When examining process ownership after installation from nginx.org packages, you'll typically see:

ps aux | grep nginx
root      1234  0.0  0.1  12345  6789 ?        Ss   12:34   0:00 nginx: master process
nginx     1235  0.0  0.2  23456  9012 ?        S    12:34   0:00 nginx: worker process

The www-data user originates from Debian/Ubuntu conventions where web services traditionally run under this shared account. Key characteristics:

• UID typically 33 across Debian-based systems
• Default group ownership for /var/www
• Used by Apache and other web services by default

Official nginx.org packages create a dedicated 'nginx' user with:

id nginx
uid=998(nginx) gid=996(nginx) groups=996(nginx)

Key security aspects:

• Isolated from other services
• Custom UID/GID outside standard ranges
• Tighter control over required permissions

Sample directory structure permissions for both users:

drwxr-xr-x 2 root    root    4096 /etc/nginx
drwxr-xr-x 3 www-data www-data 4096 /var/www/html
drwxr-x--- 2 nginx   nginx   4096 /var/cache/nginx

Recommended configuration for production environments:

user nginx;
worker_processes auto;

events {
    worker_connections 1024;
}

http {
    server {
        listen 80;
        root /var/www/html;
        
        location / {
            try_files $uri $uri/ =404;
        }
    }
}

To switch from nginx to www-data user:

sudo usermod -a -G www-data nginx
sudo chown -R www-data:www-data /var/cache/nginx
sudo sed -i 's/user nginx;/user www-data;/' /etc/nginx/nginx.conf
sudo systemctl restart nginx

For maximum security, implement separate permissions:

sudo mkdir -p /srv/example.com/{public,private}
sudo chown nginx:nginx /srv/example.com/private
sudo chown www-data:www-data /srv/example.com/public
sudo chmod 750 /srv/example.com/private

When installing Nginx from nginx.org repository on Ubuntu 16.04, the default configuration specifies:

user nginx;

This creates a dedicated system user nginx for worker processes, while Ubuntu's package manager typically uses www-data. The key differences:

Both users serve the same fundamental purpose but have different permission contexts:

# Typical www-data permissions (Debian/Ubuntu):
drwxr-xr-x  4 www-data www-data 4096 Feb 15  2023 /var/www
-rw-r--r--  1 www-data www-data  725 Jan 12  2023 /var/www/html/index.nginx-debian.html

# Typical nginx permissions (from nginx.org):
drwxr-xr-x  2 nginx    nginx    4096 Nov 30 15:22 /var/cache/nginx
drwx------  2 nginx    nginx    4096 Nov 30 15:22 /var/lib/nginx

The security difference primarily depends on:

• Directory ownership and permissions set during installation
• How your web application files are deployed
• Whether other services need to access the same files

Use www-data when:

# If using Ubuntu/Debian packages
user www-data;

# When integrating with PHP-FPM or other services expecting www-data
fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;

Use nginx when:

# For standalone Nginx installations from source or nginx.org
user nginx;

# When you want strict separation from other web services
chown -R nginx:nginx /var/www/myapp;

To switch users safely:

# Stop Nginx first
sudo systemctl stop nginx

# Change ownership if switching users
sudo chown -R www-data:www-data /var/lib/nginx
sudo chown -R www-data:www-data /var/log/nginx

# Update config file
sudo sed -i 's/user nginx;/user www-data;/' /etc/nginx/nginx.conf

# Verify permissions
sudo nginx -t
sudo systemctl start nginx

For most Ubuntu deployments, using www-data provides better compatibility with:

• System monitoring tools
• Other web services (Apache, PHP-FPM)
• Automated security updates

However, for dedicated Nginx deployments where you need strict process isolation, the nginx user may be preferable.