man s PID

pid(n)                       Tcl Built-In Commands                      pid(n)

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NAME
       pid - Retrieve process id(s)

SYNOPSIS
       pid ?fileId?
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DESCRIPTION
       If  the  fileId  argument  is  given then it should normally refer to a
       process pipeline created with the open command.  In this case  the  pid
       command  will  return a list whose elements are the process identifiers
       of all the processes in the pipeline, in order.  The list will be empty
       if  fileId refers to an open file that isn't a process pipeline.  If no
       fileId argument is given then pid returns the process identifier of the
       current  process.   All  process  identifiers  are  returned as decimal
       strings.

KEYWORDS
       file, pipeline, process identifier

Tcl                                   7.0                               pid(n)