Managing File Permissions with chmod and chown

Understanding Linux Permissions

Every file has three permission sets: owner, group, and other. Each set has three bits: read (r=4), write (w=2), execute (x=1).

ls -la /var/www/html
# -rwxr-xr-x 1 root root 1234 Jan 1 00:00 index.php
#  ^^^        = owner permissions
#     ^^^     = group permissions
#        ^^^  = other permissions

chmod — Change Permissions

Numeric (octal) mode:

chmod 755 /var/www/html          # rwxr-xr-x (directories)
chmod 644 /var/www/html/index.php # rw-r--r-- (files)
chmod 600 ~/.ssh/id_ed25519       # rw------- (private keys)
chmod -R 755 /var/www/html        # Recursive

Symbolic mode:

chmod u+x script.sh    # Add execute for owner
chmod g-w file.txt     # Remove write from group
chmod o-r file.txt     # Remove read from others
chmod a+r file.txt     # Add read for all

chown — Change Owner

chown root:root /etc/nginx/nginx.conf      # owner:group
chown www-data:www-data /var/www/html -R   # Recursive
chown -R user:user /home/user/             # Change entire home dir

Recommended Permissions for Web Hosting

# Set directories to 755, files to 644
find /var/www/mysite -type d -exec chmod 755 {} \;
find /var/www/mysite -type f -exec chmod 644 {} \;

# Set ownership to web server user
chown -R www-data:www-data /var/www/mysite

Special Permissions

• chmod 4755 — setuid (runs as owner's identity)
• chmod 1755 — sticky bit (only owner can delete files in directory, used on /tmp)