(All About) Boot into Single User

What is "Single User"?

"Single User" is a boot mode, you actually tells the unix shell that the OS should load only prompt (no GUI), and the local console login is the only login available (can't be logged on from the network),

Why would you like to do that?

sometimes the system requires an administrative actions by the administrator, such as disk checkup, partition altering, drivers uninstalling, backups, etc.

at this delicate tasks you don't want other users to touch/create/remove/use files, hardware, CPU, and use resources. this is why you enter into single user mode.

What can you do in "Single User" Mode?

since single user mode is for administrator and no GUI is available then every task you can do from terminal (command line) as root user you can do here, which is practically everything (on the current system) it wasn't meant to be used for regular work such as surfing the web (there is no network) or reading files etc. you can use all the unix commands available for OS X and all the command line utilities and applications available on that machine.

How to enter Single User Mode?

1. Restart your computer
   
2. Continuously press F8 until you see the Darwin boot prompt
3. write "-s" without the quotation marks and press enter (for real mac do this instead)
4. you will soon reach a prompt
5. note the lines written to the console about fsck and mount
6. write: /sbin/fsck -fy
7. press enter and then the system should write:
   ** The volume XXX appears to be OK
   or
   ***** FILE SYSTEM WAS MODIFIED *****
   if it is the second option, then repeat the fsck command (6) until you get the first line saying the volume is OK
8. write: /sbin/mount - uw / this will mount your file system
9. thats it you are in "Single User"
   

Enjoy.