README.md

⚙️ Chapter 10: Process Management

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📑 Table of Contents

• What is a Process?
• Viewing Processes
• Process States
• Foreground & Background
• Signals & Killing Processes
• Process Priority (nice & renice)
• The /proc Filesystem
• System Resource Monitoring
• Practice Exercises

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What is a Process?

A process is a running instance of a program. Every process has:

Attribute	Description	Example
PID	Process ID (unique number)	1234
PPID	Parent Process ID	1 (init/systemd)
UID	User who owns the process	1000 (sovon)
State	Current status	Running, Sleeping
Priority	CPU scheduling priority	20 (normal)
Memory	RAM usage	15.2 MB

┌────────────────────────────────────┐
│ PID 1: systemd (init)              │
├──────┬─────────┬───────────────────┤
│ PID  │ PID     │ PID               │
│ 200  │ 300     │ 400               │
│sshd  │ cron    │ login             │
│      │         ├──────┬────────────┤
│      │         │ PID  │ PID        │
│      │         │ 500  │ 600        │
│      │         │ bash │ bash       │
│      │         │      ├─────┬──────┤
│      │         │      │ vim │ grep │
└──────┴─────────┴──────┴─────┴──────┘

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Viewing Processes

ps — Process Snapshot

ps                              # Your processes in current terminal
ps aux                          # ALL processes, all users, detailed
ps -ef                          # ALL processes, full format
ps aux --sort=-%mem | head -10  # Top 10 memory-consuming processes
ps aux --sort=-%cpu | head -10  # Top 10 CPU-consuming processes
ps -u sovon                     # Processes owned by user sovon
ps -p 1234                      # Info about specific PID
ps -C nginx                     # Processes named "nginx"
ps --forest                     # Process tree view
ps -eo pid,ppid,user,%cpu,%mem,cmd --sort=-%cpu  # Custom columns

top — Real-Time Monitor

top

Top keyboard shortcuts:

Key	Action
q	Quit
k	Kill a process (enter PID)
r	Renice a process
M	Sort by memory
P	Sort by CPU
T	Sort by time
1	Show individual CPU cores
c	Show full command path
h	Help

htop — Better top

sudo apt install htop
htop

htop provides a colorful, interactive process viewer with:

• CPU/memory bars
• Mouse support
• Tree view (F5)
• Filter (F4)
• Search (F3)
• Sort (F6)
• Kill (F9)

pgrep & pidof

pgrep nginx                 # Find PID by name
pgrep -u sovon               # PIDs of user's processes
pgrep -a bash                # PID + full command line
pidof nginx                  # PID of exact program name

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Process States

State	Symbol	Description
Running	R	Currently executing on CPU
Sleeping	S	Waiting for event (interruptible)
Disk Sleep	D	Waiting for I/O (uninterruptible)
Stopped	T	Suspended (Ctrl+Z or signal)
Zombie	Z	Finished but parent hasn't collected exit status
Dead	X	Should never be seen

# Find zombie processes
ps aux | awk '$8 == "Z" {print}'

# Find and kill zombie parent
ps -eo pid,ppid,stat,cmd | grep "Z"
# Then kill the PARENT process (PPID) to clean zombies

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Foreground & Background

# Run in foreground (default)
sleep 100                    # Blocks your terminal

# Suspend a foreground process
Ctrl + Z                    # Sends SIGTSTP

# Resume in background
bg                           # Continue in background
bg %1                        # Resume job #1 in background

# Resume in foreground
fg                           # Bring to foreground
fg %2                        # Bring job #2 to foreground

# Start directly in background
sleep 100 &                  # The & runs it in background
echo $!                      # PID of last background process

# List background jobs
jobs                         # List all jobs
jobs -l                      # Include PIDs

# Disown a job (keeps running after logout)
sleep 1000 &
disown %1                    # Detach from terminal
disown -a                    # Disown all jobs

# nohup — survive terminal close
nohup long-running-command &
# Output goes to nohup.out

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Signals & Killing Processes

Common Signals

Signal	Number	Description	Default Action
SIGHUP	1	Terminal hangup	Terminate
SIGINT	2	Interrupt (Ctrl+C)	Terminate
SIGQUIT	3	Quit (Ctrl+\)	Terminate + core dump
SIGKILL	9	Force kill	Cannot be caught!
SIGTERM	15	Graceful termination	Terminate
SIGTSTP	20	Stop (Ctrl+Z)	Suspend
SIGCONT	18	Continue	Resume
SIGUSR1	10	User-defined	Varies

