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In the POSIX Definitions there is specific section 3.424, User Database, which defines what information should be related to each user ID. However through out the document, I couldn't find (at least not via simple Ctrl+f search) mention of either root nor any of the typical system usernames, such as syslog, bin, daemon, nobody, sys. There doesn't seem to be mention of these in newgrp utility definitions either, to which the 3.424 refers.

This begs the question: does POSIX require existence of certain system-level usernames or UIDs ? Is root user even required to be root, or is it theoretically possible to have POSIX-compliant system where the most powerful system account is not called root or has non-zero UID?

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    POSIX explicitly does not standardize system utilities and administration part. – Giacomo Catenazzi Jan 16 '19 at 11:42
  • @GiacomoCatenazzi Any source for that statement ? I am curious to which extent then POSIX does limit administration part. But utilities and interfaces are in fact standardized, at least basic utilities such as cat, /bin/sh, etc – Sergiy Kolodyazhnyy Jan 16 '19 at 11:48
  • Now I do not find it explicitly (but the first paragraph of the standard tell us that it is mostly about shell utilities and API for programmers and users (but not for administration). In fact you do not find very common "su, passwd, mount" as utilities. "init" was cited only once (and just for zombies handling, not the system initialization) – Giacomo Catenazzi Jan 16 '19 at 12:54
  • Note: you linked the old POSIX. [It doesn't matter much on this question, but I want to put it into your attention] – Giacomo Catenazzi Jan 16 '19 at 12:55
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That is a very good question. To which the simple answer would be no.

There is no guarantee, and you could even rename root to toto.

But your system would probably be unstable, since a lot of services and functions depend on certain usernames, typically, root.

  • That is a very tempting username, toto , I might consider using it in future :) What about the UID itself ? Would these be "set in stone" so to speak ? – Sergiy Kolodyazhnyy Jan 16 '19 at 11:49
  • I agree, thinking to get toto's privileges. For the UID, 0 is the only one that is really always the admin user. The up to 99 are usually reserved. Users used to start at 500, and now at 1000. And the link here : linfo.org/uid.html – jayooin Jan 16 '19 at 11:58

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