I'm new on linux server.I have 2 question that I'm confuse about this.1 1. user:group now I chown my /var/www/html like this.my nginx.conf is set server{ user www-data }and in terminal I setchown -R root:www-data /var/www/htmlfind /var/www/html -type d -exec chmod 775 {} +find /var/www/html -type f -exec chmod 664 {} +find /var/www/html/uploads/images -type d -exec chmod 775 {} +Is I'm do the right thing ? or it need to set to www-data:www-data ?2 2. about crontab it a lot of TUT but it is not clear about user who ran crontab. The question isIf I login with adam user and my server is own by root:www-data or www-data:www-data how can I give the crontab to that user not adam user ? because it need perm to write the files like backup. 
confuse about User:Group what is the best for /var/www, and Crontab user?
permissions;cron
1) In general, files in /var/www should be owned by root:www-data and chmod 644, while /var/www itself and all subdirectories should be chmod 755.They should not be writable by the www-data user unless absolutely necessary (and that goes triple for executable files) because files which are writable by www-data can be modified by an attacker who manages to compromise the web server itself or a script run by the web server (unless you're using suexec, in which case they'll have the permissions of the script's owner rather than the web server, which generally isn't much better and may be much worse).  If nothing is writable by www-data, then the damage that can be done by an attacker who gains www-data's access is substantially reduced.2) cron jobs are run by the user whose crontab invokes the job.  If it's in root's crontab, it runs as root.  If it's in adam's crontab, it runs as adam, etc.The exception to this is the system-wide crontabs under /etc (/etc/crontab, /etc/cron.d/*, and so on).  The job specification format for these crontabs includes an additional field specifying the user who the job should be run as.