Sending Signals

# kill (sends signal to process)
kill 1234                    # Send SIGTERM (graceful)
kill -9 1234                 # Send SIGKILL (force)
kill -SIGTERM 1234           # Same as kill 1234
kill -l                      # List all signals

# killall (by name)
killall nginx                # Kill all nginx processes
killall -9 firefox           # Force kill all firefox

# pkill (by pattern)
pkill -f "python server"     # Kill by command pattern
pkill -u alice               # Kill all of alice's processes
pkill -9 -t pts/0            # Kill all processes on terminal pts/0

# Best practice: always try graceful first
kill PID                     # Try SIGTERM first
sleep 5                      # Wait 5 seconds
kill -9 PID                  # Force kill if still running

The Kill Escalation Path

1. kill PID          (SIGTERM — polite request to stop)
2. kill -2 PID       (SIGINT — like Ctrl+C)
3. kill -1 PID       (SIGHUP — reload or die)
4. kill -9 PID       (SIGKILL — instant death, cannot be caught)

⚠️ SIGKILL (-9) should be a last resort. It doesn't let the process clean up (flush buffers, close files, remove temp files).

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Process Priority (nice & renice)

Priority ranges from -20 (highest) to 19 (lowest). Default is 0.

# Start a process with a specific priority
nice -n 10 ./heavy-computation.sh    # Lower priority (nice)
nice -n -10 ./important-task.sh      # Higher priority (need sudo for negative)
sudo nice -n -20 ./critical.sh       # Highest priority

# Change priority of running process
renice 15 -p 1234               # Set PID 1234 to priority 15
renice -5 -p 1234               # Set to -5 (need sudo for negative)
sudo renice -20 -u mysql        # Set all mysql processes to max priority

# View priorities
ps -eo pid,ni,cmd | head -20    # Show nice values
top                              # NI column shows nice value

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The /proc Filesystem

Every running process has a directory in /proc:

# Info about PID 1234
ls /proc/1234/
cat /proc/1234/status           # Process status
cat /proc/1234/cmdline          # Full command line
cat /proc/1234/environ          # Environment variables
ls -l /proc/1234/fd/            # Open file descriptors
ls -l /proc/1234/cwd            # Current working directory
cat /proc/1234/maps             # Memory mappings
cat /proc/1234/io               # I/O statistics

System-Wide /proc Files

cat /proc/cpuinfo               # CPU information
cat /proc/meminfo               # Memory information
cat /proc/loadavg               # Load averages
cat /proc/uptime                # System uptime
cat /proc/version               # Kernel version
cat /proc/filesystems           # Supported filesystems
cat /proc/mounts                # Mounted filesystems
cat /proc/net/dev               # Network interface stats

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System Resource Monitoring

Memory

free -h                          # Quick memory overview
# Output:
#               total    used    free   shared  buff/cache  available
# Mem:          16Gi    8.2Gi   2.1Gi   512Mi    5.7Gi      7.1Gi
# Swap:          4Gi    256Mi   3.7Gi

vmstat 1 5                       # Virtual memory stats (every 1s, 5 times)

CPU

uptime                           # Load averages
# 10:00:00 up 5 days, 3:22, 2 users, load average: 0.52, 0.48, 0.45
# Load average: 1 min, 5 min, 15 min
# < number of CPUs = not overloaded

nproc                            # Number of CPU cores
lscpu                            # Detailed CPU info
mpstat 1 5                       # Per-CPU stats (need sysstat package)

Disk I/O

iostat 1 5                       # Disk I/O stats
iotop                            # Top for disk I/O (needs root)

Open Files & Connections

lsof                             # List all open files
lsof -u sovon                    # Open files by user
lsof -p 1234                     # Open files by PID
lsof -i :80                      # Who's using port 80?
lsof -i tcp                      # All TCP connections
lsof +D /var/log                 # Open files in directory

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🏋️ Practice Exercises

1. List all running processes and sort by memory usage
2. Background: Start sleep 300, suspend it, move to background, then bring back
3. Signals: Start sleep 1000 &, then kill it gracefully with SIGTERM
4. Priority: Start a process with nice value 10, then renice to 5
5. Monitor: Use htop to find the most CPU-intensive process
6. Proc: Look at your current shell's /proc/<PID>/ directory
7. Find: Use lsof to find which process is using port 22
8. Zombie: Write a script that creates a zombie process (hint: fork without wait)

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