Creates a pipe. If fileId_var_r
and fileId_var_r are specified, then pipe will set the
variable named fileId_var_r to
contain the fileId of the side of the pipe that was opened for
reading, and set fileId_var_w to contain the fileId
of the side of the pipe that was opened for writing.
If the fileId variables are not specified, then a
list containing the read and write fileIds is returned as the
result of the command.