02. Files & Directory Permissions

Overview¶

Linux uses a permission system to control who can read, write, or execute files and directories. Understanding permissions is essential for:

System security
Shell scripting
File access control
Cybersecurity practices

Permission Structure¶

Each file/directory has permissions divided into three sections:

[ User ][ Group ][ Others ]

Displayed using:

ls -l

Example Output¶

-rwxr-xr-- 1 user group 1234 Mar 21 test.sh

Breakdown¶

Section	Meaning
`-`	File type (`-` file, `d` directory)
`rwx`	User permissions
`r-x`	Group permissions
`r--`	Others permissions

Permission Types¶

Symbol	Value	Meaning
`r`	4	Read
`w`	2	Write
`x`	1	Execute

chmod — Change Permissions¶

The chmod command is used to modify file and directory permissions.

Method 1: Symbolic Mode¶

Syntax¶

chmod [user][operator][permissions] filename

User Types¶

Symbol	Meaning
`u`	User (owner)
`g`	Group
`o`	Others
`a`	All

Operators¶

Operator	Meaning
`+`	Add permission
`-`	Remove permission
`=`	Assign exact permission

Examples¶

Assign full permission to user¶

chmod u=rwx test.sh

Explanation
Gives read, write, and execute permission to the owner.

Add execute permission¶

chmod +x test.sh

Explanation
Adds execute permission for all users.

Remove write permission from group¶

chmod g-w test.sh

Set read-only for others¶

chmod o=r test.sh

Method 2: Octal (Numeric) Mode¶

Syntax¶

chmod XYZ filename

Where:

X → User permission
Y → Group permission
Z → Others permission

Permission Values¶

Number	Permission
0	No permission
1	Execute
2	Write
3	Write + Execute
4	Read
5	Read + Execute
6	Read + Write
7	Read + Write + Execute

Examples¶

Read-only for all¶

chmod 444 test.sh

Full access to user, read-only to others¶

chmod 744 test.sh

Breakdown

7 → rwx (user)
4 → r-- (group)
4 → r-- (others)

Common Permission Sets¶

Mode	Meaning
755	Owner full, others read/execute
644	Owner read/write, others read
700	Owner full, no access to others

Directory Permissions¶

Permissions behave slightly differently for directories:

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

Examples¶

Symbolic¶

chmod u=rwx Test/

Numeric¶

chmod 744 Test/

Recursive Permission Change¶

chmod -R 744 Test/

Explanation

-R → applies changes to all files and subdirectories inside

chown — Change Ownership¶

Syntax¶

chown username filename

Example¶

chown subrat test.sh

Explanation
Changes file owner to subrat.

Change Owner and Group Together¶

chown user:group filename

chgrp — Change Group Ownership¶

Syntax¶

chgrp groupname filename

Example¶

chgrp developers test.sh

Explanation
Assigns file to a specific group.

Important Notes¶

Only root or file owner can change permissions
Be cautious with 777 (full access to everyone)
Use least privilege principle in security environments
Always verify using:

ls -l

Summary Table¶

Command	Purpose
`chmod`	Change file permissions
`chmod -R`	Change permissions recursively
`chown`	Change file owner
`chgrp`	Change group ownership

Conclusion¶

Understanding Linux permissions is critical for:

Securing systems
Managing access control
Writing safe scripts
Performing penetration testing

Mastering these concepts ensures proper control over files and prevents unauthorized access.