Have you locked yourself out of your linux machine? If by any chance you forgot the root password of your linux box and you have physical access to that machine, booting linux in single user mode will do the trick. Restart your computer using one of the following commands:

 $shutdown -r now

OR        

 $init 6  

OR       

 $reboot

Then select the Linux kernel from the boot menu, type “e” (for edit). Then select the second line which starts with the word “kernel” and type “e”. Then press “space bar” and type “single” (for linux single user mode). Finally press “Enter” and type “b” for boot. Go ahead change the password now with 

$passwd