Triggering a Raspberry PI Fan via GPIO port in python or bash

rpi-fan.py

#!/usr/bin/python3

import sys
import time
from gpiozero import LED # doc: https://gpiozero.readthedocs.io/

# define the GPIO to control the transistor's B pin
fan = LED(21)

def cpu_temp():
    with open("/sys/class/thermal/thermal_zone0/temp", 'r') as f:
        return float(f.read())/1000


def main():
    # close fan at begining
    is_close = True
    fan.off()
    while True:
        temp = cpu_temp()
        if is_close:
            if temp > 60.0: # upper bound to turn on the fan
                print(time.ctime(), temp, 'Fan ON')
                fan.on()
                is_close = False
        else:
            if temp < 45.0: # lower bound to turn off the fan
                print(time.ctime(), temp, 'Fan OFF')
                fan.off()
                is_close = True

        time.sleep(5.0)


if __name__ == '__main__':
    main()

rpi-fan.service

[Unit]
Description=RPI Fan Service
After=multi-user.target

[Service]
Type=idle
ExecStart=/usr/bin/rpi-fan.sh

[Install]
WantedBy=multi-user.target

rpi-fan.sh

#!/bin/bash

GPIO_EXPORT=/sys/class/gpio/export

if [ ! -f ${GPIO_EXPORT} ]; then
    echo "21" > ${GPIO_EXPORT}
fi
echo "out" > /sys/class/gpio/gpio21/direction

while true; do

    TEMP=$(cat /sys/class/thermal/thermal_zone0/temp)
    if [ ${TEMP} -gt 60000 ]; then
        echo "starting fan"
        echo "1" > /sys/class/gpio/gpio21/value
    elif [ ${TEMP} -lt 45000 ]; then
        echo "stopping fan"
        echo "0" > /sys/class/gpio/gpio21/value
    fi
    sleep 2
done