Ubuntu Serial Monitor

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

The cu command is used to call up another system and act as a dial in terminal. To monitor the input I have tried using (Coolterm & GTKTerm), assuming an issue with the driver I have referred and tried updating the DTR line as the solutions suggested in the following stack overflow resources Reference 1,Reference 2,Reference 3. I have tied the same on the ubuntu based computer too but the issue remain the same. wsl -list -verbose NAME STATE VERSION. docker-desktop-data Stopped 2 docker-desktop Stopped 2 Ubuntu-18.04 Stopped 2 Fedora-32 Stopped 2 Ubuntu Stopped 1 The normal recommendation is to use WSL2 for installation of all new WSL distros. Vcomconnected FALSE Ubuntu Linux 14.04 - Use USB for Serial Monitor. Use USB for Serial Monitor ‎ 11:39 AM. 241 Views lpcware. You can not get the Monitor's serial number, but you can get the make & model once X is running: you can look for it in /var/log/Xorg.0.log, e.g. Grep '-' /var/log/Xorg.0.log or in.

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

4. Execution result

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.

3 grub-mkconfig

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

Reboot Ubuntu 16.04.

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.

tty1 is displayed with press Ctrl + Alt + F1.

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.