Linux System Resource Monitoring

top -d 1
htop

uptime   # check load average (overload if higher than core count)
nproc
lscpu | grep "CPU(s):"

ps aux --sort=-%cpu | head -10

free -h
watch -n 1 free -h

ps aux --sort=-%mem | head -10
cat /proc/meminfo | grep -E "MemTotal|MemFree|MemAvailable|Cached"
swapon --show

df -h
df -h /var

du -sh /var/* | sort -rh | head -10
du -sh /*     | sort -rh | head -10

df -i   # inode usage

iostat -xz 1
iotop -ao

ss -tunlp
ss -s

iftop -i eth0
nload eth0

# check connection counts (DDoS detection)
ss -tn | awk 'NR>1 {print $5}' | cut -d: -f1 | sort | uniq -c | sort -rn | head

vmstat 1 10

sudo apt install sysstat
sar -u 1 5   # CPU
sar -r 1 5   # memory
sar -d 1 5   # disk

#!/bin/bash
THRESHOLD=90
CPU=$(top -bn1 | grep "Cpu(s)" | awk '{print $2}' | cut -d'%' -f1 | cut -d',' -f1)
CPU=${CPU%.*}

if [[ $CPU -gt $THRESHOLD ]]; then
  echo "[ALERT] CPU ${CPU}%" | mail -s "CPU Alert" admin@example.com
fi