10. Manage Security

📘Red Hat Certified System Administrator (RHCSA – EX200)

1. What Are Default File Permissions?

Default file permissions define:

What permissions are assigned automatically when:
- A new file is created
- A new directory is created

In Linux systems (including Red Hat), permissions are not random. They are controlled by:

System defaults
A special setting called umask

2. Basic Permission Types

Every file and directory has three types of permissions:

Permission	Symbol	Meaning
Read	r	View content
Write	w	Modify content
Execute	x	Run file / enter directory

3. Permission Ownership Categories

Permissions are assigned to three groups:

Category	Meaning
User (u)	Owner of the file
Group (g)	Group members
Others (o)	Everyone else

4. Default Permission Values

When a file or directory is created, the system starts with base permissions:

Default Base Values

Type	Base Permission
Files	666 (rw-rw-rw-)
Directories	777 (rwxrwxrwx)

However, these are not final permissions. They are modified by umask.

5. What Is umask?

umask (User Mask) controls which permissions are removed from the default base.

Key Idea:

umask does not add permissions, it removes permissions

6. How umask Works

Formula:

Final Permission = Default Permission - umask

Example 1: umask = 022

For files:

666 - 022 = 644

Result:

rw-r--r--

For directories:

777 - 022 = 755

Result:

rwxr-xr-x

Example 2: umask = 027

Files:

666 - 027 = 640

Directories:

777 - 027 = 750

7. Why Files Do Not Get Execute Permission by Default

Even though base is 666, files never get execute (x) by default because:

Files are not assumed to be executable programs
Execute permission must be added manually using chmod

8. Viewing Current umask

Use:

umask

Example output:

To see symbolic format:

umask -S

Example:

u=rwx,g=rx,o=rx

9. Setting umask Temporarily

To change umask for the current session:

umask 027

This change:

Applies only to the current shell
Resets after logout

10. Setting umask Permanently

To make umask persistent, configure it in:

For all users:

/etc/profile
/etc/bashrc

For a specific user:

~/.bash_profile
~/.bashrc

Example:

umask 027

11. Default Permissions in an IT Environment

In a system environment:

umask 022
- Used in general systems
- Allows others to read files
umask 027
- Used in secure environments
- Restricts access to group only
umask 077
- Highly restricted
- Only owner has access

12. Special Default Permissions for Directories

Directories behave differently:

Permission	Meaning
r	List contents
w	Create/delete files
x	Enter directory

So correct default permissions are important for:

Shared directories
Application data directories

13. Interaction with chmod

Even after default permissions are set:

You can manually modify permissions using:

chmod 755 file
chmod u+x script.sh

Default permissions only apply at creation time.

14. Important Exam Points

You should be able to:

✔ Understand how default permissions are calculated
✔ Identify base permissions (666, 777)
✔ Calculate final permissions using umask
✔ View current umask
✔ Change umask temporarily and permanently
✔ Understand differences between file and directory permissions
✔ Explain why execute permission is not set by default

15. Common Mistakes (Very Important for Exam)

❌ Thinking umask adds permissions
✔ It removes permissions

❌ Using subtraction incorrectly
✔ Always subtract digit by digit

❌ Expecting files to have execute permission by default
✔ Files never get execute automatically

16. Quick Summary

Files start with 666, directories with 777
umask removes permissions
Common umask values:
- 022 → standard access
- 027 → restricted access
- 077 → private access
Default permissions apply only when a file/directory is created