Disk Usage Threshold Alert

#!/usr/bin/env bash
#
# disk_usage_alert.sh
# Sends an email when any mounted filesystem exceeds a usage threshold.
# Designed to be run from cron, e.g.:  */15 * * * * /opt/scripts/disk_usage_alert.sh
#
set -euo pipefail

# ---- Configuration -----------------------------------------------------------
THRESHOLD="${THRESHOLD:-85}"            # Alert when usage % is greater than this.
ALERT_EMAIL="${ALERT_EMAIL:-root@localhost}"
HOSTNAME_FQDN="$(hostname -f 2>/dev/null || hostname)"
# Filesystem types to skip (pseudo / network filesystems).
EXCLUDE_FS="tmpfs|devtmpfs|squashfs|overlay|udf|iso9660"

# ---- Collect filesystems over the threshold ----------------------------------
# df -P  -> POSIX output (one line per FS, no wrapping).
# We parse the "Use%" column and strip the trailing '%'.
alert_body=""
while read -r filesystem size used avail usep mount; do
    use_pct="${usep%\%}"                # Remove the '%' sign.
    # Skip the header row and any non-numeric value.
    [[ "$use_pct" =~ ^[0-9]+$ ]] || continue
    if (( use_pct > THRESHOLD )); then
        alert_body+="${mount} (${filesystem}) is at ${use_pct}% used [${used}/${size}]"$'\n'
    fi
done < <(df -P -x tmpfs -x devtmpfs 2>/dev/null | grep -vE "${EXCLUDE_FS}")

# ---- Notify if there is anything to report -----------------------------------
if [[ -n "${alert_body}" ]]; then
    subject="[DISK ALERT] ${HOSTNAME_FQDN}: filesystem over ${THRESHOLD}%"
    {
        echo "Disk usage threshold (${THRESHOLD}%) exceeded on ${HOSTNAME_FQDN}:"
        echo
        echo "${alert_body}"
        echo "Checked at: $(date '+%Y-%m-%d %H:%M:%S %Z')"
    } | mail -s "${subject}" "${ALERT_EMAIL}"
    echo "Alert sent to ${ALERT_EMAIL}."
else
    echo "OK: all filesystems are below ${THRESHOLD}%."
fi