Control the fan speed of an 27-inch, Mid 2010, iMac. I use this after installing an SSD drive instead of the factory-supplied HDD.

fan.sh

#!/bin/sh

version="0.1"
script=$(basename $0)
smc=/Applications/smcFanControl.app/Contents/Resources/smc
smc_key="F1Mx" # HDD fan key
minimum_speed=2500
fan_speed=0

version () {
	smc_version=$($smc -v)
	echo "$script version $version ($smc_version) by Joachim Bondo <joachim@bondo.net>" >&2
}

usage () {
cat << EOF >&2
Synopsis:
  Fan control, using smcFanControl
  
Usage:
  $script [options]
  
Options:
  -s, --speed <speed>  Sets fan speed (RPM)
  -h, --help           Shows this help text
  -v, --version        Shows script, app version
  
Notes:
  - Requires the smcFanControl app to be installed.
  - With no options the script outputs the current fan setting.
  - Valid fan speeds are: 2500, 3000, 3600, 4000, 4800, 5000, 5600, 6200.
  
See:
  - http://www.ifixit.com/Answers/View/14068/#answer40908
  - https://github.com/hholtmann/smcFanControl
  - $smc -h
EOF
}

readout () {
	$smc -k $smc_key -r
}

bytes_from_speed () {
	case $1 in
		6200 ) echo "60e0";;
		5600 ) echo "5780";;
		5000 ) echo "4e20";;
		4800 ) echo "4b00";;
		4000 ) echo "3e80";;
		3600 ) echo "3840";;
		3000 ) echo "2ee0";;
		2500 ) echo "2710";;
	esac
}

if [ $# -eq 0 ]; then
	readout
	exit 0
fi

while [ $# -gt 0 ]; do
	case $1 in
		-h | --help )
			version
			echo ""
			usage
			exit 0
			;;
		-v | --version )
			version
			exit 0
			;;
		-s | --speed )
			fan_speed=$2
			shift
			;;
		* ) # Unknown option
			echo "Illegal option ($1)." >&2
			usage
			exit 1
			;;
	esac	
	shift
done

fan_speed_bytes=$(bytes_from_speed $fan_speed)

if [ -n $fan_speed_bytes ]; then
	$smc -k $smc_key -w $fan_speed_bytes
	readout
else
	echo "Illegal speed ($fan_speed). See help for a list of valid values." >&2
	exit 1;
fi

exit 0