On RHEl7 booting in single user mode is done with the use of rescue mode.
RHEL7 documentation states that in RHEL7 rescue mode is equivalent to single user mode. In this mode the system attempts to mount all local file systems and start some important system services. However, it does not activate network interfaces or allow more users to login to the system at the same time.
To boot in the single user mode (rescue mode) press e (edit) on the GRUB2 boot screen.
Scroll down to the line beginning with "linux16" (on x86_64 bit systems) For other systems refer
RHEL 7 documentation. On the following screenshot this is underlined in red. Add the following to the end of this line (underline in green in the image below).
systemd.unit=rescue.target
Press Ctrl+x to boot the system in the rescue mode. Enter the root password when prompted to gain access to the system in single user mode.
