Server operations · Lesson 18 of 36 · 8 minutes

Open files and listeners

Use lsof to relate sockets to services.

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Your objectives

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1
Show open sockets
Run lsof -i.
Hint & worked example
Inspect a synthetic association between processes and open files or sockets.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
lsof -i
```
Remember: Combine process and socket evidence rather than assuming a service from its port number.
2
Show listeners
Run ss -tulpn.
Hint & worked example
Inspect simulated listening sockets and connection information.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
ss -tulpn
```
Remember: A listening socket is not proof of correct application responses or external reachability.
3
Find sshd pid
Run pgrep sshd.
Hint & worked example
Find simulated process IDs by a name pattern.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
pgrep sshd
```
Remember: Verify a process identity before sending a signal.

Understand the pattern

Junior operators build confidence by gathering evidence before changing systems.

lsof: Inspect a synthetic association between processes and open files or sockets.
Combine process and socket evidence rather than assuming a service from its port number.
ss: Inspect simulated listening sockets and connection information.
A listening socket is not proof of correct application responses or external reachability.
pgrep: Find simulated process IDs by a name pattern.
Verify a process identity before sending a signal.

Worked example and expected result

$ lsof -i

COMMAND PID USER   FD  TYPE NAME
nginx    42 www    6u  IPv4 *:80 (LISTEN)
nginx    42 www    7u  IPv4 *:443 (LISTEN)
sshd     77 root   3u  IPv4 *:22 (LISTEN)

Exit status: 0. The command completed successfully in the training fixture.

$ ss -tulpn

Netid State  Local Address:Port  Process
udp   UNCONN 0.0.0.0:68          lab-dhcp
tcp   LISTEN 0.0.0.0:22          sshd
tcp   LISTEN 0.0.0.0:80          nginx
tcp   LISTEN 0.0.0.0:443         nginx

Exit status: 0. The command completed successfully in the training fixture.

$ pgrep sshd

Exit status: 0. The command completed successfully in the training fixture.

Try a variation

Repeat this workflow in the sandbox with a different scratch filename. Keep the system fixtures unchanged and compare the observed output with your expectation before moving on.

Explain it

In “Open files and listeners”, which interpretation is safe?A successful first check guarantees that every dependent service is healthy.A listening socket is not proof of correct application responses or external reachability.A command appearing in history proves it completed successfully.

Apply it elsewhere

Before adapting the ss workflow from “Open files and listeners” to another server, what should you do?Confirm the current directory, target identity, permissions, and the installed command behavior first.Paste the training paths unchanged into production.Skip output inspection whenever the command is familiar.

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: Mount and block devices Try an incident drill

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