Understanding Red Hat Linux recovery runlevels

If Linux system can boot but hang during starting a service, booting to “recovery runlevels” can skip the service and gain shell to troubleshoot.
If Linux system can’t boot at all,  booting from rescue CD (first installation media) and type “linux rescue” to gain shell to troubleshoot
Red Hat Linux boot order

The BIOS ->MBR->Boot Loader->Kernel->/sbin/init->
/etc/inittab->
/etc/rc.d/rc.sysinit->
/etc/rc.d/rcX.d/ #where X is run level in /etc/inittab
run script with K then script with S

Recovery runlevels
- runlevel  1
Execute up to /etc/rc.d/rc.sysinit and /etc/rc.d/rc1.d/
Runlevel 1 is identical to singleuser mode. It is switched to singleuser mode in last step, just a number of trivial scripts executed before that.
 $ls  /etc/rc.d/rc1.d/S*
 /etc/rc.d/rc1.d/S02lvm2-monitor  /etc/rc.d/rc1.d/S13cpuspeed  /etc/rc.d/rc1.d/S99singlesingleuser

- single
Execute up to /etc/rc.d/rc.sysinit

- Emergency
Does not execute /etc/rc.d/rc.sysinit.
 Because rc.sysinit is not executed, file system is mounted in read-only mode. You need run “mount –o rw,remount /” to remount it in read-write mode.
emergency runlevel is Red Hat term, it is identical to  “init=/bin/sh” in any Linux distribution
How to go to a  runlevel
In the grub menu, type “a” to append one of following options to boot line.
1  
single  
emergency   
init=/bin/sh
When Centos hung on starting up boot services, how to get to shell without rescue CD
RHCE Notes - Troubleshooting booting issue