Re: want to know more about single user mode
Single user mode is used for repair of some failure - such as the desctruction of the /etc/shadow password file.
Single user mode is an console interactive for root only. That means that usually, none of the disk mounts, other than root, may not be mounted, and root itself may be mounted read only (usually only occurs after a crash/power failure where the system is not shutdown gracefully, AND automatic repairs fail). By definition, only one user may be logged in (root).
There is no network, no general services (audio/USB/whatever). Using the root shell allows you to run most things (but not using a GUI), and is command line only. It can be used to install software, if the software is locally available (rpm/tarball/whatever). Sometimes (depending on the system) you cannot even compile things (Fedora doesn't have that restriction as it has nearly all software in the root filesystem).
Normally, when this single user account logs out (via control D or exit of the top level shell) the system will then attempt to come up in multiuser mode.