101 - Practice - Linux & Networking

1. Key Concepts

👶 Child Process

A child process is created by a parent process using fork() or similar system calls.

Example:

 

pid_t pid = fork(); if (pid == 0) {     // child } else {     // parent }

🧟 Zombie Process

A zombie process is a child that has terminated, but its parent hasn’t read its exit status via wait() or waitpid().

• It still has a PID and entry in the process table.
  
  

• Shows up as <defunct> in ps.
  
  

👻 Orphan Process

An orphan process is a child whose parent has exited before the child. It is adopted by init (PID 1) or systemd.

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🔧 2. Essential Linux Commands

Command	Purpose
ps -ef	Show all processes
ps -eo pid,ppid,stat,cmd	Show child/parent relationships
pstree	Tree of processes
top / htop	Monitor live process usage
strace -p <pid>	Trace system calls of a process
lsof -p <pid>	List open files by a process
kill -9 <pid>	Force kill a process
wait <pid>	Wait for child to finish
cat /proc/<pid>/status	Inspect process status

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🧠 3. 20 FAANG-Level Debugging Scenarios

🔸 Process Relationships & Lifecycle

1. How can you confirm if a process has become a zombie?
   
   

Use:

ps -eo pid,ppid,stat,cmd | grep Z

• 
  

2. How can you identify if a process is an orphan?
   
   

Check if the parent PID is 1:

ps -eo pid,ppid,cmd | awk '$2==1'

• 
  

3. Simulate a zombie process in C and verify via shell.
   
   

C code:

#include <stdlib.h> #include <unistd.h> int main() {     if (fork() == 0) {         exit(0); // Child exits     } else {         sleep(100); // Parent doesn't wait     } }

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Shell:

ps aux | grep defunct

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4. How can you reparent an orphaned process?
   
   

Let the parent exit before the child:

setsid ./long_running_child.sh

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5. What does the STAT column in ps output mean for zombies?
   
   

• Z = zombie
  
  

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🔸 Monitoring & Debugging Tools

Trace a running process to monitor its system calls.

strace -p <pid>

6. 
   

How do you detect file descriptors leakage in child processes?

lsof -p <pid>

7. How can you monitor orphan/zombie processes over time?

            watch "ps -eo pid,ppid,stat,cmd | egrep 'Z|^[0-9]+ +1 '"

8. Check if a parent process is cleaning up zombies.
   
   

Use:

ps -eo pid,ppid,stat,cmd

• 
  

10. Forcefully kill a zombie process.
    
    

• You can't. You must kill its parent or let it wait().
  
  

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🔸 Advanced Debugging Scenarios

11. You observe many <defunct> processes in production. What could be the cause?
    
    

• Parent not handling SIGCHLD or not calling wait().
  
  

12. Why do orphan processes not create resource issues like zombies?
    
    

• Because init or systemd reaps them properly.
  
  

Write a shell script that checks for zombie processes and alerts.

#!/bin/bash ZOMBIES=$(ps -eo stat,ppid | grep -w Z | wc -l) if [ "$ZOMBIES" -gt 0 ]; then     echo "Zombie processes detected: $ZOMBIES" fi

13. 
    

14. A child exits immediately but ps still shows it—why?
    
    

• It’s a zombie; parent hasn’t reaped it.
  
  

15. How do you make sure zombies are cleaned in C?
    
    

Use:

signal(SIGCHLD, SIG_IGN); // Auto-reaps children

• 
  

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🔸 System Design + Debugging

16. How do large-scale daemons prevent zombie processes?
    
    

• Use SIGCHLD handler + waitpid(-1, ...)
  
  

17. During a fork bomb attack, what behavior do you expect in the process tree?
    
    

• Many zombie/defunct or orphaned processes. System will slow.
  
  

18. How does systemd handle orphaned processes differently than init?
    
    

• It tracks child services and can impose cgroups-level resource limits.
  
  

19. What issue occurs if you fork but never exec or wait?
    
    

• Memory usage spikes; system can hit PID limits (e.g. 32768).
  
  

20. Design a resilient background service that doesn't leave zombie children.
    
    

• Use fork() with waitpid() + double-fork + SIGCHLD handling.
  
  

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🧪 Bonus: Create Zombie and Orphan Processes for Practice

# Create Zombie bash -c 'sleep 100 & echo $! > child.pid; exit' # Create Orphan bash -c 'setsid bash -c "sleep 100"'