The Bash Loop

A No-Dependency System Monitor You Can Run in Cron

Hey engineer,

Want basic system telemetry like CPU, RAM, disk usage, logged to a rolling file every minute, without installing anything?

Let’s build a mini agent in Bash. Drop it into cron and watch it do its thing. Perfect for low-touch servers, test boxes, or inside CI/CD runners.

---

The Script

#!/bin/bash

# Set locale for consistent command output parsing
export LANG=C

# Define log file path
LOG_FILE="/var/log/syswatch.log"

# Check if log file is writable or can be created
if [ ! -w "$LOG_FILE" ] && [ ! -e "$LOG_FILE" ]; then
    # Try to create the file
    touch "$LOG_FILE" 2>/dev/null
fi

if [ ! -w "$LOG_FILE" ]; then
    echo "Error: Cannot write to log file '$LOG_FILE'. Permission denied." >&2
    exit 1
fi

# Get timestamp
TIMESTAMP=$(date +"%Y-%m-%d %H:%M:%S")

# Get CPU usage (subtract idle percentage)
CPU=$(top -bn1 | grep "Cpu(s)" | awk -F'id,' '{ split($1, vs, ","); sub("%", "", vs[length(vs)]); print 100 - vs[length(vs)]"%"}')

# Get memory usage
MEM=$(free -m | awk '/Mem:/ {printf("%.2f%%", $3/$2 * 100.0)}')

# Get disk usage on root
DISK=$(df -h / | awk 'NR==2 {print $5}')

# Get system load average
LOAD=$(uptime | awk -F'load average:' '{ print $2 }' | sed 's/^ *//')

# Write to log file
echo "$TIMESTAMP | CPU: $CPU | MEM: $MEM | DISK: $DISK | LOAD: $LOAD" >> "$LOG_FILE"

---

What It Tracks

• CPU usage (total active)

• Memory usage (percent)

• Disk usage of root mount

• Load average

Log line example:

2025-05-19 07:45:01 | CPU: 12.5% | MEM: 62.14% | DISK: 77% | LOAD: 0.47, 0.59, 0.65

---

How To Use

1. Save it as /usr/local/bin/syswatch.sh

2. Make it executable:
   chmod +x /usr/local/bin/syswatch.sh

3. Add it to cron:

* * * * * /usr/local/bin/syswatch.sh

Logs updated every minute in /var/log/syswatch.log

---

Why It’s Useful

• Zero overhead monitoring

• Works on fresh VMs, test servers, containers

• Log file is parse-friendly (CSV or grep)

• Can be tailed with multitail or piped into Slack alerts

• Great lightweight solution before you install Prometheus

---

Simple. Scriptable. Observability, without the complexity.

---

I built the DevOps Automation Vault ,  a hand-crafted set of 60+ production-ready scripts for backups, monitoring, security, cleanup, and more.

Every script is cron-ready, supports dry-run mode, and works right out of the box.
👉 Grab your copy now →

---