Text and logs · Lesson 35 of 38 · 8 minutes

Config substitution preview

Preview a port substitution and compare it with the unchanged source.

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 a port change
Run sed s/443/8443/ data/config-a.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/ data/config-a.conf
```
Remember: Without a global flag, a substitution normally changes only the first match on each line.
2
Confirm the source remains unchanged
Run grep listen data/config-a.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 data/config-a.conf
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
3
Number the source config
Run nl data/config-a.conf.
Hint & worked example
Number input lines for a readable evidence reference.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
nl data/config-a.conf
```
Remember: Line numbers depend on the exact file version and numbering mode.

Understand the pattern

Previewing a text change before editing a config helps operators explain what they know before they change anything.

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.
nl: Number input lines for a readable evidence reference.
Line numbers depend on the exact file version and numbering mode.

Worked example and expected result

$ sed s/443/8443/ data/config-a.conf

server_name learn.clairos.ai;
listen 8443 ssl;
root /srv/www;

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

$ grep listen data/config-a.conf

listen 443 ssl;

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

$ nl data/config-a.conf

     1	server_name learn.clairos.ai;
     2	listen 443 ssl;
     3	root /srv/www;

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

Apply it elsewhere

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

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: Rotated access comparison 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