8. Embedding a configuration file into GRUB

GRUB supports a preset menu which is to be 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 good to run the commands serial (see section serial) and terminal (see section terminal) before anything else at the start-up time.

How the preset menu works is slightly complicated:

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 if the preset menu was loaded.
3. If 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 rebuild GRUB specifying a file to the configure script with the option ‘--enable-preset-menu’. The file has the same semantics as normal configuration files (see section Configuration).

Another point you should take care is that the diskless support (see section Booting from a network) diverts the preset menu. Diskless images embed a preset menu to execute the command bootp (see section 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

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