Shell scripting · Lesson 6 of 31 · 8 minutes

Sed config preview

Preview config text replacement as a script step.

Open the lab Course map

Mastery 0/3Run itExplain itApply it

learner@clairos:~ $

↑ history · Tab complete · Ctrl+C cancel · Esc, Tab to leaveExit 0

Your objectives

Hints are always available. Wrong turns never cost XP.

1
Preview port replacement
Run sed s/443/8443/ /etc/nginx/sites-available/learn.conf.
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/443/8443/ /etc/nginx/sites-available/learn.conf
```
Remember: Without a global flag, a substitution normally changes only the first match on each line.
2
Verify original listen
Run grep listen /etc/nginx/sites-available/learn.conf.
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 listen /etc/nginx/sites-available/learn.conf
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
3
Validate current config
Run nginx -t.
Hint & worked example
Check the simulated configuration using nginx -t before changing service state.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
nginx -t
```
Remember: This lab checks a limited configuration subset, not the complete Nginx parser.

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.
nginx: Check the simulated configuration using nginx -t before changing service state.
This lab checks a limited configuration subset, not the complete Nginx parser.

Worked example and expected result

$ sed s/443/8443/ /etc/nginx/sites-available/learn.conf

server {
  listen 8443 ssl;
  server_name learn.clairos.ai;
  root /srv/www;
  location ^~ /api/learn/ { proxy_pass http://127.0.0.1:8010; }
  location ^~ /api/ { return 404; }
  add_header Content-Security-Policy "default-src 'self'; connect-src 'self'" always;
  # training config only
}

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

$ grep listen /etc/nginx/sites-available/learn.conf

  listen 443 ssl;

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

$ nginx -t

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

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 config preview”, 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 “Sed config preview” 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: Awk reports Try an incident drill

Anonymous lesson and mastery progress lasts for this session. Sign in to keep synced lesson XP between sessions; mastery stays device-local. No speed bonus, lost lives, or hint penalties.

Keep learning

GNU Coreutils manual · GNU Bash manual