5.4 Special Directories and Files (Weight: 1)

📘Linux Essentials (LPI 010-160)

1. File Ownership in Linux

Every file and directory in Linux has an owner and a group.

Owner

The owner is usually the user who created the file.

Example:

-rw-r--r-- 1 student staff 1200 Mar 10 notes.txt

Here:

student → file owner
staff → group owner

Group

A group is a collection of users. Files can be assigned to a group so that multiple users can share access.

For example:

Developers group may access application code
Administrators group may access system configuration files

2. File Permission Types

Linux permissions define what actions are allowed on a file or directory.

There are three basic permission types.

Permission	Symbol	Meaning
Read	r	View file content
Write	w	Modify file content
Execute	x	Run file as a program

3. Permission Categories

Permissions are assigned to three different user categories.

Category	Symbol	Description
Owner	u	User who owns the file
Group	g	Users in the file’s group
Others	o	All other users

Example permission output:

-rwxr-xr--

Breakdown:

Section	Meaning
rwx	owner permissions
r-x	group permissions
r–	others permissions

4. Viewing File Permissions

To view file permissions, use:

ls -l

Example output:

-rw-r--r-- 1 admin developers 845 Mar 12 script.sh

Explanation:

Field	Meaning
–	file type
rw-r–r–	permissions
admin	owner
developers	group

5. Changing File Ownership

Ownership can be changed using the chown command.

Basic syntax

chown owner file

Example:

sudo chown admin report.txt

This changes the owner of report.txt to admin.

Changing owner and group together

chown owner:group file

Example:

sudo chown admin:developers project.txt

Now:

owner = admin
group = developers

Changing only the group

chown :group file

Example:

sudo chown :developers project.txt

6. Changing Group Ownership

Group ownership can also be changed using the chgrp command.

Syntax

chgrp group file

Example:

chgrp developers project.txt

This assigns the file to the developers group.

7. Changing Permissions

Permissions are changed using the chmod command.

chmod permissions file

There are two methods:

Symbolic method
Numeric method

8. Symbolic Method

The symbolic method uses letters to modify permissions.

Structure

chmod [user][operation][permission] file

Users:

Symbol	Meaning
u	owner
g	group
o	others
a	all users

Operations:

Symbol	Meaning
+	add permission
–	remove permission
=	set exact permission

Permissions:

Symbol	Meaning
r	read
w	write
x	execute

Examples

Add execute permission for owner:

chmod u+x script.sh

Remove write permission from group:

chmod g-w file.txt

Add read permission for everyone:

chmod a+r document.txt

Set owner permissions exactly:

chmod u=rwx file.sh

9. Numeric (Octal) Method

Linux also supports a numeric method for permissions.

Each permission has a number:

Permission	Value
Read	4
Write	2
Execute	1

The values are added together.

Example calculations:

Permission	Calculation	Value
rwx	4+2+1	7
rw-	4+2	6
r–	4	4
—	0	0

Example

chmod 755 script.sh

Meaning:

Category	Value	Permission
Owner	7	rwx
Group	5	r-x
Others	5	r-x

Result:

-rwxr-xr-x

Another example

chmod 644 file.txt

Permissions:

Category	Value	Permission
Owner	6	rw-
Group	4	r–
Others	4	r–

Result:

-rw-r--r--

10. Changing Permissions Recursively

When working with directories containing many files, permissions can be changed recursively.

chmod -R permissions directory

Example:

chmod -R 755 webapp/

This changes permissions for:

directory
all files inside it

11. Special Permissions

Linux supports special permission bits that provide advanced access control.

The three special permissions are:

Permission	Purpose
setuid	Run program with owner’s privileges
setgid	Run program with group privileges
sticky bit	Restrict file deletion in shared directories

12. SetUID (Set User ID)

SetUID allows a program to run with the permissions of the file owner.

Example command:

chmod u+s program

Example permission display:

-rwsr-xr-x

The s indicates SetUID.

Example IT usage

Some administrative utilities require elevated privileges to access system information. SetUID allows a standard user to run the program while the program temporarily uses the owner’s permissions.

13. SetGID (Set Group ID)

SetGID allows programs to run with the group’s permissions.

Command:

chmod g+s directory

Example display:

drwxr-sr-x

Example IT usage

In a shared project directory:

files created automatically inherit the same group
useful for development teams sharing project files

14. Sticky Bit

The sticky bit is used on shared directories.

Command:

chmod +t directory

Example display:

drwxrwxrwt

What it does

Users can only delete:

files they own
files owned by root

Example IT usage

Used in shared temporary directories to prevent users from deleting other users’ files.

A well-known example directory is:

/tmp

15. File Types in Permission Display

The first character in ls -l shows the file type.

Symbol	Type
–	regular file
d	directory
l	symbolic link
c	character device
b	block device

Example:

drwxr-xr-x

The d indicates a directory.

16. Best Practices in IT Environments

In professional Linux environments:

Restrict permissions

Files should only allow necessary access.

Example:

Configuration files:

chmod 600 config.conf

Use groups

Teams working on shared resources should use group permissions.

Avoid giving write access to everyone

Permissions like:

chmod 777

should be avoided because they allow any user to modify files.

17. Commands You Must Know for the Exam

Command	Purpose
ls -l	View permissions
chmod	Change permissions
chown	Change owner
chgrp	Change group
chmod -R	Change permissions recursively

18. Quick Exam Summary

For the Linux Essentials exam, remember:

Every file has owner, group, and others permissions
Permissions include read (r), write (w), execute (x)
ls -l displays permissions
chmod changes permissions
chown changes file ownership
chgrp changes group ownership
Permissions can be set using symbolic or numeric methods
Special permissions include setuid, setgid, and sticky bit