Shell Programming and Scripting

               str                  str   has  non-zero  length.
                                    Note  that  there   is   the
                                    potential  for  problems  if
                                    str turns out to be an oper-
                                    ator (e.g., -r) - it is gen-
                                    erally better to use a  test
                                    like
                                                  [  X"str" != X
                                                  ]
                                           instead       (double
                                           quotes  are  used  in
                                           case   str   contains
                                           spaces  or file glob-
                                           ing characters).