Although being a root in recovery mode, we can not change anything. The partitions are mounted in read-only status. So, what should we do?
Recovery mode can be accessed by selecting “Ubuntu Advance Options” of grub menu shows while computer booting up.
In general, to change system in Linux, you have to be a root user. But unfortunately, Ubuntu prohibited us changing anything in recovery mode, despite we were logged as root user, cause of system mounting the partitions in read-only status (see A).
To overcome this, an easiest step is by selecting “fsck” menu (see B). After finished, you could then select “root” menu (see C) to access system without limitation.