>My kernel version is 5.0.12.
>When i learn the manual,i didn't find any variables in the manul
>when i used "show variables".Weither there is a lack of manual

Some system variables can be read by running mysqld --help or mysqladmin variables from a command line. Some can be SET and SELECTed in a mysql client, some can be displayed, by the command SHOW VARIABLES in a mysql client.

System variables can be set in option files or on the server command line.
For how to use option files see http://dev.mysql.com/doc/refman/5.0/en/option-files.html,
for command line options see http://dev.mysql.com/doc/mysql/en/Server_options.html,
for use of such variables in db admin see http://dev.mysql.com/doc/mysql/en/MySQL_Database_Administration.html,
about System Variables see http://dev.mysql.com/doc/mysql/en/System_Variables.html
for the SHOW VARIABLES command see http://dev.mysql.com/doc/mysql/en/SHOW_VARIABLES.html and in the INNODB section http://dev.mysql.com/doc/mysql/en/InnoDB.html.