Permissions and users · Lesson 20 of 33 · 8 minutes

Permissions checkpoint

Review modes, groups, ownership, and sensitive paths.

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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
Set key mode
Run chmod 600 ~/.ssh/authorized_keys.
Hint & worked example
Change permissions using an octal mode or a symbolic adjustment. Each rwx bit belongs to owner, group, or other.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
chmod 600 ~/.ssh/authorized_keys
```
Remember: Do not use 777 as a blanket fix. Grant the smallest access needed.
2
Set key group
Run chgrp learners ~/.ssh/authorized_keys.
Hint & worked example
Change the file group within the permissions of the acting user.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
chgrp learners ~/.ssh/authorized_keys
```
Remember: A group name alone does not grant access unless membership and mode bits agree.
3
Verify key file
Run test -f ~/.ssh/authorized_keys.
Hint & worked example
Evaluate a file or string condition through its exit status rather than printed output.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
test -f ~/.ssh/authorized_keys
```
Remember: A successful test usually prints nothing; inspect $? or chain a command.
4
Inspect final key state
Run stat ~/.ssh/authorized_keys.
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 ~/.ssh/authorized_keys
```
Remember: Metadata and file contents answer different questions.

Understand the pattern

Junior operators build confidence by gathering evidence before changing systems.

chmod: Change permissions using an octal mode or a symbolic adjustment. Each rwx bit belongs to owner, group, or other.
Do not use 777 as a blanket fix. Grant the smallest access needed.
chgrp: Change the file group within the permissions of the acting user.
A group name alone does not grant access unless membership and mode bits agree.
test: Evaluate a file or string condition through its exit status rather than printed output.
A successful test usually prints nothing; inspect $? or chain a command.
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

$ chmod 600 ~/.ssh/authorized_keys

(no output; exit 0)

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

$ chgrp learners ~/.ssh/authorized_keys

group learners set on /home/learner/.ssh/authorized_keys

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

$ test -f ~/.ssh/authorized_keys

(no output; exit 0)

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

$ stat ~/.ssh/authorized_keys

Path: /home/learner/.ssh/authorized_keys
Type: file
Mode: -rw-------
Owner: learner
Group: learners
Size: 40 bytes

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 “Permissions checkpoint”, which interpretation is safe?A successful first check guarantees that every dependent service is healthy.A group name alone does not grant access unless membership and mode bits agree.A command appearing in history proves it completed successfully.

Apply it elsewhere

Before adapting the chgrp workflow from “Permissions checkpoint” 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: SSH directory audit Try an incident drill

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