How to get a new pid file of Mysql and restart Mysql Successfully?
The result you are showing:

root      6490  0.0  0.2 112720  2332 pts/0    S+   03:48   0:00 grep --color=auto mysql


is actually the pid and process for the  ps aux | grep mysql command. It is not actually a process for MySQL.

A running process for MySQL would look similar to this:

mysql     1894  0.8  9.8 1865804 1192032 ?     Sl   Jul01 300:53 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --user=mysql --log-error=/var/lib/mysql/DBNAME.err --pid-file=/var/lib/mysql/DBNAME.pid


You can use the command pkill to kill the running MySQL processes by name. However, it's not recommended to kill MySQL in a forceful manner. 

 pkill -f mysql


This will match the filename pattern for any part of the command line

To start/stop the MySQL services, you can run:

service mysqld start 
service mysqld stop 
service mysqld restart


or 

service mysql start
service mysql stop
service mysql restart