How to set the serial output of Ubuntu system (four)

Before the grub.cfg is streamlined, the system automatically starts to enter the INITRAMFS command line without mounting the root file system

What if I mount the root file system from Initramfs? Use the following command:

mkdir MNT

Mount/dev/sda1/mnt

Chroot/mnt

This is the command line after the system enters root

(INITRAMFS) chroot/mnt
Bash:cannot Set Terminal process Group ( -1): Inappropriate IOCTL for device
Bash:no job control in this shell
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However, at this time some file systems are not mounted, many system initialization is not up, but also need further tracking

How to set the serial output of Ubuntu system (four)