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Mastering Linux Processes: A Comprehensive Guide

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    Linux Bash
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Processes, Programs & Threads: The Trinity

  • Program: Static executable file (e.g., /bin/bash).
  • Process: Running instance of a program with unique PID, memory, and resources.
  • Thread: Lightweight sub-process sharing memory with parent.
ps -eLf | grep firefox  # View processes + threads

Process Lifecycle & Control

Attributes/States:

  • States: Running (R), Sleep (S), Zombie (Z), Stopped (T).
  • Permissions: Inherit user/group of parent process.

  • Limits:

    ulimit -a                 # Show current limits

ulimit -n 4096            # Set max open files

Creation & Job Control:

  1. Forking: fork() duplicates parent → exec() loads new program.

  2. Background/Foreground:

    sleep 60 &               # Run in background (output still prints)

bg %1                    # Resume stopped job in background

fg %1                    # Bring to foreground

Scheduling:

  • One-time:
    bash echo "tar -zcf backup.tar.gz ~/docs" | at 3:00 AM
  • Periodic (cron):
    cron */5 * * * * /scripts/monitor.sh # Run every 5 mins

Process Priorities & Monitoring

Priorities:

  • Nice: -20 (highest) to 19 (lowest).
    ```bash nice -n 10 ./compile.sh # Start low-priority

renice -n 5 -p 1234 # Change priority of running PID ```

Monitoring Tools:

  • ps:
ps aux                    # All processes (user, PID, %CPU, command)
ps -o pid,ppid,ni,cmd -C sshd  # Custom fields for SSH daemon
  • pstree: Visualize hierarchies:
    bash pstree -p | grep java # Show Java thread ancestry
  • top: Real-time view (press P sort by CPU, M by memory).

Memory Management

Key Metrics:

  • /proc/meminfo: Total, free, cached RAM.
  • /proc/sys/vm/swappiness: 0-100 (aggressiveness of swap usage).
    bash sysctl vm.swappiness=10 # Prefer RAM over swap

Tools:

  • vmstat: Memory, swap, I/O, CPU:
    bash vmstat 2 5 # Report every 2s, 5 times
  • Swap Management:
    bash swapon -s # List swap areas dd if=/dev/zero of=/swap2 bs=1M count=2048 && mkswap /swap2 # Create new swap
  • OOM-Killer: Kills processes when RAM exhausted (logs in /var/log/kern.log).

I/O Monitoring & Bottlenecks

Symptoms: High await (I/O wait time), sluggish system.

Tools:

  • iostat: Per-device I/O stats:
    bash iostat -dxh 2 # Extended metrics every 2s
  • iotop: Real-time I/O by process (run as root):
    bash iotop -o # Show active I/O only

User vs. Kernel Mode

  • User Mode: Apps run restricted (cannot access hardware directly).
  • Kernel Mode: OS/core drivers run with full privileges.
    bash dmesg | grep "segfault" # Catch crashes from user-mode violations

Daemons: Background Guardians

  • Detached from terminal (e.g., sshd, nginx).
  • Managed via systemd:
    bash systemctl status cron # View daemon state

Troubleshooting Checklist

  1. High CPU? → top → Identify runaway process.
  2. High RAM? → vmstat → Check swap usage.
  3. I/O Bottleneck? → iotop → Find disk-hogging process.
  4. Zombies? → ps aux | grep 'Z' → Kill parent process.

Pro Tip: Use strace to trace system calls:

strace -p 1234          # Debug process 1234

Key Takeaways

  • Processes ≠ Programs: Processes are running entities with resources.
  • Prioritize wisely: nice prevents resource starvation.
  • Memory/swap tuning avoids OOM-killer chaos.
  • I/O monitoring reveals hidden bottlenecks.
# Generate a process snapshot for debugging: 
(echo "==== $(date) ===="; ps aux; vmstat; iostat) > system_snapshot.txt

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