The double-equal is the correct syntax for ksh93, although it will tolerate the single-equal to be backward-compatable with ksh88. In ksh88 (the default AIX shell) the single-equal is correct, but the double-equal is sometimes tolerated. By "sometimes" I mean the currently supported AIX versions. I don't believe it will work with ksh under AIX43 (or even AIX51) but these have both been out of support for some time.