What is the procedure for booting in rescue mode on RHEL (or CentOS)?

The procedure for booting RHEL (or CentOS) currently in use on a virtual server in rescue mode is as follows.

1. Log in to the cloud computing control panel.

2. Go ahead [Server]-[Virtual Server]-[Instance]-[Console] and open the console screen of the virtual server instance.

3. Press [Send CtrlAltDelete] button on the console screen.

4. Press [e] key on the GRUB Boot screen (OS selection screen for booting) immediately after starting the virtual server instance.

5. Delete "console = ttyS0 console = ttyS1,115200n8 quiet" at the end of the line starting with "linux16" on the edit screen, enter "systemd.unit = rescue.target" and enter "Ctrl" "x" at the same time.
# When using the Japanese keyboard,  it may be difficult to enter "=". In this case, please leave "=" when deleting "console = ttyS0 console = ttyS1,115200n8 quiet".

6. Log in using the root user password on the Red Hat Enterprise Linux or CentOS login screen.

Reference

Red Hat - Product Documentation
URL : https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/sec-terminal_menu_editing_during_boot