#!/usr/bin/ksh
# choose a threshold level to which you want to fill your FS
threshold=80
FStoFill=/tmp

while :
do

set -- $(df -k $FStoFill | tail -1)
# Now:
# $1 = FS
# $2 = total size in 1k-block
# $3 = Used size in k block
# $4 = available size
# $5 = %used    ${5%?} will remove the char "%"
# $6 = mount point

# if current %used has reached of exceed threshold, then break
[[ ${5%?} -ge $threshold ]] && break

# setup the remaining number of 1k block to fill
nb=$((($threshold*$2/100)-$3))

# if the remaining number of 1k block to fill represent more than 4Gb,
# setup the remaining number of 1k block to fill a 4Gb file
[[ $nb -gt $((4*1024*1024)) ]] && nb=$((4*1024*1024))

# create a file of the remaining $nb number of 1k block to fill
dd if=/dev/zero of=$FStoFill/zerofile.$$ bs=1k count=$nb

done

threshold=80
FStoFill=/tmp
set -- $(df -k $FStoFill | tail -1)
dd if=/dev/zero of=$FStoFill/zerofile.$$ bs=1k count=$((($threshold*$2/100)-$3))