Text and logs · Lesson 34 of 38 · 8 minutes

Three-stage evidence pipeline

Search, number, and limit log evidence in one bounded pipeline.

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Your objectives

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1
Find denied events
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
Number denied events
Run grep denied /var/log/auth.log | nl.
Hint & worked example
Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection. Number input lines for a readable evidence reference.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
grep denied /var/log/auth.log | nl
```
Remember: No matches is exit status 1, which is different from a malformed invocation. Line numbers depend on the exact file version and numbering mode.
3
Limit the evidence view
Run grep denied /var/log/auth.log | nl | head -n 3.
Hint & worked example
Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection. Number input lines for a readable evidence reference. Show the beginning of an input; -n sets how many lines to include.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
grep denied /var/log/auth.log | nl | head -n 3
```
Remember: No matches is exit status 1, which is different from a malformed invocation. Line numbers depend on the exact file version and numbering mode. A small sample may omit the failure. Choose the time window you actually need.

Understand the pattern

Building a readable multi-stage text pipeline helps operators explain what they know before they change anything.

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.
nl: Number input lines for a readable evidence reference.
Line numbers depend on the exact file version and numbering mode.
head: Show the beginning of an input; -n sets how many lines to include.
A small sample may omit the failure. Choose the time window you actually need.

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.

$ grep denied /var/log/auth.log | nl

     1	May 26 08:18:33 clairos-learn sshd[118]: denied password for root
     2	May 26 08:21:44 clairos-learn sshd[141]: denied password for admin
     3

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

$ grep denied /var/log/auth.log | nl | head -n 3

     1	May 26 08:18:33 clairos-learn sshd[118]: denied password for root
     2	May 26 08:21:44 clairos-learn sshd[141]: denied password for admin
     3

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

Apply it elsewhere

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

Keep the momentum.

Complete the lab, then explain and apply the pattern.

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