GRUB's user interface
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GRUB has both a simple menu interface for choosing preset entries
from a configuration file, and a highly flexible command-line for
performing any desired combination of boot commands.
GRUB looks for its configuration file as soon as it is loaded. If one
is found, then the full menu interface is activated using whatever
entries were found in the file. If you choose the "command-line" menu
option, or if the configuration file was not found, then GRUB drops to
the command-line interface.