Since the user and group arguments to these commands
may be specified as names or numeric IDs, there is an
apparent ambiguity.
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?
(Using a number as a user name is common in some environments.)
POSIX requires that these commands
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.
Simply invoking chown 42 F, will set Fs owner ID to
1000 – not what you intended.
GNU chown, chgrp, chroot, and id
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 /
The name look-up process is skipped 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.