07-17-2019
I don't know of any OS that randomly picks the next PID to use when creating a new process. But, it is theoretically possible to do it that way.
Most of the systems I have worked on have a maximum PID value that is a configuration parameter. They start out assigning PIDs from 1 and increment the PID they want to assign by one every time a new process is created (e.g., by fork()), but before assigning that value, they check to see if that PID is currently in use. If it is in use, the OS tries again with the next possible PID until it finds one that is not in use. Using this method, you won't run into problems with PID recycling unless there are so many processes running on your system that you don't have many possible PIDs available that aren't in use.
Furthermore, if you have a ps utility that conforms to POSIX requirements, you can ask ps to just print the fields you want to see and not have to worry about some unexpected value in a field you aren't interested in containing what looks like a PID. (Look at your man page for the ps -o format option.)
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pid(3tcl) Tcl Built-In Commands pid(3tcl)
__________________________________________________________________________________________________________________________________________________
NAME
pid - Retrieve process identifiers
SYNOPSIS
pid ?fileId?
_________________________________________________________________
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 is not a process pipeline. If no fileId argument is given then pid returns the process identi-
fier of the current process. All process identifiers are returned as decimal strings.
EXAMPLE
Print process information about the processes in a pipeline using the SysV ps program before reading the output of that pipeline:
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
SEE ALSO
exec(3tcl), open(3tcl)
KEYWORDS
file, pipeline, process identifier
Tcl 7.0 pid(3tcl)