I was looking for where mysql.sock was since running rake db:bootstrap was complaining with "No such file or directory - /tmp/mysql.sock". Rails looks for the mysql.sock file under /tmp/mysql.sock by default. Ubuntu using an apt-get install of mysql puts the mysql.sock file at /var/run/mysqld/mysqld.sock.
I always forget where it is. So, I'm writing it down.
/var/run/mysqld/mysqld.sock
You can also grep for it from mysql like so
>> mysql -? | grep mysqld.sock socket /var/run/mysqld/mysqld.sock