Bash snippet to see if something is listening on a port?

Question

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?

Solution 1

lsof -i :14881

Solution 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.

Solution 3

If you are root:

netstat -lnp | grep ':14881 '

Solution 4

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..

View more solutions

Comments

[Admin]

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?

[Admin]

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"

[Dan Carley]

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]

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.

[msanford]

This one provides lots of great data.

[Kyle Brandt]

On my desktop system this one requires root.

[Justin]

lsof -i :14881 | grep ":14881" then you can use echo $?