Permissions and users · Lesson 23 of 33 · 8 minutes

Service user ownership

Compare service users with files they serve.

Open the lab Course map

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Your objectives

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1
Find service user
Run grep www-data /etc/passwd.
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 www-data /etc/passwd
```
Remember: No matches is exit status 1, which is different from a malformed invocation.
2
Inspect web file
Run stat /srv/www/index.html.
Hint & worked example
Inspect a file or directory metadata record, including ownership, permissions, and size.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
stat /srv/www/index.html
```
Remember: Metadata and file contents answer different questions.
3
Find service group
Run grep www-data /etc/group.
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 www-data /etc/group
```
Remember: No matches is exit status 1, which is different from a malformed invocation.

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.
stat: Inspect a file or directory metadata record, including ownership, permissions, and size.
Metadata and file contents answer different questions.

Worked example and expected result

$ grep www-data /etc/passwd

www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin

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

$ stat /srv/www/index.html

Path: /srv/www/index.html
Type: file
Mode: -rw-r--r--
Owner: www-data
Group: www-data
Size: 72 bytes

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

$ grep www-data /etc/group

www-data:x:33:

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 “Service user ownership”, which interpretation is safe?A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.Metadata and file contents answer different questions.

Apply it elsewhere

Before adapting the stat workflow from “Service user ownership” 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: Shared directory risk Try an incident drill

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