Auto-Reboot

Auto Reboot Server in Linux

Sometimes a Linux server can get overwhelmed by sustained high CPU / Memory load – due to runaway processes, DDoS attacks or rogue scripts. Manually catching and fixing it in real-time isn’t always possible. This guide will show you how to automatically reboot your server when CPU load is critically high for multiple minutes – but safely.

We’ll create a lightweight bash script, track load over time, and schedule it with cron to run every minute.

📜 Step 1: Create the Auto-Reboot Script
Let’s start by writing the script that checks system load and initiates a reboot after 3 consecutive high-load readings.

✏️ Create or Edit the Script

sudo nano /usr/local/bin/reboot-on-high-load.sh

Paste this logic into the file:

#!/bin/bash # Configuration CPU_THRESHOLD=0.8 # 80% load per CPU core MEM_THRESHOLD=90 # 90% memory usage threshold MAX_RETRIES=3 # Reboot if high load/memory persists N times CHECK_FILE="/tmp/highload.counter" LOG_FILE="/var/log/reboot-load.log" MAX_LOG_SIZE=$((10 * 1024 * 1024)) # 10MB # Auto-truncate log if too large if [ -f "$LOG_FILE" ] && [ "$(stat -c%s "$LOG_FILE")" -ge "$MAX_LOG_SIZE" ]; then echo "[!] Log file exceeded 10MB. Truncating..." >> "$LOG_FILE" truncate -s 0 "$LOG_FILE" fi # Detect CPU cores and load threshold CPU_CORES=$(nproc) LOAD_THRESHOLD=$(echo "$CPU_CORES * $CPU_THRESHOLD" | bc) # CPU Load Check LOAD_AVG=$(awk '{print $1}' /proc/loadavg) LOAD_OK=$(echo "$LOAD_AVG < $LOAD_THRESHOLD" | bc) # Memory Check MEM_USED_PERCENT=$(free | awk '/Mem:/ { printf("%.0f", $3/$2 * 100) }') MEM_OK=$( [ "$MEM_USED_PERCENT" -lt "$MEM_THRESHOLD" ] && echo 1 || echo 0 ) # Initialize counter file if needed if [ ! -f "$CHECK_FILE" ]; then echo 0 > "$CHECK_FILE" fi COUNTER=$(cat "$CHECK_FILE") # Evaluate system status if [ "$LOAD_OK" -eq 0 ] || [ "$MEM_OK" -eq 0 ]; then echo "[!] High resource usage detected at $(date):" >> "$LOG_FILE" [ "$LOAD_OK" -eq 0 ] && echo " - CPU Load: $LOAD_AVG / Threshold: $LOAD_THRESHOLD" >> "$LOG_FILE" [ "$MEM_OK" -eq 0 ] && echo " - Memory Usage: $MEM_USED_PERCENT% / Threshold: $MEM_THRESHOLD%" >> "$LOG_FILE" COUNTER=$((COUNTER + 1)) echo "$COUNTER" > "$CHECK_FILE" else if [ "$COUNTER" -ne 0 ]; then echo "[✓] Resources back to normal at $(date): Load = $LOAD_AVG, Mem = $MEM_USED_PERCENT%" >> "$LOG_FILE" fi echo 0 > "$CHECK_FILE" fi # Reboot if over threshold for too long if [ "$COUNTER" -ge "$MAX_RETRIES" ]; then echo "[!!!] High resource usage sustained for $MAX_RETRIES checks. Rebooting at $(date)..." >> "$LOG_FILE" rm -f "$CHECK_FILE" /sbin/shutdown -r now fi

🔓 Step 2: Make the Script Executable

sudo chmod +x /usr/local/bin/reboot-on-high-load.sh ⏰ Step 3: Schedule It to Run Every Minute
Open root crontab: sudo crontab -e

Add this line to the bottom:

* * * * * /usr/local/bin/reboot-on-high-load.sh >> /var/log/reboot-load.log 2>&1

This schedules the script to run every minute and logs its output to /var/log/reboot-load.log.

🛡️ Why This Is Safe
✅ No reboots on single spikes – It only reboots if load stays high for 3 checks.

✅ Self-resetting – If load normalizes, the counter resets.

✅ Persistent state tracking – Uses /tmp/highload.counter.

✅ Simple logging – Outputs to /var/log/reboot-load.log.

✅ Cron-scheduled – Lightweight, runs every 60 seconds.

🧪 Optional: Test the Script by Simulating High Load
🛠️ 1. Install a CPU Stress Tool
On Ubuntu/Debian: sudo apt update && sudo apt install -y stress

On Alpine Linux:

apk add stress

If stress is not available, use yes as a simple CPU loader (see below).

🚀 2. Simulate High Load for Over 3 Minutes
Option A: With stress (preferred) stress --cpu $(nproc) --timeout 200

–cpu $(nproc) starts 1 thread per core.

–timeout 200 runs for 200 seconds (~3.3 minutes).

Option B: With yes Command

for i in $(seq 1 $(nproc)); do yes > /dev/null & done

Let it run for at least 3 minutes, then stop it:

killall yes 📝 3. Monitor the Logs
In a second terminal, run: tail -f /var/log/reboot-load.log

You’ll see output like:

yaml Copy Edit [!] Load is high: 7.23 / Threshold: 6.40 [!] Load is high: 7.45 / Threshold: 6.40 [!!!] High load sustained for 3 checks. Rebooting... Then the system will automatically reboot. ✅ Recap
By following this guide, you’ve set up a safe and efficient way to auto-reboot your Linux server during persistent high-load events:

✅ Script with thresholds and state tracking

✅ Cronjob for regular checks

✅ No false positives from single spikes

✅ Full log output for auditing