Linux in practice: processes, threads and zombies

On a Linux machine:

1. ps -ef --forest | head -40 and pstree -p | head -20 — find PID 1 and your shell. Which process is the parent of your shell?
2. Run sleep 300 &, then cat /proc/$!/status | head -12 — report State, PPid, Threads, VmRSS and explain each.
3. Make a zombie. Save and run this Python program (or write the C/Java equivalent):

   import os, time
   pid = os.fork()
   if pid == 0:
       os._exit(0)          # child exits immediately
   time.sleep(60)           # parent does NOT call wait()
   

   While it sleeps, run ps -o pid,ppid,stat,cmd -p <childpid> — what state do you see? What happens after the parent ends?
4. Run a multithreaded program (e.g., python3 -c "import threading,time; [threading.Thread(target=time.sleep,args=(60,)).start() for _ in range(5)]") and inspect it with ps -eLf | grep python or top -H -p <pid>. How many kernel threads (LWP) do you see, and why?
5. nice -n 10 yes > /dev/null & then top — what are the NI and PR values? Stop it with kill <pid>; which system call did the kill command use, and which signal is sent by default?

Report key output lines and explain them with this chapter's concepts.