Troubleshooting · Lesson 22 of 26 · 8 minutes

Incident rollback evidence

Capture rollback evidence before writing a note.

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learner@clairos:~ $

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Your objectives

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1
Capture warning
Run grep warn /var/log/nginx-error.log | tee lab/rollback-evidence.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 warn /var/log/nginx-error.log | tee lab/rollback-evidence.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
Hash evidence
Run sha256sum lab/rollback-evidence.txt.
Hint & worked example
Compare content digests to detect byte changes, not to establish who authored the file.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
sha256sum lab/rollback-evidence.txt
```
Remember: A checksum detects changes only relative to a trusted expected digest.
3
Read template
Run cat data/incident-template.md.
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 data/incident-template.md
```
Remember: Reading a source file should precede changing it; avoid printing real secrets.

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.
sha256sum: Compare content digests to detect byte changes, not to establish who authored the file.
A checksum detects changes only relative to a trusted expected digest.
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.

Worked example and expected result

$ grep warn /var/log/nginx-error.log | tee lab/rollback-evidence.txt

May 26 08:20:05 [warn] nginx: /api/test returned 404 by design

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

$ sha256sum lab/rollback-evidence.txt

ae6cd90dee9f6887e6d56961b870414e1677df32ff46b38d09251ac605248531  /home/learner/lab/rollback-evidence.txt

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

$ cat data/incident-template.md

summary:
evidence:
impact:
next action:
rollback:

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 “Incident rollback evidence”, which interpretation is safe?A checksum detects changes only relative to a trusted expected digest.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 sha256sum workflow from “Incident rollback evidence” 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.

Next: Incident evidence bundle Try an incident drill

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GNU Coreutils manual · GNU Bash manual