Text and logs · Lesson 20 of 38 · 8 minutes

Capture with tee

Capture pipeline output to a lab note.

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Your objectives

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1
Capture denied lines
Run grep denied /var/log/auth.log | tee lab/denied-review.txt.
Hint & worked example
Select input lines that match a pattern; -i ignores case, -n adds line numbers, and -v inverts selection. Copy a stream both to a file and standard output; -a appends.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
grep denied /var/log/auth.log | tee lab/denied-review.txt
```
Remember: No matches is exit status 1, which is different from a malformed invocation. tee can overwrite a file even though the output is also visible on screen.
2
Read capture
Run cat lab/denied-review.txt.
Hint & worked example
Read one or more files to standard output without editing their contents.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
cat lab/denied-review.txt
```
Remember: Reading a source file should precede changing it; avoid printing real secrets.
3
Count capture
Run wc -l lab/denied-review.txt.
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 lab/denied-review.txt
```
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.

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.
tee: Copy a stream both to a file and standard output; -a appends.
tee can overwrite a file even though the output is also visible on screen.
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.
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

$ grep denied /var/log/auth.log | tee lab/denied-review.txt

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.

$ cat lab/denied-review.txt

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 lab/denied-review.txt

2 lab/denied-review.txt

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 “Capture with tee”, which interpretation is safe?Reading a source file should precede changing it; avoid printing real secrets.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 cat workflow from “Capture with tee” 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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