second_perf_script

#!/bin/bash
#This script controls the green LED of the Pi according to the CPU usage

# exits the script when any command fails
set -e

# specifying all excutables' paths
readonly TOP=/usr/bin/top
readonly GREP=/bin/grep
readonly CUT=/usr/bin/cut
readonly BC=/usr/bin/bc
readonly SUDO=/usr/bin/sudo
readonly TEE=/usr/bin/tee
readonly SLEEP=/bin/sleep

# if the scripts catches a "USR2" signal, or an interruption signal, it will call the reset act led function
trap reset_act_led USR2 SIGHUP SIGINT SIGTERM

# resets led trigger state to default and exits the script
reset_act_led() {
    echo mmc0 | $SUDO $TEE /sys/class/leds/led0/trigger >/dev/null
    exit
}

# turns on green LED
turn_on_act_led() {
    echo 1 | $SUDO $TEE /sys/class/leds/led0/brightness >/dev/null
}

# turns off green LED
turn_off_act_led() {
    echo 0 | $SUDO $TEE /sys/class/leds/led0/brightness >/dev/null
}

# removes green LED trigger before going into the while loop so we can manually turn on and off the LED
echo none | $SUDO $TEE /sys/class/leds/led0/trigger >/dev/null

while true #creates an infinite loop until the script exits itself
do 

    cpu_total=100
    one_second=1

    # get the percentage of cpu idle using top command, and get the numeric value using grep and cut
    cpu_idle=$($TOP -b -n 1 | $GREP "%Cpu(s):" | $CUT -d ',' -f 4 | $GREP -o '[0-9]*\.[0-9]*')

    # get the percentage of cpu usage from all users and processes
    cpu_usage=$(echo "$cpu_total - $cpu_idle" | $BC -l)

    # fraction of the second that the LED will stay on
    led_on_time=$(echo "$cpu_usage / $cpu_total" | $BC -l)

    # fraction of the second that the LED will stay off
    led_off_time=$(echo "$one_second - $led_on_time" | $BC -l)

    # turns on led, and sleeps for the calculated fraction of a second
    turn_on_act_led
    $SLEEP "$led_on_time"

    # turns off led, and sleeps for the rest of the second
    turn_off_act_led
    $SLEEP "$led_off_time"
done