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I followed these instructions to build Shadow, which provides the groupadd command. I am now getting an error when trying this:

$ groupadd automake1.10
groupadd: 'automake1.10' is not a valid group name

I checked alphanumeric names, and they work okay.

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2 Answers 2

up vote 9 down vote accepted

See the source code, specifically libmisc/chkname.c. Shadow is pretty conservative: names must match the regexp [_a-z][-0-9_a-z]*\$? and may be at most GROUP_NAME_MAX_LENGTH characters long (configure option, default 16; user names can usually go up to 32 characters, subject to compile-time determination).

Debian relaxes the check a lot. As of squeeze, anything but whitespace and : is allowed. See bug #264879 and bug #377844.

POSIX requires allowing letters of either case, digits and ._- (like in file names). POSIX doesn't set any restriction if you don't care about portability. A number of recommended restrictions come from usage:

  • Colons, newlines and nulls are right out; you just can't use them in /etc/passwd or /etc/group.
  • An name consisting solely of digits is a bad idea — chown and chgrp are supposed to treat a digit sequence as a name if it's in the user/group database, but other applications may treat any number as a numerical id.
  • An initial - or a . in a user name is strongly not recommended, because many applications expect to be able to pass $user.$group to an external utility (e.g. chown $user.$group /path/to/file)¹. A . in a group name should cause less trouble, but I'd still recommend against it.
  • / is likely to cause trouble too, because some programs expect to be able to use user names in file names.
  • Any character that the shell would expand is probably risky.
  • Non-ASCII characters should be ok if you don't care about sharing with systems that may use different encodings.

¹ All modern implementations expect chown $user:$group, but support chown $user.$group for backward compatibility, and there are too many applications out there that pass a dot to remove that compatibility support.

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Re chown argument: the current syntax, at least in GNU coreutils, is user:group, with dot being accepted only for compatibility. One can use j.smith:j.smith. –  grawity Apr 17 '11 at 10:03
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@grawity: It's not just GNU coreutils, but the problem isn't chown itself, it's existing scripts and other programs that call chown $user.$group instead of chown $user:$group — even if the chown implementation tries to do the right thing, some cases are intrinsically ambiguous. –  Gilles Apr 17 '11 at 11:58
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If you're feeling adventurous, you can edit /etc/group directly and put in whatever group name you like. Also, this has the added bonus that when you encounter one of the problems @Gilles mentioned, you may not be able to load an editor to fix the problem, or even log in at all - giving you valuable experience in recovering a broken system!

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Don't forget to edit /etc/gshadow when adding groups. Also, use vigr(8) rather than directly editing the files. –  camh Apr 17 '11 at 5:30
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