Troubleshooting · Lesson 3 of 26 · 8 minutes

Log correlation

Search multiple logs for related clues.

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Your objectives

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1
Search auth
Run grep denied /var/log/auth.log.
Hint & worked example
Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
grep denied /var/log/auth.log
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
2
Search SSH journal
Run journalctl -u ssh | grep refused.
Hint & worked example
Read the training journal by unit or a bounded line window. Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
journalctl -u ssh | grep refused
```
Remember: A relevant unit and time range are more useful than an unfiltered wall of logs. No matches is exit status 1, which is different from a malformed invocation.
3
Search nginx warnings
Run grep warn /var/log/nginx-error.log.
Hint & worked example
Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
grep warn /var/log/nginx-error.log
```
Remember: No matches is exit status 1, which is different from a malformed invocation.

Understand the pattern

Junior operators build confidence by gathering evidence before changing systems.

grep: Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection.
No matches is exit status 1, which is different from a malformed invocation.
journalctl: Read the training journal by unit or a bounded line window.
A relevant unit and time range are more useful than an unfiltered wall of logs.

Worked example and expected result

$ grep denied /var/log/auth.log

May 26 08:18:33 clairos-learn sshd[118]: denied password for root
May 26 08:21:44 clairos-learn sshd[141]: denied password for admin

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

$ journalctl -u ssh | grep refused

May 26 08:18:33 clairos-learn sshd[118]: password authentication refused
May 26 08:21:44 clairos-learn sshd[141]: root login refused by policy

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

$ grep warn /var/log/nginx-error.log

May 26 08:20:05 [warn] nginx: /api/test returned 404 by design

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 “Log correlation”, which interpretation is safe?A successful first check guarantees that every dependent service is healthy.A relevant unit and time range are more useful than an unfiltered wall of logs.A command appearing in history proves it completed successfully.

Apply it elsewhere

Before adapting the journalctl workflow from “Log correlation” 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: Config diff triage Try an incident drill

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