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What's the origin of root account? Where did it come from and why is it called root anyway?

(Originally asked by @lizztheblizz on Twitter.)

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I'm going with Ken Thompson's appreciation of root beer :D –  user5090 Feb 24 '11 at 20:14
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up vote 19 down vote accepted

The original home directory of the root user was the root of the filesystem / (http://minnie.tuhs.org/cgi-bin/utree.pl?file=V5/etc/passwd). I think the user was indeed named after that directory. But why 'root' and not 'start' or 'origin' or something else? Well, before Ken Thompson and Dennis Ritchie wrote UNIX, they were (also at Bell Labs) developing Multics. If you take a look at Multics history, you will find that ROOT existed there too (http://web.mit.edu/multics-history/source/Multics_Internet_Server/Multics_mdds.html). So the name must come from Multics.

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I just looked at the Multics documents you linked to. They refer to the root directory (and the root volume), but there's no reference to a root user. –  Keith Thompson Jul 3 '12 at 22:47
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According to Linux Info, the origin of the name may have come from file system layout / permissions:

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.

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