Linux File Permissions Overview

Understanding Linux File Permissions

Overview of File Ownership

• Types of Owners: In Linux, every file or directory is assigned three types of owners:
• User: The individual who created the file, holds the most control.
• Group: Consists of users who share similar permissions to access the file.
• Others: Users who are neither the file owner nor part of the group, having the least control.

Types of Permissions

• Read (r): Permission to open and read the contents of a file.
• Write (w): Permission to modify or delete the file.
• Execute (x): Permission to run the file as a program.

Permission Representation

1. Binary Representation: Each permission is represented by a bit:

• Read: 1,
• Write: 1,
• Execute: 1
• Example: rwx is represented as 111 (binary).

1. Octal Representation: Each permission value is assigned a numerical value:

• Read: 4
• Write: 2
• Execute: 1
• Combined, rwx equals 7 (4+2+1).

1. String Representation: Represents the permissions in a simple format such as rwx.

Practical Example of Permissions

• Given a file with the following permissions:
• Owner: rwx (7)
• Group: rw- (6)
• Others: r-- (4)
• Octal Representation: 764
• Setting: Use chmod 764 filename to set permissions.

Special Permission Bits

1. setuid:

• Allows an executable file to run with the permissions of the owner, not the user running it.
• Example: passwd command uses setuid to enable regular users to change their passwords.
• When set, the execute bit for owner shows as s.

1. setgid:

• Allows an executable to run with the permissions of the group's owner.
• On directories, ensures new files inherit the group ownership, beneficial for team project directories.
• When set, the execute bit for group shows as s.

1. Sticky Bit:

• Ensures that only the file owner, directory owner, or root can delete or rename files within a directory.
• Commonly used in directories like /tmp to prevent file deletion by other users.
• When set, the execute bit for others shows as t.

Setting Special Permissions with chmod

• Example Command to set permissions:
• chmod 4755 important_script
  • Here, 4 sets the setuid bit; 7 gives owner read, write, and execute; 5 gives group and others read and execute.

Importance of Understanding Permissions

• Proper file permissions management is essential for maintaining security.
• Correct settings prevent unauthorized access and accidental modifications, safeguarding data and promoting collaboration in a Linux environment.