Using a serial console on Ubuntu 20.04

If GRUB_TIMEOUT is 1 or more (or -1), you can bring up the menu by pressing the Escape </ kbd> key even if GRUB_TIMEOUT_STYLE = hidden ([ref.](Https: /) /www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html)).

By adding GRUB_TERMINAL =" console serial ", the GRUB menu will be displayed on both the VGA console and the serial terminal.

If you don't see the output of the serial console, see the Manual (https://www.gnu.org/software/grub/manual/grub/html_node/serial.html) for your hardware and GRUB_SERIAL_COMMAND = Try changing "serial --speed = 115200" .

GRUB_CMDLINE_LINUX =" console = tty1 console = ttyS0,115200 " is the Linux kernel command-line parameter )is. Similar to GRUB_TERMINAL =" console serial ", this option allows tty input / output on both the VGA console (tty1) and the serial console (ttyS0).

This can also be changed in the GRUB menu at boot time.

Original (depending on the environment, this is from QEMU):

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT=""
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

Revised:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_TIMEOUT=2
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT=""
GRUB_CMDLINE_LINUX="console=tty1 console=ttyS0,115200"

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console
#GRUB_TERMINAL="console serial"
GRUB_TERMINAL="console serial"
GRUB_SERIAL_COMMAND="serial --speed=115200"

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

diff:

sudo apt install python3-pip
pip3 install icdiff
~/.local/bin/icdiff /etc/default/grub.orig   /etc/default/grub
~/.local/bin/icdiff /boot/grub/grub.cfg.orig /boot/grub/grub.cfg

The diff of /boot/grub/grub.cfg is large, so please check it with your own eyes.