Text and logs · Lesson 7 of 38 · 8 minutes

Pipeline counts

Filter logs through safe text pipelines.

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learner@clairos:~ $

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Your objectives

Hints are always available. Wrong turns never cost XP.

1
Count denied lines
Run cat /var/log/auth.log | grep denied | wc -l.
Hint & worked example
Read one or more files to standard output without editing their contents. Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection. 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.
```
cat /var/log/auth.log | grep denied | wc -l
```
Remember: Reading a source file should precede changing it; avoid printing real secrets. No matches is exit status 1, which is different from a malformed invocation. Text without a final newline may have fewer counted lines than visible rows.
2
Filter 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
Sort ports
Run cat /home/learner/data/ports.txt | sort | uniq.
Hint & worked example
Read one or more files to standard output without editing their contents. Order lines before comparing or grouping them; -n compares numerically and -r reverses order. Collapse adjacent duplicate lines; use sort first when duplicates are not already together.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
cat /home/learner/data/ports.txt | sort | uniq
```
Remember: Reading a source file should precede changing it; avoid printing real secrets. Alphabetical sorting is not numeric sorting. uniq does not find every duplicate in an unsorted stream.

Understand the pattern

Pipelines are a daily Unix skill for triage and reporting.

cat: Read one or more files to standard output without editing their contents.
Reading a source file should precede changing it; avoid printing real secrets.
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.
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.
sort: Order lines before comparing or grouping them; -n compares numerically and -r reverses order.
Alphabetical sorting is not numeric sorting.
uniq: Collapse adjacent duplicate lines; use sort first when duplicates are not already together.
uniq does not find every duplicate in an unsorted stream.

Worked example and expected result

$ cat /var/log/auth.log | grep denied | wc -l

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.

$ cat /home/learner/data/ports.txt | sort | uniq

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 “Pipeline counts”, 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 “Pipeline counts” 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: Port text review Try an incident drill

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