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MySQL 8.4 Reference Manual

7.4.2.2 Default Error Log Destination Configuration

This section describes which server options configure the default error log destination, which can be the console or a named file. It also indicates which log sink components base their own output destination on the default destination.

In this discussion,“console” meansstderr, the standard error output. This is your terminal or console window unless the standard error output has been redirected to a different destination.

The server interprets options that determine the default error log destination somewhat differently for Windows and Unix systems. Be sure to configure the destination using the information appropriate to your platform. After the server interprets the default error log destination options, it sets thelog_error system variable to indicate the default destination, which affects where several log sink components write error messages. The following sections address these topics.

Default Error Log Destination on Windows

On Windows,mysqld uses the--log-error,--pid-file, and--console options to determine whether the default error log destination is the console or a file, and, if a file, the file name:

If--console is given, the default destination is the console. (--console takes precedence over--log-error if both are given, and the following items regarding--log-error do not apply.)
If--log-error is not given, or is given without naming a file, the default destination is a file namedhost_name.err in the data directory, unless the--pid-file option is specified. In that case, the file name is the PID file base name with a suffix of.err in the data directory.
If--log-error is given to name a file, the default destination is that file (with an.err suffix added if the name has no suffix). The file location is under the data directory unless an absolute path name is given to specify a different location.

If the default error log destination is the console, the server sets thelog_error system variable tostderr. Otherwise, the default destination is a file and the server setslog_error to the file name.

Default Error Log Destination on Unix and Unix-Like Systems

On Unix and Unix-like systems,mysqld uses the--log-error option to determine whether the default error log destination is the console or a file, and, if a file, the file name:

If--log-error is not given, the default destination is the console.
If--log-error is given without naming a file, the default destination is a file namedhost_name.err in the data directory.
If--log-error is given to name a file, the default destination is that file (with an.err suffix added if the name has no suffix). The file location is under the data directory unless an absolute path name is given to specify a different location.
If--log-error is given in an option file in a[mysqld],[server], or[mysqld_safe] section, on systems that usemysqld_safe to start the server,mysqld_safe finds and uses the option, and passes it tomysqld.

Note

It is common for Yum or APT package installations to configure an error log file location under/var/log with an option likelog-error=/var/log/mysqld.log in a server configuration file. Removing the path name from the option causes thehost_name.err file in the data directory to be used.

How the Default Error Log Destination Affects Log Sinks

After the server interprets the error log destination configuration options, it sets thelog_error system variable to indicate the default error log destination. Log sink components may base their own output destination on thelog_error value, or determine their destination independently oflog_error

Iflog_error isstderr, the default error log destination is the console, and log sinks that base their output destination on the default destination also write to the console:

log_sink_internal,log_sink_json,log_sink_test: These sinks write to the console. This is true even for sinks such aslog_sink_json that can be enabled multiple times; all instances write to the console.
log_sink_syseventlog: This sink writes to the system log, regardless of thelog_error value.

Iflog_error is notstderr, the default error log destination is a file andlog_error indicates the file name. Log sinks that base their output destination on the default destination base output file naming on that file name. (A sink might use exactly that name, or it might use some variant thereof.) Suppose that thelog_error valuefile_name. Then log sinks use the name like this:

log_sink_internal,log_sink_test: These sinks write tofile_name.
log_sink_json: Successive instances of this sink named in thelog_error_services value write to files namedfile_name plus a numbered.NN.json suffix:file_name.00.json,file_name.01.json, and so forth.
log_sink_syseventlog: This sink writes to the system log, regardless of thelog_error value.

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