As of MySQL 8.0.16, MySQL Server supports a --validate-config option that enables the startup configuration to be checked for problems without running the server in normal operational mode:

mysqld --validate-config

If no errors are found, the server terminates with an exit code of 0. If an error is found, the server displays a diagnostic message and terminates with an exit code of 1. For example:

$> mysqld --validate-config --no-such-option
2018-11-05T17:50:12.738919Z 0 [ERROR] [MY-000068] [Server] unknown
option '--no-such-option'.
2018-11-05T17:50:12.738962Z 0 [ERROR] [MY-010119] [Server] Aborting

The server terminates as soon as any error is found. For additional checks to occur, correct the initial problem and run the server with --validate-config again.

For the preceding example, where use of --validate-config results in display of an error message, the server exit code is 1. Warning and information messages may also be displayed, depending on the log_error_verbosity value, but do not produce immediate validation termination or an exit code of 1. For example, this command produces multiple warnings, both of which are displayed. But no error occurs, so the exit code is 0:

$> mysqld --validate-config --log_error_verbosity=2
         --read-only=s --transaction_read_only=s
2018-11-05T15:43:18.445863Z 0 [Warning] [MY-000076] [Server] option
'read_only': boolean value 's' was not recognized. Set to OFF.
2018-11-05T15:43:18.445882Z 0 [Warning] [MY-000076] [Server] option
'transaction-read-only': boolean value 's' was not recognized. Set to OFF.

This command produces the same warnings, but also an error, so the error message is displayed along with the warnings and the exit code is 1:

$> mysqld --validate-config --log_error_verbosity=2
         --no-such-option --read-only=s --transaction_read_only=s
2018-11-05T15:43:53.152886Z 0 [Warning] [MY-000076] [Server] option
'read_only': boolean value 's' was not recognized. Set to OFF.
2018-11-05T15:43:53.152913Z 0 [Warning] [MY-000076] [Server] option
'transaction-read-only': boolean value 's' was not recognized. Set to OFF.
2018-11-05T15:43:53.164889Z 0 [ERROR] [MY-000068] [Server] unknown
option '--no-such-option'.
2018-11-05T15:43:53.165053Z 0 [ERROR] [MY-010119] [Server] Aborting

The scope of the --validate-config option is limited to configuration checking that the server can perform without undergoing its normal startup process. As such, the configuration check does not initialize storage engines and other plugins, components, and so forth, and does not validate options associated with those uninitialized subsystems.

--validate-config can be used any time, but is particularly useful after an upgrade, to check whether any options previously used with the older server are considered by the upgraded server to be deprecated or obsolete. For example, the tx_read_only system variable was deprecated in MySQL 5.7 and removed in 8.0. Suppose that a MySQL 5.7 server was run using that system variable in its my.cnf file and then upgraded to MySQL 8.0. Running the upgraded server with --validate-config to check the configuration produces this result:

$> mysqld --validate-config
2018-11-05T10:40:02.712141Z 0 [ERROR] [MY-000067] [Server] unknown variable
'tx_read_only=ON'.
2018-11-05T10:40:02.712178Z 0 [ERROR] [MY-010119] [Server] Aborting

--validate-config can be used with the --defaults-file option to validate only the options in a specific file:

$> mysqld --defaults-file=./my.cnf-test --validate-config
2018-11-05T10:40:02.712141Z 0 [ERROR] [MY-000067] [Server] unknown variable
'tx_read_only=ON'.
2018-11-05T10:40:02.712178Z 0 [ERROR] [MY-010119] [Server] Aborting

Remember that --defaults-file, if specified, must be the first option on the command line. (Executing the preceding example with the option order reversed produces a message that --defaults-file itself is unknown.)