Text and logs · Lesson 16 of 38 · 8 minutes

Sed substitution

Preview text replacement without editing a log.

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learner@clairos:~ $

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Your objectives

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1
Preview replacement
Run sed s/denied/blocked/ /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 s/denied/blocked/ /var/log/auth.log
```
Remember: Without a global flag, a substitution normally changes only the first match on each line.
2
Verify original log
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.
3
Count log lines
Run wc -l /var/log/auth.log.
Hint & worked example
Count input. -l counts newline characters, -w counts whitespace-delimited words, and -c counts bytes.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
wc -l /var/log/auth.log
```
Remember: Text without a final newline may have fewer counted lines than visible rows.

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.
wc: Count input. -l counts newline characters, -w counts whitespace-delimited words, and -c counts bytes.
Text without a final newline may have fewer counted lines than visible rows.

Worked example and expected result

$ sed s/denied/blocked/ /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]: blocked password for root
May 26 08:19:10 clairos-learn sudo[126]: learner : command=list services
May 26 08:21:44 clairos-learn sshd[141]: blocked password for admin

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.

$ wc -l /var/log/auth.log

3 /var/log/auth.log

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 substitution”, which interpretation is safe?A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.No matches is exit status 1, which is different from a malformed invocation.

Apply it elsewhere

Before adapting the grep workflow from “Sed substitution” 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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