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(grub.info)Preset Menu

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Embedding a configuration file into GRUB
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   GRUB supports "preset menu" which is always loaded before starting.
The preset menu feature is useful, for example, when your computer has
no console but a serial cable. In this case, it is critical to set up
the serial terminal as soon as possible, since you cannot see any
message until the serial terminal begins to work. So it is nice to run
the commands `serial' (Note: serial) and `terminal' (Note:
terminal) sooner than anything else at the start-up time.

   It is slightly complicated how the preset menu works:

  1. GRUB checks if the preset menu feature is used, and loads the
     preset menu, if available. This includes running commands and
     reading boot entries, like an ordinary configuration file.

  2. GRUB checks if the configuration file is available. Note that this
     check is performed *regardless of the existence of the preset
     menu*. The configuration file is loaded, even after the preset
     menu was loaded.

  3. When the preset menu includes any boot entries, they are cleared
     when the configuration file is loaded. It doesn't matter whether
     the configuration file has any entries or no entry. The boot
     entries in the preset menu are used only when GRUB fails in
     loading the configuration file.

   To enable the preset menu feature, you must specify a file to the
configure script with the option `--enable-preset-menu'. The file has
the same semantics as normal configuration files (Note:
Configuration).

   Another point you should take care is that the diskless support
(Note: Diskless) diverts the preset menu. Diskless images embed a
preset menu to execute the command `bootp' (Note: bootp)
automatically, unless you specify your own preset menu to the configure
script. This means that you must put commands to initialize a network in
the preset menu yourself, because diskless images don't set it up
implicitly, when you use the preset menu explicitly.

   Therefore, a typical preset menu used with diskless support would be
like this:

     # Set up the serial terminal, first of all.
     serial --unit=0 --speed=19200
     terminal --timeout=0 serial
     
     # Initialize the network.
     dhcp