Find The Web Server User:

The first step to finding the web server user in Apache or Nginx server, go ahead and ssh to your server by using terminal and run the below command:

Apache Server

sudo ps aux | egrep '(apache|httpd)';

You should get an output similar to this:

www-data 19760  0.0  4.5 349176 46152 ?        S    07:59   0:00 /usr/sbin/apache2 -k start                            
www-data 20008  0.0  0.8 342060  8912 ?        S    08:07   0:00 /usr/sbin/apache2 -k start                            
root     20120  0.0  0.0  14992   976 pts/0    S+   08:08   0:00 grep -E --color=auto (apache|httpd)

This output clearly says that Apache is running by www-data, so you get your webserver user.

Nginx Server

sudo ps aux| grep nginx

You should get an output similar to this:

root      139271  0.0  0.0  66580  1800 ?        Ss   15:18   0:00 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
www-data  139272  0.0  0.1  66820  5540 ?        S    15:18   0:00 nginx: worker process
www-data  139273  0.0  0.1  66820  5540 ?        S    15:18   0:00 nginx: worker process

This output clearly says that Apache is running by www-data, so you get your webserver user.