Linux : Red Hat 7 boot to single user mode

1 Append rd.break  to kernel cmdline and Ctrl-x to boot to single user mode

kernel cmdline is the line contains (linux16 /vmlinuz-3.10.0-2........)

After boot up in single mode, run
mount -o remount,rw /sysroot
chroot /sysroot
Then we can modify necessary files

Or

2 Append init=/bin/bash to kernel cmdline and Ctrl-x to boot to single user mode
After boot up in single mode, run
mount -o remount,rw /
Then we can modify necessary files