Rescue mode provides the ability to boot a small Linux environment from an external bootable device like a CD-ROM, or USB drive instead of the system's hard drive.Rescue mode is provided to help you with your system from repairing the file system or fixing certain issues which prevent your normal operations.
In order to get into the rescue mode, change the BIOS settings of the machine to boot from the external media. Once the system started booting using bootable disk, add the keyword rescue as a kernel parameter or else you can give the parameter "linux rescue" in the graphical boot interface.
In single-user mode, the system boots to runlevel 1, but it will have many more additional functionalities compared to switching to runlevel 1 from other levels.
The local file systems can be mounted in this mode, but the network is not activated.
Use the following steps to boot into single-user mode:
1) At the GRUB splash screen during the booting process, press any key to enter the GRUB interactive menu.
2) Select the proper version of kernel that you wish to boot and type "a" to append the line.
3) Go to the end of the line and type "single" as a separate word.
4) Press Enter to exit edit mode and type "b" to boot into single usermode now.
In emergency mode, you are booting 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 the init is corrupted , you can still mount file systems to recover data that could be lost during a re-installation. To boot into emergency mode, use the same method as described for single-user mode, with one exception, replace the keyword single with the keyword "emergency".