I noticed that the word separator hyphen (-) is mixed with the underscore (_) in the configuration item in the MySQL configuration file my.conf. I wonder if there is a difference between the two?
like
log-bin = /var/log/mysql/mysql-bin.log
log_bin = /var/log/mysql/mysql-bin.log
From the official reference guide, and the section "Using Options on the Command Line":
Within option names, dash (-) and underscore (_) may be used interchangeably in most cases [...]
In then gives an example:
In this Manual, we use dashes in option names, except where underscores are significant. This is the case with, for example, --log-bin and --log_bin, which are different options. We encourage you to do so as well.
So, unless you're referring to a configuration variable where there actually is a difference between two similarly named variables (which you do in your example), you can use either - or _.
The recommendation is to use - unless _ is necessary to avoid that exact problem of suddenly configuring something different from what you intended.