Ubuntu 16.04: GRUB2 and Linux with serial console

This article will describe changing GRUB2 and Linux to serial console.

This is not for desktop environment but embedded system and virtual machine environment.

1 Why using serial console

The serial console is helpful for CUI environment.

An embedded system which has no monitor display like router can be operated by serial console with RS232C.
A virtual machine like KVM + libvirt can be operated by serial console with CUI tools like virsh.

2 /etc/default/grub

Change /etc/default/grub as below.

Change GRUB terminal to console and ttyS0. This will provide one GRUB to a monitor display and serial console.
Remove hidden parameter for avoiding "no video mode activated" error. And change GRUB timeout from 10 seconds to 1 second.
Change linux kernel console to tty1 and ttyS0. This setting will be taken over to userland, and there will be two login prompt for tty1 and ttyS0.

$ diff -uprN /etc/default/grub{.org,}
--- /etc/default/grub.org       2016-06-07 08:26:05.869583873 +0900
+++ /etc/default/grub   2016-06-07 22:02:51.288594112 +0900
@@ -4,12 +4,10 @@
 #   info -f grub -n 'Simple configuration'

 GRUB_DEFAULT=0
-GRUB_HIDDEN_TIMEOUT=0
-GRUB_HIDDEN_TIMEOUT_QUIET=true
-GRUB_TIMEOUT=10
+GRUB_TIMEOUT=1
 GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
-GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
-GRUB_CMDLINE_LINUX=""
+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
@@ -17,7 +15,8 @@ GRUB_CMDLINE_LINUX=""
 #GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

 # Uncomment to disable graphical terminal (grub-pc only)
-#GRUB_TERMINAL=console
+GRUB_TERMINAL="console serial"
+GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"

 # The resolution used on graphical terminal
 # note that you can use only modes which your graphic card supports via VBE

3 grub-mkconfig

Update /boot/grub/grub.cfg with grub-mkconfig.

$ sudo grub-mkconfig -o /boot/grub/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.4.0-22-generic
Found initrd image: /boot/initrd.img-4.4.0-22-generic
Found linux image: /boot/vmlinuz-4.4.0-21-generic
Found initrd image: /boot/initrd.img-4.4.0-21-generic
Found memtest86+ image: /memtest86+.elf
Found memtest86+ image: /memtest86+.bin
done

Reboot Ubuntu 16.04.

$ sudo reboot

4 Execution result

GRUB to monitor display is as below.

GRUB to serial console is as below.

This article used "sudo virsh console <vmname>" for connecting serial console.

Login prompt to monitor display is as below.

tty1 is displayed with press Ctrl + Alt + F1.

Login prompt to serial console is as below.

4.1 Single user mode / Recovery mode

When running single user mode, there will be two root prompt for tty1 and ttyS0.
When running recovery mode, there will be one root prompt for ttyS0. If you want to switch prompt to tty1, you need to move "console=ttyS0,115200" to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub because recovery mode does not user GRUB_CMDLINE_LINUX_DEFAULT.
When running single user mode or recovery mode, root prompt for ttyS0 will be corrupted, e.g. noecho. Please append "stty sane" to root.bashrc as below.

case "$TERM" in
  linux|vt*) stty sane ;;
esac

A root prompt for ttyS0 with using "stty sane" is as below.