FileNameJoin
[{"$dir_1$“, ”$dir_2$", ...}]FileNameJoin[..., OperatingSystem->“os”]
FileNameJoin[{"dir1", "dir2", "dir3"}]
FileNameJoin[{"dir1", "dir2", "dir3"}, OperatingSystem -> "Unix"]
FileNameJoin[{"dir1", "dir2", "dir3"}, OperatingSystem -> "Windows"]