ParallelMap returns incorrect results over Map

Question

I've written a function to pull in the useful column of a large data file:

ImportNthColumn[fname_, n_] := With[{
  command = "!awk '{ print $" <> ToString[n] <> " }' "
 },
  Flatten@Import[command <> fname, "Table"]
]

and I'd like to apply this function to pull in data from every subdirectory, like so:

ImportNthColumnFromDirectories[dirs_, fname_, n_] := With[{
  targets = Map[# <> "/" <> fname&, dirs]
},
  Map[ImportNthColumn[#,n]&, targets]
]

When I use ImportNthColumnFromDirectories with a serial map, everything works perfectly. When I use ParallelMap, however, the data in the first and last elements of dirs import correctly but all other iterations return a zero-length list. What could cause this strange behavior?

Answer 1

Documentation say:

Import["!prog","format"] imports data from a pipe.

I suggest that ParallelMap sent many requests to external program in one time and it return errors, but Mathematica interpret errors as empty list.

PS: sorry for my english