Monitor server load, disk, CPU and memory with email alerts

A small shell script, run from cron every 5 minutes, that checks four health numbers and emails you when one goes over its limit. It doesn't need an agent or a monitoring server. 
 What it checks 
 
 
 
 Check 
 How it is read 
 Default limit 
 
 
 
 
 1-minute load average 
 /proc/loadavg 
 10.00 
 
 
 CPU use 
 vmstat 1 2 (100 minus idle) 
 98 % 
 
 
 Root disk use 
 df --output=pcent / 
 98 % 
 
 
 Memory use 
 free (used ÷ total) 
 98 % 
 
 
 
 Each limit can be overridden with an environment variable, so one script works on small and large machines: 
 LOAD_THRESHOLD=4 DISK_THRESHOLD=90 RECIPIENTS="you@example.com" /usr/local/bin/loadmon.sh
 
 Requirements 
 awk , df , free , vmstat (procps), bc , logger , a mail command (s-nail) with a working mail relay, and optionally sar (sysstat) to include recent history in the alert email. 
 Check for every tool at start-up, including bc , so a missing tool fails loudly instead of silently skipping alerts: 
 for c in awk df free vmstat bc mail logger; do
 command -v "$c" >/dev/null || { logger -t loadmon "missing $c"; exit 1; }
done
 
 Reading the numbers 
 load=$(cut -d' ' -f1 /proc/loadavg)
cpu=$(vmstat 1 2 | awk 'END {print 100 - $15}')
disk=$(df --output=pcent / | tail -1 | tr -dc '0-9')
mem=$(free | awk '/^Mem:/ {printf "%d", $3 * 100 / $2}')
if (( $(echo "$load > $LOAD_THRESHOLD" | bc -l) )); then ... fi
 
 Avoid parsing top -bn1 for CPU. Its first frame is an average since boot, and its column positions shift. 
 Schedule it 
 /etc/cron.d/loadmon : 
 */5 * * * * root /usr/local/bin/loadmon.sh >/dev/null 2>&1
 
 Tips 
 
 
 Throttle alerts. Write a timestamp file per check (for example /var/lib/loadmon/disk.last ), and only email again after an hour. Otherwise a full disk sends 12 emails an hour. 
 
 Rotate the log. One line every 5 minutes is about 100,000 lines a year. Add /etc/logrotate.d/loadmon . 
 
 Check more than / . Loop over /var , /home and data volumes too. 
 Use hostname -s in the subject line so you can tell servers apart.