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In the grub bootloader if you press esc it will take you to a command line interface that looks like

grub> _  

What is this promt used for? It doesn't look like it's connected to Linux. Is it a command shell provided by the bootloader?

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GRUB includes its own built-in shell where you can type commands similar to those you can type in Bash or other Linux text-mode shells. The GRUB shell is simple by Linux standards, but it's adequate for many emergency maintenance tasks. – Ramesh Chand Feb 9 at 6:43
    
Or when your auto-generated config didn't work and you urgently need to try a few things manually. – Shadur Feb 9 at 9:25

Yes, it is provided by GRUB. The GRUB command shell is just as powerful as the shell. You can use it to discover boot images, kernels, and root filesystems. When you're at the grub> prompt, you have a lot of functionality similar to any command shell such as history and tab-completion. The grub rescue> mode is more limited, with no history and no tab-completion.

  • It is mainly used as a rescue shell in case of non booting GRUB

You can get lot of inputs in here

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GRUB provides following interface:

  • Menu interface which provides a list of boot entries to the user to choose from.
  • Command-line interface which provides a prompt for performing boot commands

From GRUB’s user interface:

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.

Visit: The list of command-line and menu entry commands to get the purpose of specific command. For more visit the list of available commands.

Note: you can run help command to display information about builtin commands or to display short descriptions of all available commands

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