2

I want to see if something is listening on a port on localhost. I was going to use nc and check the exit code.

Something like this:

echo "" | nc localhost 14881
echo $?

Any other suggestions?

  • there are many reasons why you'd want to do this, but I'm curious as to your reason here... it may be possible you can avoid the port-check altogether. Do you have a 'slow start' scenario? where the application daemonizes but takes another minute or two before it actually opens up a listener? or are you just trying to avoid a lengthy timeout situation? or are you unable to handle the case where you get connection refused? – ericslaw Jul 6 '09 at 14:12
  • with nc correct way will be nc -z "$host" "$port", in bash though the proper way would be echo '123' > /dev/tcp/localhost/port" and if there is nothing listens it will says: "bash: connect: Connection refused" – Danila Ladner Mar 10 '17 at 15:37
7

lsof -i :14881

  • This one provides lots of great data. – msanford Jul 6 '09 at 14:26
  • On my desktop system this one requires root. – Kyle Brandt Jul 6 '09 at 14:58
  • lsof -i :14881 | grep ":14881" then you can use echo $? – Justin Sep 27 '16 at 23:14
2

If you are root:

netstat -lnp | grep ':14881 '
  • Just to be clear: it will still work if you aren't root but you won't benefit from seeing the process name bound to it. – Dan Carley Jul 6 '09 at 13:47
2

Maybe netstat would be better because the port might not be listening on localhost or it might be blocked by iptables:

netstat -ln  | grep :14881
echo $?

Grep will exit with 1 if there is no match. If you want just tcp and/or udp , add the -u or -t switches to netstat.

0

netstat -ano | egrep LISTEN | egrep tcp | egrep $PORTNUMBER

  • 3
    Woah, lay off the pipes. You can replace the first two egreps with -lt instead of -ao and a normal grep on the port. Or, if you wished, perform everything as a single egrep. – Dan Carley Jul 6 '09 at 13:52
0

sudo ss -lntup will tell you what is listening for TCP/UDP connections - you can filter the output as might be suitable (e.g. listening on 127.0.0.1, ::1, 0.0.0.0).

If you have python around, you can also create a dumb portscanner (only really worthwhile for TCP, though):

import socket

for p in range(1,2**16):
   try:
     s = socket.create_connection(('127.0.0.1',p))
     print "Listener on tcp/{}".format(p)
   except:
     continue
0

I use this in bash for exiting when no-one listening to this port.

$port="14881"
if [[ $(netstat -ltn | grep ":${port} " | wc -l) -eq "0" ]] ; then echo "Port $port not listened to" && exit 1; fi
  • Using bash double brackets and comparing against wc for easy reading..
  • Passing the "-c" argument to grep obviates the need for "wc -l" – parkamark Mar 10 '17 at 17:53

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