'''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]

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[Category Command] for [TclX] package.