Linux Process Management: Mastering ps, top, kill, and Job Control

# Basic ps (only current shell processes)
ps

# All processes
ps aux              # BSD syntax (most common)
ps -ef              # System V syntax

# Common ps options explained
ps aux --sort=-%cpu # Sort by CPU usage (highest first)
ps aux --sort=-%mem # Sort by memory usage
ps -u username      # Processes for specific user
ps -C process_name  # Processes with specific command
ps -p 1234          # Specific PID

# Custom output format
ps -eo pid,ppid,user,%cpu,%mem,etime,cmd --sort=-%cpu | head -20

# Top commands
# ------------
top                    # Start top
# Inside top:
# h - help
# q - quit
# P - sort by CPU
# M - sort by Memory
# k - kill process (prompts for PID)
# r - renice (change priority)
# u - show only specific user
# 1 - show each CPU core individually
# c - show full command line

# Batch mode (for scripts)
top -b -n 1 | head -20

# htop (better alternative, install first)
# sudo apt install htop
htop
# Inside htop:
# F2 - setup/configuration
# F3 - search
# F5 - tree view
# F6 - sort by column
# F9 - kill process
# F10 - quit

# Kill signals (most common)
# 1  (SIGHUP)  - Hang up (reload config)
# 2  (SIGINT)  - Interrupt (Ctrl+C)
# 9  (SIGKILL) - Kill (force, cannot be ignored)
# 15 (SIGTERM) - Terminate (graceful, default)
# 18 (SIGCONT) - Continue (resume stopped process)
# 19 (SIGSTOP) - Stop (pause process)

# Send signals
kill PID              # SIGTERM (graceful kill)
kill -9 PID           # SIGKILL (force kill)
kill -15 PID          # Same as default
kill -1 PID           # SIGHUP (reload config)
kill -STOP PID        # Pause process
kill -CONT PID        # Resume process

# Kill by name
pkill process_name    # Kill by name
pkill -9 process_name # Force kill by name
killall process_name  # Kill all instances

# Example: Reload nginx configuration
sudo kill -1 $(cat /var/run/nginx.pid)
# or
sudo systemctl reload nginx

# Run command in background
long_running_command &

# View background jobs
jobs
# Output: [1]  Running    long_running_command &

# Bring background job to foreground
fg %1

# Suspend foreground job (Ctrl+Z) then background it
# 1. Run command
sleep 100
# 2. Press Ctrl+Z
# 3. Put in background
bg

# Start multiple background commands
command1 & command2 & command3 &
jobs

# Kill specific background job
kill %1

# Method 1: nohup (no hang up)
nohup long_running_command &
# Output goes to nohup.out
nohup python script.py > output.log 2>&1 &

# Method 2: disown
long_running_command &
disown %1  # Remove from shell's job table

# Method 3: screen (best for long sessions)
screen -S my_session
# Run commands inside screen
# Detach: Ctrl+A, then D
screen -ls                    # List sessions
screen -r my_session          # Reattach

# Method 4: tmux (modern alternative)
tmux new -s my_session
tmux detach                   # Ctrl+B, D
tmux ls
tmux attach -t my_session

# Create a test script
cat > ~/test_process.sh << 'EOF'
#!/bin/bash
echo "Process started with PID: $$"
while true; do
    echo "Running... $(date)"
    sleep 5
done
EOF
chmod +x ~/test_process.sh

# Start test process in background
~/test_process.sh &

# Find its PID
ps aux | grep test_process

# Check its state and resource usage
top -p $PID

# View process tree
pstree -p $PPID

# Check open files
lsof -p $PID

# Check process environment
cat /proc/$PID/environ | tr '\0' '\n'

# Create CPU stress test
yes > /dev/null &

# Monitor CPU usage
htop
# or
top -bn1 | grep -E "%Cpu|yes"

# Kill the stress process
pkill yes
# or
kill -9 $(pgrep yes)

#!/bin/bash
# system_monitor.sh - Monitor system processes and alert on issues

# Configuration
CPU_THRESHOLD=90
MEM_THRESHOLD=90
ALERT_EMAIL="admin@example.com"
LOG_FILE="/var/log/system_monitor.log"

# Colors for output
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
NC='\033[0m'

echo "=== System Monitor Started at $(date) ==="

# Function: Find top CPU processes
top_cpu() {
    echo -e "\n${YELLOW}Top 5 CPU-consuming processes:${NC}"
    ps aux --sort=-%cpu | head -6 | tail -5
}

# Function: Find top memory processes
top_memory() {
    echo -e "\n${YELLOW}Top 5 Memory-consuming processes:${NC}"
    ps aux --sort=-%mem | head -6 | tail -5
}

# Function: Check for zombie processes
check_zombies() {
    zombies=$(ps aux | awk '$8=="Z"' | wc -l)
    if [ "$zombies" -gt 0 ]; then
        echo -e "${RED}⚠️ Warning: $zombies zombie processes found!${NC}"
        ps aux | awk '$8=="Z"'
    fi
}

# Function: Check high CPU usage
check_high_cpu() {
    high_cpu=$(ps aux | awk -v threshold="$CPU_THRESHOLD" '$3 > threshold {print "  PID:" $2 " CPU:" $3 "% CMD:" $11}')
    if [ -n "$high_cpu" ]; then
        echo -e "${RED}⚠️ High CPU usage detected:${NC}"
        echo "$high_cpu"
    fi
}

# Function: Check high memory usage
check_high_memory() {
    high_mem=$(ps aux | awk -v threshold="$MEM_THRESHOLD" '$4 > threshold {print "  PID:" $2 " MEM:" $4 "% CMD:" $11}')
    if [ -n "$high_mem" ]; then
        echo -e "${RED}⚠️ High Memory usage detected:${NC}"
        echo "$high_mem"
    fi
}

# Function: Check system load
check_load() {
    load=$(uptime | awk -F'load average:' '{print $2}')
    cores=$(nproc)
    echo -e "${YELLOW}System Load:${NC} $load (Cores: $cores)"
}

# Main execution
top_cpu
top_memory
check_load
check_zombies
check_high_cpu
check_high_memory

echo -e "\n${GREEN}=== Monitor completed ===${NC}"

# Log results
{
    echo "[$(date)] System Monitor Report"
    top_cpu
    top_memory
    check_load
} >> "$LOG_FILE"

echo "Log saved to $LOG_FILE"