Quoting "Miner, Jonathan W (CSC) (US SSA)"
<jonathan.w.miner baesystems.com>:
> The chown(1) man page says that a colon ":"
is the delimiting character:
>
> NAME
> chown - change file owner and group
>
> SYNOPSIS
> chown [OPTION]... [OWNER][:[GROUP]] FILE...
Couple of lines down in the same man page is the following
sentence:
"If the user name is followed by a colon or dot and a
group name (or
numeric group ID), with no spaces between them, the group
ownership of
the files is changed as well."
Chown in Linux uses dot as alternative separator between
user name and
group name for compatibility with some Unix systems that use
dot
separator. Dot was choosen as separator since it is not
allowed in
user names. On Unix systems, it is advisable to limit
yourself to
8-chars usernames (most utilities will work fine with longer
usernames, but not all of them). And also to limit yourself
what
characters you are using.
Without looking in the manual page or trying it out on
command line,
could you answer these questions.
1. If you have users foo and foo.bar, and group bar, what
will be
result of "chown foo.bar filename"?
A. chown will exit with error
B. chown will change file owner to foo.bar, and leave the
group unchanged
C. chown will change file owner to foo and group to bar
D. chown will change file owner to foo.bar and group to
bar
2. If you have users foo and foo.bar, but there is no group
bar, what
will be the result of "chown foo.bar filename":
A. chown will complain group bar does not exist
B. chown will change file owner to foo.bar
C. chown will change file owner to foo
3. Same as above, but you have only user foo.bar.
Any program that validates user supplied data could (and
should)
reject usernames with dots if supplied in argument list
(just like
useradd does). This should especially be the case for Web
applications (CGI scripts, PHP, ...).
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I think it
is for this.
You can have elegant, long personal names, and short ones
for login! 
Good luck,
Balázs
--
>>> When i create username like this
>>> "useradd user.name"
>>> It shows the error message saying that the name
contains invalid
>>> characters
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