The nginx -g "daemon off;" command is used to start the NGINX server in the foreground without forking into background as a daemon. This is generally useful for development or debugging purposes when you want to see the server logs and interact with it in real-time.
Normally, when you start NGINX, it runs as a background process and continues to run even after you close the terminal or log out of the server. By using the daemon off; directive in the NGINX configuration, you can instruct NGINX to stay in the foreground and log its activities directly to the terminal.
Here is an example of how you can use this command:
$ sudo nginx -g "daemon off;"
nginx: [alert] could not open error log file: open() "logs/error.log" failed (2: No such file or directory)
2021/09/10 10:00:00 [emerg] 1234#1234: bind() to 0.0.0.0:80 failed (98: Address already in use)
... more NGINX log messages ...
In the example above, the NGINX server is started using the daemon off; directive. It logs an alert about the unavailability of the error log file and an emergency error message indicating that the bind to the default HTTP port 80 failed because it’s already in use by another process.
By keeping the server in the foreground, you can observe these log messages directly in the terminal. This can be helpful for troubleshooting configuration issues, testing changes, or debugging any errors that may occur during NGINX startup or runtime.