File: coreutils.info,  Node: Disambiguating names and IDs,  Next: Random sources,  Prev: Signal specifications,  Up: Common options

2.4 chown, chgrp, chroot, id: Disambiguating user names and IDs
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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 ‘F’s 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.