GRUB supports embedding a configuration file directly into the core image, so that it is loaded before entering normal mode. This is useful, for example, when it is not straightforward to find the real configuration file, or when you need to debug problems with loading that file. grub-install uses this feature when it is not using BIOS disk functions or when installing to a different disk from the one containing /boot/grub, in which case it needs to use the search command (see search) to find /boot/grub.
To embed a configuration file, use the -c option to grub-mkimage. The file is copied into the core image, so it may reside anywhere on the file system, and may be removed after running grub-mkimage.
After the embedded configuration file (if any) is executed, GRUB will load
the ‘normal’ module (see normal), which will then read the real
configuration file from $prefix/grub.cfg. By this point, the
root variable will also have been set to the root device name. For
prefix might be set to ‘(hd0,1)/boot/grub’, and
root might be set to ‘hd0,1’. Thus, in most cases, the embedded
configuration file only needs to set the
variables, and then drop through to GRUB's normal processing. A typical
example of this might look like this:
search.fs_uuid 01234567-89ab-cdef-0123-456789abcdef root set prefix=($root)/boot/grub
(The ‘search_fs_uuid’ module must be included in the core image for this example to work.)
In more complex cases, it may be useful to read other configuration files directly from the embedded configuration file. This allows such things as reading files not called grub.cfg, or reading files from a directory other than that where GRUB's loadable modules are installed. To do this, include the ‘configfile’ and ‘normal’ modules in the core image, and embed a configuration file that uses the configfile command to load another file. The following example of this also requires the echo, search_label, and test modules to be included in the core image:
search.fs_label grub root if [ -e /boot/grub/example/test1.cfg ]; then set prefix=($root)/boot/grub configfile /boot/grub/example/test1.cfg else if [ -e /boot/grub/example/test2.cfg ]; then set prefix=($root)/boot/grub configfile /boot/grub/example/test2.cfg else echo "Could not find an example configuration file!" fi fi
The embedded configuration file may not contain menu entries directly, but may only read them from elsewhere using configfile.