MySQL manages connections to the database server through the use of a socket file, a special kind of file that facilitates communications between different processes. The MySQL server’s socket file is named mysqld.sock and on Ubuntu systems it’s usually stored in the /var/run/mysqld/ directory. This file is created by the MySQL service automatically.

Sometimes, changes to your system or your MySQL configuration can result in MySQL being unable to read the socket file, preventing you from gaining access to your databases. The most common socket error looks like this:

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

One common cause of this error is that the MySQL service is stopped or did not start to begin with, meaning that it was unable to create the socket file in the first place. To find out if this is the reason you’re seeing this error, try starting the service with systemctl:

Then try accessing the MySQL prompt again. If you still receive the socket error, double check the location where your MySQL installation is looking for the socket file. This information can be found in the mysqld.cnf file.

Look for the socket parameter in the [mysqld] section of this file. It will look like this:

Close this file, then ensure that the mysqld.sock file exists by running an ls command on the directory where MySQL expects to find it:

Output
. .. mysqld.pid mysqld.sock mysqld.sock.lock

If the file does not exist, the reason may be that MySQL is trying to create it, but does not have adequate permissions to do so. You can ensure that the correct permissions are in place by changing the directory’s ownership to the mysql user (user as in mysqld.cnf file shows above) and group:

Then ensure that the mysql user has the appropriate permissions over the directory. Setting these to 775 will work in most cases:

Finally, restart the MySQL service so it can attempt to create the socket file again:

Then try accessing the MySQL prompt once again. If you still encounter the socket error, there’s likely a deeper issue with your MySQL instance, in which case you should review the error log.