#!/usr/bin/env bash
set -euo pipefail

threshold="${1:-80}"

if [[ ! "$threshold" =~ ^[0-9]+$ ]] || (( threshold < 1 || threshold > 100 )); then
  printf 'CRITICAL: invalid threshold "%s"; provide an integer from 1 to 100\n' "$threshold" >&2
  exit 2
fi

status=0
warning_threshold=$(( threshold > 5 ? threshold - 5 : threshold ))

while read -r filesystem _size _used avail use_percent mountpoint; do
  usage="${use_percent%\%}"

  if (( usage >= threshold )); then
    printf 'CRITICAL: %s mounted on %s is %s used; threshold is %s%% (%s free)\n' "$filesystem" "$mountpoint" "$use_percent" "$threshold" "$avail"
    status=1
  elif (( usage >= warning_threshold )); then
    printf 'WARNING: %s mounted on %s is %s used; threshold is %s%%\n' "$filesystem" "$mountpoint" "$use_percent" "$threshold"
  else
    printf 'OK: %s mounted on %s is %s used\n' "$filesystem" "$mountpoint" "$use_percent"
  fi
done < <(df -P -x tmpfs -x devtmpfs | awk 'NR > 1 {print $1, $2, $3, $4, $5, $6}')

exit "$status"