Info Catalog
grub: config_file
grub: Special environment variables
grub: default
15.1.9 debug ------------ This variable may be set to enable debugging output from various components of GRUB. The value is a list of debug facility names separated by whitespace or ',', or 'all' to enable all available debugging output. The facility names are the first argument to grub_dprintf. Consult source for more details.