Text and logs · Lesson 3 of 38 · 9 minutes

Log triage

Inspect access, error, auth, and service logs in a cautious order.

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Your objectives

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1
Tail web access
Run tail -n 2 /var/log/nginx-access.log.
Hint & worked example
Show the end of an input; -n selects the number of lines.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
tail -n 2 /var/log/nginx-access.log
```
Remember: This browser lab snapshots output; it does not keep a live tail -f stream.
2
Read service journal
Run journalctl -u nginx.
Hint & worked example
Read the training journal by unit or a bounded line window.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
journalctl -u nginx
```
Remember: A relevant unit and time range are more useful than an unfiltered wall of logs.
3
Search auth denials
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.

Understand the pattern

Incident triage is evidence gathering before action.

tail: Show the end of an input; -n selects the number of lines.
This browser lab snapshots output; it does not keep a live tail -f stream.
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.
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.

Worked example and expected result

$ tail -n 2 /var/log/nginx-access.log

127.0.0.1 GET /api/test 404
127.0.0.1 POST / 405

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

$ journalctl -u nginx

May 26 08:10:00 clairos-learn nginx[42]: configuration file syntax is ok
May 26 08:10:01 clairos-learn nginx[42]: started static training site
May 26 08:20:05 clairos-learn nginx[42]: access denied for /api/test by static rule

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

$ 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.

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 triage”, which interpretation is safe?A command appearing in history proves it completed successfully.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.

Apply it elsewhere

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

Keep the momentum.

Complete the lab, then explain and apply the pattern.

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