This is a simple script for changing fan speed on iMac running Linux (ensure applesmc is active)

fans_start.sh

#!/bin/bash

# Root?
if [ "$(id -u)" != "0" ]; then
  exec sudo "$0" "$@"
fi

fan_speeds()
{
  echo "Available fan speeds on this iMac:"
  echo "1) Fan speed: $1 - $2"
  echo "2) Fan speed: $3 - $4"
  echo "3) Fan speed: $5 - $6"
}

fan_check() 
{
  if [ $1 > $2 ]
  then
    if [ $1 > $3 ]
    then
      echo "Fan is speed invalid, please set values between:"
      echo ""
      fan_speeds $one_min $one_max $two_min $two_max $three_min $three_max
      exit
     else
      echo "Fan speed within range."
  else
    echo "Fan is speed invalid, please set values between:"
    echo ""
    fan_speeds $one_min $one_max $two_min $two_max $three_min $three_max
    exit
  fi
}

fan_manual()
{
  # Set fan control to manual
  echo 1 > /sys/devices/platform/applesmc.768/fan1_manual 
  echo 1 > /sys/devices/platform/applesmc.768/fan2_manual                                                                                  
  echo 1 > /sys/devices/platform/applesmc.768/fan3_manual
}

fan1_speed()
{
  # Set fan speed
  echo $1 > /sys/devices/platform/applesmc.768/fan1_output
}

fan2_speed()
{
  # Set fan speed
  echo $1 > /sys/devices/platform/applesmc.768/fan2_output
}

fan3_speed()
{
  # Set fan speed
  echo $1 > /sys/devices/platform/applesmc.768/fan3_output
}

help_message()
{
  echo "fan_start.sh - a script to set fan speeds manually on an iMac in Linux"
  echo ""
  echo "How to use:"
  echo "'fan_start.sh 3000' - This sets speed of all fans to entered value."
  echo "'fan_start.sh 3000 3000 3000' - This individually sets speed of each fan to entered values (fan_1 fan_2 fan_3)."
  echo ""
  fan_speeds $one_min $one_max $two_min $two_max $three_min $three_max
  echo ""
  echo "To see this again use 'fan_start.sh help'."
  
}

# Variables
one_min=$(cat /sys/devices/platform/applesmc.768/fan1_min)
one_max=$(cat /sys/devices/platform/applesmc.768/fan1_max)
two_min=$(cat /sys/devices/platform/applesmc.768/fan2_min)
two_max=$(cat /sys/devices/platform/applesmc.768/fan2_max)
three_min=$(cat /sys/devices/platform/applesmc.768/fan3_min)
three_max=$(cat /sys/devices/platform/applesmc.768/fan3_max)

# Main
if [ $# = 0 ]
then
  help_message
elif [ $# = 1 ]
then
  if [ $1 = "help" ]
  then	
    help_message
  else
   fan_manual
   fan_check $1 $one_min $one_max
   fan_check $2 $two_min $two_max
   fan_check $3 $three_min $three_max
   fan1_speed $1
   fan2_speed $1
   fan3_speed $1
  fi
elif [ $# = 2 ]
then
  help_message
elif [ $# = 3 ]
then
  fan_manual
  fan_check $1 $one_min $one_max
  fan_check $2 $two_min $two_max
  fan_check $3 $three_min $three_max
  fan1_speed $1
  fan2_speed $2
  fan3_speed $3
else
  help_message
fi