Linux file permissions are important for managing access to files and folders. There are three main permission types: read (r), write (w), and execute (x). These permissions are given to three groups: the owner, the group, and others (everyone else).

Here's a breakdown of Linux file permissions:

Symbolic Representation:

• r: Read permission

• w: Write permission

• x: Execute permission

• -: No permission

Numeric Representation:

• 4: Read permission

• 2: Write permission

• 1: Execute permission

• 0: No permission

Permission Categories:

1. Owner (u):

   • r: Read permission (4)

   • w: Write permission (2)

   • x: Execute permission (1)

2. Group (g):

   • r: Read permission (4)

   • w: Write permission (2)

   • x: Execute permission (1)

3. Others (o):

   • r: Read permission (4)

   • w: Write permission (2)

   • x: Execute permission (1)

Examples:

1. Symbolic Representation:

   • rwxr-xr--: Read, write, and execute for the owner; read and execute for the group; read-only for others.

   • rw-r--r--: Read and write for the owner; read-only for the group and others.

   • rwxrwxr-x: Read, write, and execute for the owner and group; read and execute for others.

2. Numeric Representation:

   • 700: Read, write, and execute for the owner; no permissions for the group and others.

   • 644: Read and write for the owner; read-only for the group and others.

   • 775: Read, write, and execute for the owner and group; read and execute for others.

Changing Permissions:

• chmod Command:

  • To add permission, use +.

  • To remove permission, use -.

  • To set permission explicitly, use =.

Example:

    chmod +x file.txt     # Add execute permission to file.txt
    chmod u=rw,go=r file.txt  # Set explicit permissions

• chown Command:

  • Change file owner: chown new_owner:new_group file.txt

  • Change only the group: chown :new_group file.txt

• chgrp Command:

  • Change the group: chgrp new_group file.txt

View Permissions:

• ls Command:

  • ls -l: Displays detailed information, including permissions.

  • ls -la: Displays hidden files as well.

Special Permissions:

• Setuid (SUID):

  • Allows a user to execute a file with the permissions of the file owner.

  • Displayed as an s in the owner's execute position: -rwsr-xr-x.

• Setgid (SGID):

  • Similar to SUID but applies to directories. Files created in the directory inherit the group of the parent directory.

  • Displayed as an s in the group's execute position: -rwxr-sr-x.

• Sticky Bit:

  • Restricts the deletion of files in a directory to the file owner, the directory owner, or the root user.

  • Displayed as a t in the others' execute position: -rwxr-xr-t.

Examples of Setting Special Permissions:

• chmod u+s file.txt: Set the setuid bit.

• chmod g+s directory: Set the setgid bit for a directory.

• chmod +t directory: Set the sticky bit for a directory.

These basic ideas are important for knowing how to handle file access in Linux systems. Be careful when changing permissions, especially for important system files and folders.