You have a keyboard, it backlits at night, but not in day. You save a bit of energy, and your keyboard looks cool at night. Great!

But... wait a minute.

I'm learning Bash lately, and I guess sharing my learning experience with you will be fruitful on my writing experience as well. Bear with me.

Going back...

So I used a simple algorithm in making my keyboard a little bit smarter with Bash. Bash's syntax is a bit strange for me, and took me a day before getting used to it. But why Bash when I can do it with a language I'm comfortable with, like Python? Well, exposing yourself to new habits from time to time exercises your brain's neuroplasticity, which is good.

So here's how the backlit's simple algorithm works,

Living near the equator gives us full 12 hours of daylight/darkness everyday.
Therefore I shall conclude, that my keyboard must be cool in between 6pm and 6am, and eco-friendly between 6am and 6pm.

Thus, giving us this logic flow:

Okay then, so how do we compute the next 6 hours?

We need to use military time, in order to simplify things. Calculating up to 24th hour is easier to do. But since time is presented in 12hr format in my computer, I adjusted my formulas...

But wait, why those formulas?

Because 6pm is the 18th hour of day. The second formula is the same as the first, I just put it into the context of time resetting back to 0 at 12am. I guess, for the sake of a clearer view of the algorithm.

Okay, so now the keyboard knows when it should switch. Great.

But wait...

How should it switch... the scroll lock?

Well, for some keyboards, pressing the Scroll Lock key triggers backlit.

And for most Linux machines, you can control your keyboard LEDs using xset command.

From xset man page:

led

The led option controls the keyboard LEDs. This controls the turning on or off of one or all of the LEDs. It accepts an optional integer, a preceding dash(-) or an 'on/off' flag...

...For example, to turn on the Scroll Lock LED:
xset led named "Scroll Lock"

Cool.

So now we have all we need, we can now write the code.

while true; do

    current_hr=$((10#$(date +'%H')))
    current_min=$((10#$(date +'%M')))
    six_am=6
    six_pm=18

    # sunrise
    if [[ current_hr -eq six_am ]]; then

        next_check=43200 # 12 hrs in s
        xset -led named 'Scroll Lock'

        echo 'Next check at' $(date -d "now + $next_check seconds")',' 'approx' $(($next_check/60/60)) 'hrs from now.'
        sleep $next_check


    # sunset
    elif [[ current_hr -eq six_pm ]]; then

        next_check=43200 # 12 hrs in s
        xset led named 'Scroll Lock'

        echo 'Next check at' $(date -d "now + $next_check seconds")',' 'approx' $(($next_check/60/60)) 'hrs from now.'
        sleep $next_check

    # midnight till before sunrise
    elif [[ current_hr -lt six_am ]]; then

        min_to_six=$(($((10#$six_am*60 + 10#0)) - $((10#$current_hr*60 + 10#$current_min))))
        next_check=$(($min_to_six * 60))
        xset led named 'Scroll Lock'

        echo 'Next check at' $(date -d "now + $next_check seconds")',' 'approx' $(($next_check/60/60)) 'hrs from now.'
        sleep $next_check

    # after sunrise till sunset
    elif [[ current_hr -lt six_pm ]]; then

        min_to_six=$(($((10#$six_pm*60 + 10#0)) - $((10#$current_hr*60 + 10#$current_min))))
        next_check=$(($min_to_six * 60))
        xset -led named 'Scroll Lock'

        echo 'Next check at' $(date -d "now + $next_check seconds")',' 'approx' $(($next_check/60/60)) 'hrs from now.'
        sleep $next_check

    # after sunset till before sunrise
    elif [[ current_hr -gt six_pm ]]; then

        hrs_to_twelve_am=$(($((10#24*60 + 10#0)) - $((10#$current_hr*60 + 10#$current_min))))
        min_to_six=$(($((10#$hrs_to_twelve_am + 10#0)) + $((10#6*60 + 10#0))))

        next_check=$(($min_to_six * 60))
        xset led named 'Scroll Lock'

        echo 'Next check at' $(date -d "now + $next_check seconds")',' 'approx' $(($next_check/60/60)) 'hrs from now.'
        sleep $next_check
    fi

done

Feel free to correct me or suggest simpler ways. I'll be glad and grateful to learn from you. ;)

This script is hosted in Github as well, as part of my personal and personalized Bash scripts.
https://github.com/raymelon/bash-wrappers/blob/master/auto-scrlck

Cheers!

This article is posted in Medium as well.