:~$ hadoop dfs -chown aaa_bbb /user/knoguchi/test.txt
chown: 'aaa_bbb' does not match expected pattern for [owner][:group].
in 0.16.1, only alphabets and numbers are allowed. Shouldn't '_' be allowed?
I couldn't find any standard, but in Solaris10, it's defined as
The login (login) and role (role) fields accept a string of no more than eight bytes consisting of characters from the set of alphabetic characters, numeric characters, period (.), underscore (_), and hyphen . The first character should be alphabetic and the field should contain at least one lower case alphabetic character. A warning message is displayed if these restrictions are not met.