Ruby on Rails 3 Can't connect to local MySQL server through socket '/tmp/mysql.sock' on OSX

First, to find your socket file:

mysqladmin variables | grep socket

For me, this gives:

| socket                                            | /tmp/mysql.sock                                                                                                        |

Then, add a line to your config/database.yml:

development:
  adapter: mysql2
  host: localhost
  username: root
  password: xxxx
  database: xxxx
  socket: /tmp/mysql.sock

Found it!

Change host: localhost in config/database.yml to host: 127.0.0.1 to make rails connect over TCP/IP instead of local socket.

development:
  adapter: mysql2
  host: 127.0.0.1
  username: root
  password: xxxx
  database: xxxx

Your mysql server may not be running. Below explains how to start the server. This is an excerpt from the README file that comes with the mysql download.

After the installation, you can start up MySQL by running the following commands in a terminal window. You must have administrator privileges to perform this task.

If you have installed the Startup Item, use this command:

 shell> sudo /Library/StartupItems/MySQLCOM/MySQLCOM start
 (ENTER YOUR PASSWORD, IF NECESSARY)
 (PRESS CONTROL-D OR ENTER "EXIT" TO EXIT THE SHELL)

If you don't use the Startup Item, enter the following command sequence:

 shell> cd /usr/local/mysql
 shell> sudo ./bin/mysqld_safe
 (ENTER YOUR PASSWORD, IF NECESSARY)
 (PRESS CONTROL-Z)
 shell> bg
 (PRESS CONTROL-D OR ENTER "EXIT" TO EXIT THE SHELL)

These are options to fix this problem:

Option 1: change you host into 127.0.0.1

staging:
  adapter: mysql2
  host: 127.0.0.1
  username: root
  password: xxxx
  database: xxxx
  socket: your-location-socket

Option 2: It seems like you have 2 connections into you server MySql. To find your socket file location do this:

mysqladmin variables | grep socket

for me gives:

mysqladmin: connect to server at 'localhost' failed
error: 'Can't connect to local MySQL server through socket '/Applications/XAMPP/xamppfiles/var/mysql/mysql.sock' (2)'
Check that mysqld is running and that the socket: '/Applications/XAMPP/xamppfiles/var/mysql/mysql.sock' exists!

or

mysql --help 

I get this error because I installed XAMPP in my OS X Version 10.9.5 for PHP application. Choose one of the default socket location here.

I choose for default rails apps:

socket: /tmp/mysql.sock

For my PHP apps, I install XAMPP so I set my socket here:

socket: /Applications/XAMPP/xamppfiles/var/mysql/mysql.sock

OTHERS Socket Location in OS X

For MAMPP:

socket: /Applications/MAMP/tmp/mysql/mysql.sock

For Package Installer from MySQL:

socket: /tmp/mysql.sock

For MySQL Bundled with Mac OS X Server:

socket: /var/mysql/mysql.sock

For Ubuntu:

socket: /var/run/mysqld/mysql.sock

Option 3: If all those setting doesn't work you can remove your socket location:

staging:
  # socket: /var/run/mysqld/mysql.sock

I hope this help you.

"/tmp/mysql.sock" will be created automatically when you start the MySQL server. So remember to do that before starting the rails server.