You can easily determine if a port is in use with a few simple Linux commands.
Whenever you're trouble shooting a network or an application running on a network, your first step can be to see what ports are open. For example you may have an apache server running a web service and that web service isn't responding. The first thing you might do is to check if the apache is running or simply check to see if anything is listening on the port. Since it's a web service you might think it's on 80 but it doesn't have to be. Instead it might be on 8080, 5555 or some other port.
Everything in Unix/Linux is considered to be a file or folder, which means you can use the lsof or List Of Open File command to inspect for applications with open ports.
$ sudo lsof -i -n
If nothing returns be sure to check that you used sudo. If you leave sudo off, you won't get an error but there's a good chance that nothing will come back either
According to the man pages for lsof, the -i, essentially filters on any internet
-i selects the listing of files any of whose Internet address matches the address specified in i. If no address is specified, this option selects the listing of all Internet and x.25 (HP-UX) network files.
-n inhibits any conversion to a hostnames. So leave that off if you are looking for host name information
-n inhibits the conversion of network numbers to host names for network files. Inhibiting conversion may make lsof run faster. It is also useful when host name lookup is not working properly.
Depending on what's running on your system you may get a lot of information back. If you're only concerned with what ports are listening for connections, the you can use grep to filter it even further.
sh-4.2$ sudo lsof -i | grep LISTEN
rpcbind 1714 rpc 8u IPv4 14917 0t0 TCP *:sunrpc (LISTEN)
rpcbind 1714 rpc 11u IPv6 14920 0t0 TCP *:sunrpc (LISTEN)
master 2168 root 13u IPv4 17098 0t0 TCP localhost:smtp (LISTEN)
sshd 3106 root 3u IPv4 27586 0t0 TCP *:ssh (LISTEN)
sshd 3106 root 4u IPv6 27588 0t0 TCP *:ssh (LISTEN)
container 7355 root 14u IPv4 34814 0t0 TCP localhost:43529 (LISTEN)
docker-pr 8755 root 4u IPv4 40667 0t0 TCP *:commplex-main (LISTEN)
docker-pr 8849 root 4u IPv4 40766 0t0 TCP *:http (LISTEN)
In the example above you can see that I have TCP: *:http listening from the docker-pr, which is the docker proxy hosting my application. There are also other services running like ssh listening for connections, rpc connections, ect.
The netstat command displays active TCP connections, ports the host is listening on, ethernet stats, the IP routing table, IPv4 stats (for IP, ICMP, TCP and UPD) as well as IPv6 stats for (IPv6, ICMPv6, TCP over IPv6 and UPD over IPv6). If you omit any of the parameters it typically displays active TCP connections
sudo netstat -anp | grep tcp | grep LISTEN
Using the command above we supplied the -a for all, chained with n for numeric (like n argument for lsof it won't resolve names) and p to display the PID/program name for the sockets.
Running the netstat command I have output like the following
sh-4.2$ sudo netstat -anp | grep tcp | grep LISTEN
tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN 1714/rpcbind
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN 8849/docker-proxy
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 3106/sshd
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 2168/master
tcp 0 0 0.0.0.0:5000 0.0.0.0:* LISTEN 8755/docker-proxy
tcp 0 0 127.0.0.1:43529 0.0.0.0:* LISTEN 7355/containerd
tcp6 0 0 :::111 :::* LISTEN 1714/rpcbind
tcp6 0 0 :::22 :::* LISTEN 3106/sshd
If you forget the sudo, you should still get some information back but you'll also get a warning that it couldn't get the PID/ program information. See how the following example is missing all the information at the end.
sh-4.2$ netstat -anp | grep tcp | grep LISTEN
(No info could be read for "-p": geteuid()=1001 but you should be root.)
tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN -
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN -
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN -
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN -
tcp 0 0 0.0.0.0:5000 0.0.0.0:* LISTEN -
tcp 0 0 127.0.0.1:43529 0.0.0.0:* LISTEN -
tcp6 0 0 :::111 :::* LISTEN -
tcp6 0 0 :::22 :::* LISTEN -
There actually several other commands out there like nmap, but not all of them come installed by default, so you'll need to install them in order to use them. But you can pretty much guarantee that lsof and netstat will be available.