How do I determine if a port is in use, e.g. via netstat?

Question

I am trying to check port availability and get a return value using shell script. Example: if port 8080 is free then return true, else return false. Can anyone help? I tried with netstat.

Solution 1

Assuming you are using netstat from net-tools, this is a working example:

function is_port_free { 

    netstat -ntpl | grep [0-9]:${1:-8080} -q ; 

    if [ $? -eq 1 ]
    then 
        echo yes 
    else 
        echo no
    fi
}

• ${1:-8080} means use first argument as port and 8080 if no first argument
• grep -q [0-9]:port means match a number followed by a colon followed by port
• $? is the exit value of the previous command. Zero means all went well. Exit values above 0 indicate an error condition. In the context of grep, exit code 1 means no match. The -q means don't do anything but return the exit value.
• netstat -ltnp means list numeric ports and IPs for all programs that are listening on a tcp port.
• a | b means process standard output of a with b

eg.

$ is_port_free 8080
yes

$ is_port_free 22
no

Solution 2

lsof is your friend:

# lsof -i:8080      # free on my machine
# echo $?
1
# lsof -i:5353      # occupied
COMMAND   PID           USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
mDNSRespo  64 _mdnsresponder    8u  IPv4 0x9853f646e2fecbb7      0t0  UDP *:mdns
mDNSRespo  64 _mdnsresponder    9u  IPv6 0x9853f646e2fec9cf      0t0  UDP *:mdns
# echo $?
0

So in a script, you could use ! to negate the value to test for availability:

if ! lsof -i:8080
then
    echo 8080 is free
else
    echo 8080 is occupied
fi

Solution 3

How about something simple:

netstat -an|egrep '[0-9]:8080 .+LISTENING'

Comments

[ivant]

And one can also redirect the output of lsof to /dev/null to make it more "scripty".

[wsams]

It may also be useful to grep for LISTEN. Something like if ! lsof -i:8080 | grep LISTEN > /dev/null. This may be useful if you have a server running locally as well as an outgoing connection. For example, you're connected to a remote MySQL server while you also have a local MySQL server running. The outgoing connection would be listed with ESTABLISHED. These would need to be filtered out if you only want to know if there's a local port being used.

[gentooboontoo]

Instead of filtering with grep, one could use lsof -i:8080 -sTCP:LISTEN > /dev/null.