Differents paths of MySQL Server

Database datas

The database datas are located on /var/lib/mysql path, by default.
This path is defined on /etc/mysql/mysql.cnf file. It's "datadir" parameter.

datadir         = /var/lib/mysql

MySQL working directory

MySQL working directory is located on /tmp, by default.
This path is defined on /etc/mysql/mysql.cnf. It's "tmpdir" parameter.

tmpdir          = /tmp

PID File

MySQL PID File (file which contains the process number for MySQL server) is /var/run/mysqld/mysqld.pid