Dots are at least forbidden charactes in usernames and groupnames as chown will fail with such names.
The Linux Standard Base (LSB) uses the Single UNIX Specification (SUS)
with some overriding differences for defining the behavior of commands and utilities like "chown".
UNIX long ago used the period ("."), also known as the "full stop", to separate the user name from the group name in "chown", as in
"USER.GROUP".
The SUS now defines user and group names to be composed of characters from the "Portable Filename Character Set" (PFCS) which consists of uppercase alphabetics ("A" to "Z"), lowercase alphabetics ("a" to "z"), decimal numerals ("0" to "9"), the period ("."), the underscore ("_"), and the hyphen ("-"), except that the first character of the user or group name can not be a hyphen so as to avoid having the name interpreted as a commandline option. When the SUS allowed the period in user and group names, the colon (":") was chosen as the separator in "chown" instead of the period because the colon was not in the PFCS and the colon was not allowed in traditional user or group names since it was the separator in the user and group databases ("/etc/passwd" and "/
etc/group").
FreeBSD, NetBSD, and OpenBSD now use the colon to separate the user name from the group name in "chown" and allow periods in user and group names just as in the SUS. Note that the new BSDs recommend against using periods (as well as uppercase alphabetics) in user or group names.
The LSB currently follows both the old UNIX use of the period to separate the user and group names in "chown" and also the SUS definition of allowing periods in user and group names. Thus, either the colon or the period is used as the user name and group name separator in "chown". I think that the use of the period as the LSB "chown" separator should be deprecated.
To summarize the current situation:
----------------------------------------- | tradition | separator | period | | | in "chown" | in name | ----------------------------------------- | old UNIX | period | NO | | new BSDs | colon | YES | | SUS | colon | YES | | LSB | colon or period | YES | -----------------------------------------
The LSB web page is "http://www.linuxbase.org/";.
The SUS web page is "http://www.opengroup.org/products/publications/catalog/un.htm";
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