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Why must a user in POSIX-compliance have a Primary Group? Why can't they just belong to groups?

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The primary group is assigned to files created by that user...

  • So touch a file, and by default it will have the group of the Primary Group for that user. – leeand00 Feb 26 at 16:59
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    … leading the questioner to come back with Why do files need to have primary groups? Why can't they just have group ACL entries?. (-: – JdeBP Feb 26 at 17:07
  • Yes (at least on Linux, BSDs are somewhat different, and a directory with setgid bit set behaves differently.) I'd say that's the most apparent effect of the primary group. – filbranden Feb 26 at 17:08

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