File and Directory Permissions in Linux: Using chmod and chown

In Linux, file and directory permissions are crucial for system security and user access control. Incorrect permissions can lead to unauthorized access, data loss, or serious security vulnerabilities.

1. Importance of File Permissions in Linux

Each file and directory in Linux has specific permissions and ownership. You can view a file’s permissions using the ls -l command:

ls -l file.txt

Example Output:

-rw-r--r--  1 user group  1024 Feb 28 12:00 file.txt

• -rw-r--r-- → File permissions
• user → File owner
• group → Group owner
• 1024 → File size (bytes)
• Feb 28 12:00 → Last modified date

2. Setting File Permissions with chmod

Basic Permission Types

Numeric Method

chmod 755 file.txt

• 7 (rwx): Owner has all permissions
• 5 (r-x): Group can read/execute
• 5 (r-x): Others can read/execute

Common Settings

Symbolic Method

chmod u+rwx,g+rx,o-r file.txt

• u+rwx: Add all permissions to user
• g+rx: Add read/execute to group
• o-r: Remove read from others

chmod o-w file.txt  # Remove write from others
chmod u+x script.sh # Give execute to user
  

3. Changing Ownership with chown

Change File Owner

chown ahmet file.txt

Change Owner and Group

chown ahmet:www-data file.txt

Change Ownership Recursively

chown -R apache:apache /var/www/html

4. Real-World Examples & Security Tips

Web Server Setup

chown -R www-data:www-data /var/www/html
chmod -R 755 /var/www/html
  

SSH Key Permissions

chmod 600 ~/.ssh/id_rsa
chmod 700 ~/.ssh
chmod 644 ~/.ssh/id_rsa.pub
  

Restrict User Home Access

chmod 750 /home/username

Best Practices

• Avoid 777 unless necessary
• Use 444 if write is not needed
• Protect system files like /etc/passwd
• Run ls -l before applying chmod/chown