Troubleshooting · Lesson 4 of 26 · 8 minutes

Config diff triage

Compare configs before guessing at a routing issue.

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Your objectives

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1
Compare configs
Run diff data/config-a.conf data/config-b.conf.
Hint & worked example
Compare two text files; a difference is a meaningful nonzero status, not necessarily an execution error.
Inspect the differences. diff exits 1 when files differ and 0 when they match.
```
diff data/config-a.conf data/config-b.conf
```
Remember: Status 1 means differences for diff; shell status must be interpreted in command context.
2
Search listen
Run grep listen data/config-b.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-b.conf
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
3
Validate 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.

diff: Compare two text files; a difference is a meaningful nonzero status, not necessarily an execution error.
Status 1 means differences for diff; shell status must be interpreted in command context.
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

$ diff data/config-a.conf data/config-b.conf

--- /home/learner/data/config-a.conf
+++ /home/learner/data/config-b.conf
-listen 443 ssl;
+listen 8443 ssl;

Exit status: 1. This nonzero result is an expected part of this exercise.

$ grep listen data/config-b.conf

listen 8443 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 “Config diff triage”, which interpretation is safe?A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.No matches is exit status 1, which is different from a malformed invocation.

Apply it elsewhere

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

Keep the momentum.

Complete the lab, then explain and apply the pattern.

Next: Disk pressure review Try an incident drill

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