code: public static final void setGroup(String name, Path... files) throws IOException
      {
          if (supports("posix"))
          {
              for (Path file : files)
              {
                  FileSystem fs = file.getFileSystem();
                  UserPrincipalLookupService upls = fs.getUserPrincipalLookupService();
                  GroupPrincipal group = upls.lookupPrincipalByGroupName(name);
                  PosixFileAttributeView view = Files.getFileAttributeView(file, PosixFileAttributeView.class);
                  view.setGroup(group);
              }
          }
          else
          {
              JavaLogging.getLogger(PosixHelp.class).warning("no posix support. setGroup(%s)", name);
          }
      }
docs: Change group of given files. Works only in linux
  @param name
  @param files
  @throws IOException
  @see org.vesalainen.util.OSProcess#call(java.lang.String...)