#!/bin/bash
# Tested Under FreeBSD and OS X

FS="/usr"
THRESHOLD=90
OUTPUT=($(LC_ALL=C df -P ${FS}))
CURRENT=$(echo ${OUTPUT[11]} | sed 's/%//')
[ $CURRENT -gt $THRESHOLD ] && /usr/bin/zenity  --warning  --text="The disk $FS ($CURRENT% used) is almost full. Delete some files or add a new disk." --title="df Warning"