Bash monitor disk usage

#!/bin/bash
source /etc/profile

# Device to check
devname="/dev/sdb1"

let p=`df -k $devname | grep -v ^File | awk '{printf ("%i",$3*100 / $2); }'`
if [ $p -ge 90 ]
then
  df -h $devname | mail -s "Low on space" my@email.com
fi

Crontab this to run however often you want an alert

EDIT: For multiple disks

#!/bin/bash
source /etc/profile

# Devices to check
devnames="/dev/sdb1 /dev/sda1"

for devname in $devnames
do
  let p=`df -k $devname | grep -v ^File | awk '{printf ("%i",$3*100 / $2); }'`
  if [ $p -ge 90 ]
  then
    df -h $devname | mail -s "$devname is low on space" my@email.com
  fi
done

I tried to use Erik's answer but had issues with devices having long names which wraps the numbers and causes script to fail, also the math looked wrong to me and didn't match the percentages reported by df itself.

Here's an update to his script:

#!/bin/bash
source /etc/profile

# Devices to check
devnames="/dev/sda1 /dev/md1 /dev/mapper/vg1-mysqldisk1 /dev/mapper/vg4-ctsshare1 /dev/mapper/vg2-jbossdisk1 /dev/mapper/vg5-ctsarchive1 /dev/mapper/vg3-muledisk1"


for devname in $devnames
do
  let p=`df -Pk $devname | grep -v ^File | awk '{printf ("%i", $5) }'`
  if [ $p -ge 70 ]
  then
    df -h $devname | mail -s "$devname is low on space" my@email.com
  fi
done

Key changes are changed df -k to df -Pk to avoid line wrapping and simplified the awk to use pre-calc'd percent instead of recalcing.