44.Linux_Permission_Types.md

Linux vs Security

The virus files are usually created by others.
Others are not given execute permission on our directories, thus they cannot add virus files.
Hackers are also not granted execute permission on our directories, so they cannot read our file data.
Due to these reasons, Linux is considered a more secure operating system.

Linux Security Levels

Linux follows two levels of security:

1. First Level: Login with credentials
2. Second Level: File and Directory permissions

Permission Types

Linux uses symbolic permissions: r (read), w (write), x (execute).
Alternatively, permissions can be specified using octal numbers, called numeric permissions.

Numeric Permissions

Numeric (Octal) permissions are represented by base-8 numbers (0-7).
The mapping is as follows:

Octal	Binary	Permission
0	000	No Permission
1	001	Execute Permission
2	010	Write Permission
3	011	Write and Execute Permissions
4	100	Read Permission
5	101	Read and Execute Permissions
6	110	Read and Write Permission
7	111	Read, Write, and Execute Permissions

Breaking it Down:

• 4: Read Permission
• 2: Write Permission
• 1: Execute Permission

It’s easier to remember:

• 5 = 4+1 → r-x
• 3 = 2+1 → -wx
• 6 = 4+2 → rw-

Example Command

For the following permissions:

• User: Read and Write (6)
• Group: Write and Execute (3)
• Others: Write (2)

Execution of commands:

1. Initial State:

   ls -l demo.txt
   # -rw-rw-rw-  (Owner, Group, Others have read & write)

2. Changed Permissions to 777 (Full permissions for everyone):

   chmod 777 demo.txt
   ls -l demo.txt
   # -rwxrwxrwx  (Owner, Group, Others have read, write, execute)

3. Changed Permissions to 77 (Owner has no permissions, Group and Others have full permissions):

   chmod 77 demo.txt
   ls -l demo.txt
   # ----rwxrwx  (Owner has no permissions, Group/Others have rwx)

4. Changed Permissions to 7 (Only Others have full permissions):

   chmod 7 demo.txt
   ls -l demo.txt
   # -------rwx  (Owner/Group have no permissions, Others have rwx)

5. Error when Running chmod demo.txt:

   chmod demo.txt
   # Error: Missing operand (permission mode).

Explanation:

• chmod 777: Gives full permissions (read, write, execute) to owner, group, and others.
• chmod 77: Owner has no permissions, group and others have full permissions (rwx).
• chmod 7: Only others have full permissions (rwx), owner and group have no permissions.
• Error: chmod requires both the permission mode and the file name to work properly. You didn't specify a mode, which caused the error.

Correct Command Syntax:

chmod <permissions> <filename>