pipe [fileId_var_r fileId_var_w]
Creates a pipe. If fileId_var_r and fileId_var_w 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.