Permissions and users · Lesson 4 of 33 · 8 minutes

Numeric modes

Set and inspect numeric file modes.

Open the lab Course map

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learner@clairos:~ $

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Your objectives

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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
Inspect key mode
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.
3
List SSH folder
Run ls -la ~/.ssh.
Hint & worked example
List directory entries; -a includes hidden names and -l adds ownership and permission detail.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
ls -la ~/.ssh
```
Remember: A listing is not proof that you can read or modify every listed file.

Understand the pattern

SSH key files often require restrictive permissions.

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.
stat: Inspect a file or directory metadata record, including ownership, permissions, and size.
Metadata and file contents answer different questions.
ls: List directory entries; -a includes hidden names and -l adds ownership and permission detail.
A listing is not proof that you can read or modify every listed file.

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.

$ 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.

$ ls -la ~/.ssh

--rw------- learner learners    40 authorized_keys
--rw------- learner learners    87 config
--rw------- learner learners    84 known_hosts

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 “Numeric modes”, 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 “Numeric modes” 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: Directory modes Try an incident drill

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