'''pipe ?fileId_var_r ?fileId_var_w'''

Create  a pipe.  If fileId_var_r and fileId_var_w 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 containing
the read and write fileIdw is returned as the result of the command.

There is a proposal [tclx file commands for core]
[[seems like something is missing here...]]

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[[We also could talk about the abstract notion of "pipe", and how it comes up in conversation
far more than just its use in reference to this command.]]

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Can somebody give an example for this? My attempt to use

 package require Tclx
 pipe rpipe wpipe;
 puts $wpipe "test\n";
 read $wpipe

simply leads to a lock up. Any help appreciated ;). 

The problem is that read tries to read to the end of file. Since a pipe does not have 
and end of file indication ( unless one of the ends is closed) read never returns. If 
you use the following  or used read with a specific capture size as seen in the 2nd 
example 

----
#this seems to work 
package require Tclx
pipe rpipe wpipe;
puts $wpipe "test"
flush $wpipe
puts "[gets $rpipe]"

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and so does this more elaborate version using read with a capture size of 1000 bytes
the larger the capture size the less likely you will have to loop more than once. Also critical
was using fconfigure to make it non-blocking.  
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fconfigure $rpipe -blocking 0 
puts $wpipe "test"
flush $wpipe
set buff ""
set done 0 
while { !$done } {
    set appstr [ read $rpipe 1000 ]
    append buff $appstr
    if { [ string length $appstr ] != 1000 } {
	set done 1;
    } 
}
puts "$buff"
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[Category Command] for [TclX] package.