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What is Single User mode used for? Can it override the root Password? Do you need a password to acces the SU mode?

In a normal session when you run SU command does it stands for Single User or Super User?

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  • su means switch user, not superuser. The command for running something as superuser is sudo
    – Wes Sayeed
    Apr 5, 2016 at 2:29

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Single user mode and su aren't exactly the same, if you're referring to the boot option single.

Single user mode (from adding single to grub's boot commands or add init="/bin/bash" to the kernel line) should basically let you log in with root access & change just about anything. Basically:

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 can not be mounted successfully. You can not use single-user mode if the runlevel 1 configuration on your system is corrupted.

(from Red Hat docs)

su looks like it could stand for super user... but I'm not holding my breath on finding reliable definitive info on the origin of su's name. Probably like other "old school" unix/linux names, it could be shorthand for something, or just for fun, self-referencing acronym, a couple's names or initials (debian), just about anything really.

According to the (sometimes?) reliable Wikipedia:

The Unix command su, also referred to as substitute user, super user, or switch user, is used by a computer user to execute commands with the privileges of another user account. When executed it invokes a shell without changing the current working directory or the user environment.

When the command is used without specifying the new user id as a command line argument, it defaults to using the superuser account (user id 0) of the system.

Also interesting on sudo:

The command sudo is related, and executes a command as another user but observes a set of constraints about which users can execute which commands as which other users. Unlike su, sudo authenticates users against their own password rather than that of the target user

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