UNIX Get An Alert When Disk Is Full

I want to get an alert when my disk is full under UNIX and Mac OS X? How do I set a a specified threshold and run the script via cron?

The df command report file system disk space usage including the amount of disk space available on the file system containing each file name argument. Disk space is shown in 1K blocks by default, unless the environment variable POSIXLY_CORRECT is set, in which case 512-byte blocks are used.

Use df command and pass the -P option which make df output POSIX compliant (i.e. 512-byte blocks rather than the default. Note that this overrides the BLOCKSIZE specification from the environment).
# df -P /
OR
# df -P /usr
Sample Outputs:

Filesystem    512-blocks     Used     Avail Capacity  Mounted on
/dev/aacd0s1e  162491344 21988048 127503992    15%    /usr

You can now simply grep /usr file system and print out used capacity:
# df -P /usr | grep /usr | awk '{ print $5}' | sed 's/%//g'
15
Or assign value to a variable:
# output=$(df -P /usr | grep /usr | awk '{ print $5}' | sed 's/%//g')
# echo $output
Under BASH or KornShell you can use arrays indexed by a numerical expression to make code small:
# output=($(df -P /))
# echo "${output[11]}"

A Sample Shell Script

#!/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 $THRESHOLD ] && echo "$FS file system usage $CURRENT" | mail -s "$FS file system" you@example.com

You need to modify syntax, if you are using KSH or TCSH / CSH instead of BASH. Save this script and run as a cron job:
@daily /path/to/your.df.script.sh

GUI Notification

Display warning dialog using /usr/bin/zenity

#!/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"

DF GUI Warning Notification

Finally update your cronjob as follows (you need to use DISPLAY variable to display output window):

36 19 * * *  DISPLAY=:0.0 /path/to/script.sh