How do I find the location of MySQL daemon error log?

Question

Many MySQL problems require examining MySQL daemon error log, but it is not always obvious where it is. One way to find it is to issue SELECT @@log_error query. But

Answer 1

We can use the fact that mysqld uses file descriptor 2 (stderr) for the error log and the /proc file system like this:

sudo readlink /proc/$(pidof mysqld)/fd/2

this, of course, requires a currently running server. File descriptor is guaranteed to be 2 because MySQL writes to the error log via the redirection of the stderr stream to the specified log on startup, and then writing to stderr - at least in versions 5.5 and earlier, likely in later versions as well.