Why is the most powerful user on a Unix/Linux system called “root?”
The ancient precursor to the precursor of Linux (Unix) was Multics operating system and in Multics the privileged user’s home directory was the root directory.
And some super-creative type came up with the name root for the privileged user. And it stuck, because Unix was created by people who had worked on Multics, e.g., Dennis Ritchie or Ken Thompson, the original writers of Unix.
Probably there is not a reliable source for it, there are only assumptions as mine, similar to the following: “The name root may have originated because root is the only user account with permission to modify the root directory of a Unix system.” from Wikipedia: Superuser.
Some premises
- It is not advised (really not advised) but you can change that name.
- It was not always so [1]
some early UNIX systems (particularly ones from CMU) called the user "avatar" - to designate the special state and try to dissuade people from logging in as it. In those same old days, since BSD systems used /bin/csh as the default root shell, sometimes you would see systems with the login sroot and kroot as a second & third line in the password file with /bin/sh and /bin/ksh as the default shell.
Simply It becomes the prevailing one.
It's related to the fact that the root user is the only to have the ownership of the root folder (/) on which all the file system resides (all other files and directories, including their subdirectories, and files), so de facto as root of all powers and privileges, of all processes (remember that init is the root of the processes and it is owned by root too) and of the critical processes, the daemons ...
It's possible to find some support more on the Linux Information Project states in its root page [2]
The use of the term root for the all-powerful administrative user may have arisen from the fact that root is the only account having write permissions (i.e., permission to modify files) in the root directory.
Stripping some text more from the root page of The Linux Information Project [2]
root is the user name or account that by default has access to all commands and files on a Linux or other Unix-like operating system. It is also referred to as the root account, root user and the superuser.
...
Root privileges are the powers that the root account has on the system. The root account is the most privileged on the system and has absolute power over it (i.e., complete access to all files and commands). Among root's powers are the ability to modify the system in any way desired and to grant and revoke access permissions (i.e., the ability to read, modify and execute specific files and directories) for other users, including any of those that are by default reserved for root.
...
The use of the term root for the all-powerful administrative user may have arisen from the fact that root is the only account having write permissions (i.e., permission to modify files) in the root directory. The root directory, in turn, takes its name from the fact that the filesystems (i.e., the entire hierarchy of directories that is used to organize files) in Unix-like operating systems have been designed with a tree-like (although inverted) structure in which all directories branch off from a single directory that is analogous to the root of a tree.... (speaking about the Unix beginning)
It was also necessary to have a means for enabling a system administrator to perform such tasks as entering user directories and files to correct individual problems, granting and revoking powers for ordinary users, and accessing critical system files to repair or upgrade the system.
(Here it is implied the need of a hierarchy and a root point from which to start from.)
BTW it's seems that the name God was just taken and could stir up resentments and protests...