pipe ?fileId_var_r ?fileId_var_w
Create a pipe. If fileId_var_r and fileId_var_r are specified, then pipe will set the a variable named fileId_var_r to contain the fileId of the side of the pipe that was opened for reading, and fileId_var_w will contain the fileId of the side of the pipe that was opened for writing.
If the fileId variables are not specified, then a list contain- ing the read and write fileIdw is returned as the result of the command.
There is a proposal tclx file commands for core
Category Command for TclX package.