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Start with ss to identify the socket, use lsof to cross-check it or retrieve its PID, then use ps to inspect the process. For example, to check for a listening TCP socket on port 8080, run sudo ss -ltnp 'sport = :8080'. A result identifies a process to investigate—not one that is automatically safe to stop.
Find a listening TCP port with ss
On Linux, ss gives you a socket-oriented view: it can show the local address, port, connection state and, when visible, the process associated with a socket. To look for a listener on TCP port 8080, run:
sudo ss -ltnp 'sport = :8080'-lshows listening sockets.-tselects TCP sockets.-nkeeps addresses and port numbers numeric.-prequests process information.'sport = :8080'filters for local source port 8080.
sudo may be needed to see process details for sockets owned by other users. Use it only if you are authorized to inspect the system; it is not a universal requirement. Filter syntax and available options can vary with the installed ss version, so check man ss if the command is rejected or behaves differently.
For UDP, query UDP sockets rather than assuming a TCP check covers them. For example, to show UDP sockets bound locally to port 8080, use sudo ss -lunp 'sport = :8080'. UDP does not have TCP’s listening state, so do not add -l expecting the same state semantics as a TCP listener.
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Cross-check the port or retrieve its PID with lsof
lsof takes an open-file-oriented view; network sockets are among the open files it can report. To list processes associated with Internet sockets using port 8080, run:
sudo lsof -nP -i :8080-i :8080 selects Internet files matching that port, while -nP avoids converting addresses and port numbers to names. For a terse list of process IDs instead of the readable listing, use:
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sudo lsof -t -i :8080lsof -i :PORT can include sockets in states other than listening. If you need to distinguish a listener from another connection, check the state and local address in the output; do not treat every match as a server accepting new connections. The lsof manual describes an open file as potentially including “a network file (Internet socket, NFS file or UNIX domain socket.)”
Inspect the process with ps
Once you have a PID, use ps to see who owns the process and the command arguments visible to your user. Replace 1234 with the PID reported by ss or lsof:
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ps -p 1234 -o pid,user,argsps provides process context; it does not independently map a port to its owner. The procps-ng ps manual documents its process-display options and implementation details. Check man ps on the machine you are using for local option behavior.
Interpret the result before taking action
Check the protocol, socket state, local address, PID and command together. A port number alone does not tell you whether a service is reachable from outside the host.
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- A service bound to
127.0.0.1or::1listens on loopback, rather than on all network interfaces. - A wildcard address such as
0.0.0.0or::indicates a bind across local interfaces for that address family; actual reachability can still depend on firewall rules, routing and other system configuration. - A process listed for a port may own an established or other non-listening socket. Confirm the state before concluding that it is accepting new connections.
Do not terminate a process solely because it appears in a port listing. First establish what application or service it is, whether it is expected, and what depends on it. If it needs to be stopped, use the service manager or the application’s normal shutdown procedure where possible.
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- Protocol: Run a TCP query for a TCP service and a UDP query for a UDP socket; one does not substitute for the other.
- State: A listening-only query omits established or other non-listening sockets. Use a query suited to the state you are investigating.
- Filter: Confirm the port filter and the installed command’s syntax. Compare with
sudo lsof -nP -i :8080if appropriate. - Permissions: Your account may be able to see a socket but not its owning process details. Authorized use of
sudomay reveal more. - Container or network namespace: A command run on the host may not show the same sockets as one run inside the relevant container or namespace. Inspect from the context where the service runs.
- Timing: Processes and connections can start or stop between commands, so repeated checks can differ.
The lsof manual documents access warnings and errors. Treat missing output as a clue to investigate rather than proof that the port is unused.
How ss, lsof, and ps fit together
| Command | Best use in this workflow | What it contributes |
|---|---|---|
ss |
Start with the socket | Socket address, port, state and, when visible, process information; supports protocol, state and other filters. |
lsof |
Cross-check sockets or list matching PIDs | Open-file view that includes Internet sockets; -t emits terse process IDs. |
ps |
Investigate a PID already found | Selected process details such as PID, user and command arguments; does not identify socket ownership by itself. |
These are Linux examples, not a guarantee that every distribution ships identical command versions or options. Check the installed manuals—man ss, man lsof and man ps—when local behavior differs.
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