Since the owner and group arguments to chown and
chgrp may be specified as names or numeric IDs, there is an
What if a user or group name is a string of digits?
Should the command interpret it as a user name or as an ID?
POSIX requires that chown and chgrp
first attempt to resolve the specified string as a name, and
only once that fails, then try to interpret it as an ID.
This is troublesome when you want to specify a numeric ID, say 42,
and it must work even in a pathological situation where
‘42’ is a user name that maps to some other user ID, say 1000.
chown 42 F, will set Fs owner ID to
1000—not what you intended.
GNU chown and chgrp provide a way to work around this, that at the same time may result in a significant performance improvement by eliminating a database look-up. Simply precede each numeric user ID and/or group ID with a ‘+’, in order to force its interpretation as an integer:
chown +42 F chgrp +$numeric_group_id another-file chown +0:+0 /
GNU chown and chgrp skip the name look-up process for each ‘+’-prefixed string, because a string containing ‘+’ is never a valid user or group name. This syntax is accepted on most common Unix systems, but not on Solaris 10.
 Using a number as a user name is common in some environments.