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.
set pipeline [open "| zcat somefile.gz | grep foobar | sort -u"] # Print process information exec ps -fp [pid $pipeline] >@stdout # Print a separator and then the output of the pipeline puts [string repeat - 70] puts [read $pipeline] close $pipeline