This is a howto about booting Linux without a traditional screen and through a serial line.

It is often the only method for servers. But it can be generally useful for debugging, by allowing logging of the boot messages, which scroll too fast on a VGA screen and are not fully kept in /var/log/messages.

The method is unlikely to be useful for out of the box modern desktops/laptops/notebooks, as there have no more a serial port. Emulation of a serial port through USB can't work here, as we are too early in the boot process. In that case, and if your hardware allows, you will have to plug in some serial port extension card.

I will in the following limit the scope to what I use: the IA-32 and derived architectures, but any information about other ones is welcome.

Controlling boot through the serial line

Serial line configuration at the booting computer side

To have a complete boot experience through the serial line, you have to:

  1. manage to get the BIOS I/O go through the serial line
  2. configure a bootloader like GRUB to use the serial line
  3. configure the kernel to output boot messages to the serial line
  4. get a login prompt on the serial line

Manage to get the BIOS I/O go through the serial line

On servers it may be the only method, but it is unlikely to exist on a desktop BIOS. In that case, you will miss the BIOS messages on the serial line.

Configure a bootloader like GRUB to use the serial line

On GRUB 1 or GRUB 2 you simply have to add these lines in the GRUB configuration file:

# insmod is GRUB 2 specific, comment out for GRUB 1
insmod serial
serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1
terminal serial

Configure the kernel to output boot messages to the serial line

To get kernel messages on the serial line, you have to add a new option to the kernel command line, like:


which means 9600 bauds, no parity and 8 bits, to fit the previous section 4 lines. This will also involve editing GRUB configuration file. It may be unnecessary to do that if you use a server without a VGA card, Linux is clever enough to try a serial port in that case.

If you are using Debian, you can configure items 2 and 3 the Debian way, see for instance:

Notice that you can't use arrows to navigate the GRUB menus when on serial line, other standard keys are offered to that effect on the greeting page.

Get a login prompt on the serial line

It is covered in that page: recovering when locked out from your computer.

The other end of the serial line: hardware terminal or emulation

In any case, you will need a null modem to link the booting host and the terminal (real dumb serial terminal or computer running an emulator).

Even if your computer emulating a terminal has no serial port, you can use an USB port with a hardware adapter and a Linux driver, look here: recovering when locked out from your computer.

Recording the boot messages

At that point, you should be able to get boot messages through the serial line and then a login prompt.

When you boot through a traditional VGA screen, you can only study back at most aroung 50 lines, depending on screen definition. While through an emulated serial terminal, you are free to record the full booting process.

Minicom (see: recovering when locked out from your computer) can record what is sent/received on the serial line, see option "Capture on/off" in the menu, it has hotkey L.

But other tools are more convenient. I suggest you use ttyrec. To register in file ttyrecord what happens during a minicom session, run:

ttyrec -e minicom

When you are done with minicom, you can replay the session with:

ttyplay ttyrecord

To stop replay at some point, hit Ctrl-S; to resume hit Ctrl-Q. See ttyrec man page for how to tune the general speed of the replay.