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grub: Embedded configuration

 
 6.5 Embedding a configuration file into GRUB
 ============================================
 
 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 (⇒
 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 (⇒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 example,
 '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 'prefix' and 'root' 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'.
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