Remova as duas barras invertidas na linha useradd
ou, em outras palavras, não escape das aspas.
useradd $id -m -c "$fn $ln" -G students
PS: o ponto
GNU Coreutils: What information is listed
-l, --format=long, --format=verbose
...
Following the file mode bits is a single character that specifies whether an alternate access method such as an access control list applies to the file. When the character following the file mode bits is a space, there is no alternate access method. When it is a printing character, then there is such a method.
GNU ls uses a ‘.’ character to indicate a file with a security context, but no other alternate access method.
A file with any other combination of alternate access methods is marked with a ‘+’ character.