sudo service mysql start
https://dev.mysql.com/doc/refman/5.7/en/problems-with-mysql-sock.html
B.5.3.6 How to Protect or Change the MySQL Unix Socket File
The default location for the Unix socket file that the server uses for communication with local clients is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)
On some versions of Unix, anyone can delete files in the /tmp directory or other similar directories used for temporary files. If the socket file is located in such a directory on your system, this might cause problems.
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https://dev.mysql.com/doc/refman/5.7/en/mysqld.html
mysqld, also known as MySQL Server, is the main program that does most of the work in a MySQL installation. MySQL Server manages access to the MySQL data directory that contains databases and tables. The data directory is also the default location for other information such as log files and status files.
When MySQL server starts, it listens for network connections from client programs and manages access to databases on behalf of those clients.
The mysqld program has many options that can be specified at startup. For a complete list of options, run this command: