Over the years, I have seen a couple of variations with the use of the chown command. The ":" (colon) vs "." (dot). While studying for the RHEL6 exam sometime ago, the books that I had used each had one or the other in their documentation. I have read that the posix standard is using chown with the ":" (colon).
A quick test reveals why the ":" (colon) is the better fit.
chown user.group /path chown user:group /path
Nothing unusual there, both commands should work with the same predictable result.
This will work if both the user and the group are the same.
chown user. /path chown user: /path
How about a username with a "." (dot) as part of the name?
chown first.last. /path chown first.last.group /path
Neither one of those will work.
However, this one will.
chown first.last: /path chown first.last:group /path