Shell Programming and Scripting

       [[ expression ]]
              Similar to the test and [ ... ] commands (described later), with
              the following exceptions:
                ·    Field splitting and file name  generation  are  not  per-
                     formed on arguments.
                ·    The  -a  (and) and -o (or) operators are replaced with &&
                     and ||, respectively.
                ·    Operators (e.g., -f, =, !, etc.) must be unquoted.
                ·    The second operand of != and = expressions  are  patterns
                     (e.g., the comparison in
                                        [[ foobar = f*r ]]
                     succeeds).
                ·    There  are two additional binary operators: < and > which
                     return true if their first string operand is  less  than,
                     or  greater  than,  their  second string operand, respec-
                     tively.
                ·    The single argument form of  test,  which  tests  if  the
                     argument  has  non-zero  length,  is not valid - explicit
                     operators must be always be used, e.g., instead of
                                              [ str ]
                     use
                                           [[ -n str ]]
                ·    Parameter, command and arithmetic substitutions are  per-
                     formed  as  expressions are evaluated and lazy expression
                     evaluation is used for the &&  and  ||  operators.   This
                     means that in the statement
                                  [[ -r foo && $(< foo) = b*r ]]
                     the  $(<  foo)  is  evaluated if and only if the file foo
                     exists and is readable.