How to Reset Root Password in CentOS 7

Resetting the root password is not as easy as what you did for CentOS /RHEL 6 or older versions. 

It is no longer possible to change the password in single-user mode because the root password is now required to operate in single-user target. 
Emergency mode also has the same issue.

To reset the password on a CentOS / RHEL 7 box follow these steps:

• Restart the machine. 
• On the boot screen, press E key. As you may guess E stand for Edit!
• Use down arrow key until you reach the line starts with linux16 (if the machine has UEFI instead of legacy BIOS, you see linuxefi instead of linuxe16)
• Delete rhgb and quiet parameters from the end of the line. (You can use Ctrl+a and Ctrl+e to jump over to the start and end of a line.)
• Add the following parameter at the end of the linux16 line (or linuxefi): 

      init=/bin/sh

• Now use Ctrl+x to boot the system. Then shell prompt will appear.
• Load the initial SELinux policy into the kernel in order to preserve the SELinux context of the files that are to be modified:

# /usr/sbin/load_policy -i

• As the Linux file system is mounted read-only, you have to remount the file system to make it writable:

# mount -o remount, rw 

• Now it is ready to change the root password:  

# passwd root

• Do not forget to mount the Linux file system as read only again:

# mount -o remount, ro

• Run the following command to finish the system boot:

# exec /sbin/init

• Do not forget to remove init=/bin/sh from the kernel parameters line and put rhgb and quite again.