7. Using GRUB via a serial line

This chapter describes how to use the serial terminal support in GRUB.

If you have many computers or computers with no display/keyboard, it could be very useful to control the computers through serial communications. To connect one computer with another via a serial line, you need to prepare a null-modem (cross) serial cable, and you may need to have multiport serial boards, if your computer doesn’t have extra serial ports. In addition, a terminal emulator is also required, such as minicom. Refer to a manual of your operating system, for more information.

As for GRUB, the instruction to set up a serial terminal is quite simple. First of all, make sure that you haven’t specified the option ‘--disable-serial’ to the configure script when you built your GRUB images. If you get them in binary form, probably they have serial terminal support already.

Then, initialize your serial terminal after GRUB starts up. Here is an example:

grub> serial --unit=0 --speed=9600
grub> terminal serial

The command serial initializes the serial unit 0 with the speed 9600bps. The serial unit 0 is usually called ‘COM1’, so, if you want to use COM2, you must specify ‘--unit=1’ instead. This command accepts many other options, so please refer to serial, for more details.

The command terminal (see section terminal) chooses which type of terminal you want to use. In the case above, the terminal will be a serial terminal, but you can also pass console to the command, as ‘terminal serial console’. In this case, a terminal in which you press any key will be selected as a GRUB terminal.

However, note that GRUB assumes that your terminal emulator is compatible with VT100 by default. This is true for most terminal emulators nowadays, but you should pass the option ‘--dumb’ to the command if your terminal emulator is not VT100-compatible or implements few VT100 escape sequences. If you specify this option then GRUB provides you with an alternative menu interface, because the normal menu requires several fancy features of your terminal.

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