http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/pid.htm 

    :   '''pid''' ?''channel''?

Returns the process ID of the current process, or the process IDs of the programs running in the pipeline (created by [open]) that ''channel'' “names”.

When you have used [open] to open
a pipeline command - you can then get the process id to do a kill, etc.


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