How to Build a Simple Bash Script to Monitor CPU, Memory, and Disk Usage

1. Monitor Memory

free -m | awk 'NR==2{printf "%.2f%%\t\t", $3*100/$2 }'

free -m

displays used and free memory. Using awk we extract total and used values from the second line.

2. Monitor Disk

df -h | awk '$NF=="/"{printf "%s\t\t", $5}'

df -h

shows disk usage; the awk command selects the line for the root filesystem and prints the usage percentage.

3. Monitor CPU

top -bn1 | grep load | awk '{printf "%.2f%%\t\t
", $(NF-2)}'

The top -bn1 command runs once in batch mode, grep load finds the load line, and awk extracts the load average as a CPU usage percentage.

These commands are stored in variables:

MEMORY=$(free -m | awk 'NR==2{printf "%.2f%%\t\t", $3*100/$2 }')
DISK=$(df -h | awk '$NF=="/"{printf "%s\t\t", $5}')
CPU=$(top -bn1 | grep load | awk '{printf "%.2f%%\t\t
", $(NF-2)}')

A loop runs for a defined period (e.g., one hour) and prints the values every five seconds:

end=$((SECONDS+3600))
while [ $SECONDS -lt $end ]; do
  echo "$MEMORY $DISK $CPU"
  sleep 5
done

The complete script prints a header and then the three columns:

#!/bin/bash
printf "Memory\t\tDisk\t\tCPU
"
end=$((SECONDS+3600))
while [ $SECONDS -lt $end ]; do
  MEMORY=$(free -m | awk 'NR==2{printf "%.2f%%\t\t", $3*100/$2 }')
  DISK=$(df -h | awk '$NF=="/"{printf "%s\t\t", $5}')
  CPU=$(top -bn1 | grep load | awk '{printf "%.2f%%\t\t
", $(NF-2)}')
  echo "$MEMORY $DISK $CPU"
  sleep 5
done

Running the script outputs lines such as "9.34% 7% 0.00%". The output can be redirected to a log file.

Stress Test

Install the stress tool (e.g., yum install stress) and run a load test, for example: stress -c 2 -i 1 -m 1 --vm-bytes 128M -t 3600s While the stress test runs, the monitoring script shows increased CPU and memory usage.