Text and logs · Lesson 27 of 38 · 8 minutes

Sed window preview

Preview a small text window before searching a large file.

Open the lab Course map

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learner@clairos:~ $

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Your objectives

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1
Preview auth window
Run sed 1,3p /var/log/auth.log.
Hint & worked example
Preview a supported substitution or line range without changing the source file.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
sed 1,3p /var/log/auth.log
```
Remember: Without a global flag, a substitution normally changes only the first match on each line.
2
Preview access window
Run sed 1,3p /var/log/nginx-access.log.
Hint & worked example
Preview a supported substitution or line range without changing the source file.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
sed 1,3p /var/log/nginx-access.log
```
Remember: Without a global flag, a substitution normally changes only the first match on each line.
3
Search POST
Run grep POST /var/log/nginx-access.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 POST /var/log/nginx-access.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.

sed: Preview a supported substitution or line range without changing the source file.
Without a global flag, a substitution normally changes only the first match on each line.
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

$ sed 1,3p /var/log/auth.log

May 26 08:14:02 clairos-learn sshd[104]: accepted publickey for learner
May 26 08:18:33 clairos-learn sshd[118]: denied password for root
May 26 08:19:10 clairos-learn sudo[126]: learner : command=list services

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

$ sed 1,3p /var/log/nginx-access.log

127.0.0.1 GET / 200
127.0.0.1 GET /assets/learn-terminal.js 200
127.0.0.1 GET /api/test 404

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

$ grep POST /var/log/nginx-access.log

127.0.0.1 POST / 405

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 “Sed window preview”, which interpretation is safe?A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.Without a global flag, a substitution normally changes only the first match on each line.

Apply it elsewhere

Before adapting the sed workflow from “Sed window preview” 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.

Next: Awk service summary Try an incident drill

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