this cruft
-i secs delay interval for lines sent, ports scanned
-l listen mode, for inbound connects
-n numeric-only IP addresses, no DNS
-o file hex dump of traffic
-p port local port number
-r randomize local and remote ports
-q secs quit after EOF on stdin and delay of secs
-s addr local source address
-t answer TELNET negotiation
-u UDP mode
-v verbose [use twice to be more verbose]
-w secs timeout for connects and final net reads
-z zero-I/O mode [used for scanning]
port numbers can be individual or ranges: lo-hi [inclusive];
hyphens in port names must be backslash escaped (e.g. 'ftp\-data').
I'm trying but I get no result.
My netcat is dated. The nc version number is 1.10
EDIT
#VictorLee gives me some alternatives. I made a thing.
Here there's a little server script that listen and logs every new different access.
If someone want to use or modify I put the code below
#!/bin/bash
unset PIDTMP; rm -rf tmplog.log 2>/dev/null
while true; do
if [[ "$PIDTMP" == "" ]]; then
nc -vv -l -p <YOURPORT> > tmplog.log 2>&1 & PIDTMP=$!;
fi
if [[ "$PIDTMP" != "" ]]; then
if [[ -f tmplog.log ]]; then
thisip="$(cat -v tmplog.log 2> /dev/null | tr -d '\0' | grep -aiv "failed" | grep -ioE -m2 "\\[([0-9]{1,3}\.){3}[0-9]{1,3}\\]" | tail -1 | sed 's/^.\(.*\).$/\1/')" 2> /dev/null
#uncomment if u want output to screen
#if [[ "$thisip" != "" ]]; then cat tmplog.log 2> /dev/null; fi;
fi
if [[ "$thisip" != "" ]]; then
kill $PIDTMP 2>/dev/null
wait $PIDTMP 2>/dev/null; unset PIDTMP;
if [[ "$(grep -rnw log.log -e "$thisip" 2> /dev/null)" == "" ]]; then
echo "$thisip" >> log.log
fi
unset thisip
fi
fi
sleep 2
done

Answer 1

Try this:
nc -vv -l -p <portnum> >>/tmp/nc.log 2>&1 & sleep <timeout>;kill -9 $!
If you want to get the only connection ip, could run this grep -oP "(?<=Connection from \[)[\w\.]*(?=])" /tmp/nc.log, the one line is:
nc -vv -l -p <portnum> >>/tmp/nc.log 2>&1 & sleep <timeout>;kill -9 $!;grep -oP "(?<=Connection from \[)[\w\.]*(?=])" /tmp/nc.log
First collect the nc log to nc.log and force kill the nc progress until the time out, then get the connection ip by grep.