The main parameter in Nginx's access_log directive refers to a predefined log format name. When you see configuration like:

access_log /var/log/nginx/access.log main;

It means Nginx will use the log format named "main" to structure the access log entries. This format isn't magical - it's simply a commonly used default name that's preconfigured in most Nginx installations.

The main log format is typically defined earlier in your Nginx configuration using the log_format directive. A standard definition looks like:

log_format main '$remote_addr - $remote_user [$time_local] '
               '"$request" $status $body_bytes_sent '
               '"$http_referer" "$http_user_agent"';

You can create multiple log formats and reference them by name. For example:

log_format main '$remote_addr - $remote_user [$time_local] "$request" '
               '$status $body_bytes_sent "$http_referer" '
               '"$http_user_agent" "$http_x_forwarded_for"';

log_format minimal '$remote_addr [$time_local] "$request" $status';

Then use them selectively:

access_log /var/log/nginx/access.log main;
access_log /var/log/nginx/minimal.log minimal;

Here are some common scenarios where you might modify or create new log formats:

# For analytics tracking
log_format analytics '$remote_addr - $http_x_forwarded_for - $remote_user [$time_local] '
                    '"$request" $status $body_bytes_sent '
                    '"$http_referer" "$http_user_agent" '
                    '$request_time $upstream_response_time';

# For security monitoring
log_format security '$remote_addr - $remote_user [$time_local] '
                   '"$request" $status $body_bytes_sent '
                   '$http_cookie "$http_user_agent"';

• Always define your log formats before using them in access_log directives
• Consider creating separate log files for different purposes (analytics, debugging, security)
• For high-traffic servers, evaluate if you need all the log data or can use a minimal format
• Remember that more detailed logs consume more disk space and I/O resources

If you encounter errors about undefined log formats, check:

1. The log format is defined before it's used
2. There are no typos in the format name
3. You haven't accidentally commented out the log_format directive

When configuring Nginx logging, you might encounter this syntax:

access_log /var/log/nginx/access.log main

The main parameter isn't explicitly documented in the official Nginx docs, which can be confusing for developers.

The main parameter refers to a predefined log format. Nginx comes with a default log format called "combined", but you can define custom formats in your configuration:

log_format main '$remote_addr - $remote_user [$time_local] '
               '"$request" $status $body_bytes_sent '
               '"$http_referer" "$http_user_agent"';

Here's how you might use it in a complete server block:

http {
    log_format main '$remote_addr - $remote_user [$time_local] '
                   '"$request" $status $body_bytes_sent '
                   '"$http_referer" "$http_user_agent"';
    
    server {
        access_log /var/log/nginx/access.log main;
        
        location / {
            root /var/www/html;
        }
    }
}

Understanding this parameter is crucial when:

• Creating custom log formats for specific analytics needs
• Setting up log parsing for monitoring tools
• Debugging complex request flows

You can define multiple log formats and use them conditionally:

log_format main '$remote_addr - $remote_user [$time_local] '
               '"$request" $status $body_bytes_sent';
               
log_format debug '$remote_addr - $remote_user [$time_local] '
                '"$request" $status $body_bytes_sent '
                '$request_time $upstream_response_time';
                
server {
    access_log /var/log/nginx/access.log main;
    access_log /var/log/nginx/debug.log debug if=$debug;
}