If you have ever had to deal with a sick Redhat server, you may be familiar with the rescue, emergency and singler-user modes of operation. I have heard people refer to rescue modes incorrectly, which can sometimes lead to some interesting stories (there are several slight subtleties between them). To clear up any confusion surrounding these terms, here are the official descriptions from the Redhat administration guide:
Rescue mode provides the ability to boot a small Red Hat Enterprise Linux environment entirely from CD-ROM, or some other boot method, instead of the system’s hard drive. As the name implies, rescue mode is provided to rescue you from something. During normal operation, your Red Hat Enterprise Linux system uses files located on your system’s hard drive to do everything â€” run programs, store your files, and more.
In emergency mode, you are booted into the most minimal environment possible. The root file system is mounted read-only and almost nothing is set up. The main advantage of emergency mode over single-user mode is that the init files are not loaded. If init is corrupted or not working, you can still mount file systems to recover data that could be lost during a re-installation.
In single-user mode, your computer boots to runlevel 1. Your local file systems are mounted, but your network is not activated. You have a usable system maintenance shell. Unlike rescue mode, single-user mode automatically tries to mount your file system. Do not use single-user mode if your file system cannot be mounted successfully. You cannot use single-user mode if the runlevel 1 configuration on your system is corrupted.