ls /var/run/mysqld/ – should show below

-rw-r-----  1 mysql mysql    8 Apr 11 07:34 mysqld.pid
srwxrwxrwx  1 mysql mysql    0 Apr 11 07:34 mysqld.sock=
-rw-------  1 mysql mysql    7 Apr 11 07:34 mysqld.sock.lock
srwxrwxrwx  1 mysql mysql    0 Apr 11 07:34 mysqlx.sock=
-rw-------  1 mysql mysql    7 Apr 11 07:34 mysqlx.sock.lock

If not

cd /etc/mysql/mysql.conf.d
sudo vi mysqld.cnf

#unhash the single line - its right near the top below the user = mysql line.
pid-file        = /var/run/mysqld/mysqld.pid

sudo service mysql stop

sudo service mysql start

ls /var/run/mysqld/
should now contain the PID file as above

Oddly the following has happened to me, I do not know why and am not really phased.

Do not be in the folder /var/run/mysqld when doing the restart this for what ever reason has ended with none of the files get created not even the lock files. However the databases were up.

After leaving the directory and restarting the Mysql service all files created perfectly on multiple restarts.