A simulation of The Ultimate Machine built by Claude Shannon https://en.wikipedia.org/wiki/Useless_machine

the-ultimate-machine.sh

#!/bin/bash

IS_ON=0

loading(){
    for _ in {1..3}; do
        printf '.'
        sleep 0.2
    done
}

handle_usr1(){
    ((IS_ON^=1))
}

work(){
    printf "\n  => Sending USR1 to %d" $$
    loading
    kill -USR1 $$
    printf "\n"
}

main() {
    printf "This is a simulation of The Ultimate Machine built by Claude Shannon\n"
    printf "https://en.wikipedia.org/wiki/Useless_machine\n"
    printf "\n"
    printf "Usage:\n"
    printf "    Send USR1 to this process(%d) to toggle the switch:\n" "$$"
    printf "    kill -s SIGUSR1 %d\n\n" "$$"

    trap 'handle_usr1' USR1

    while true; do
        printf "\rThe Ultimate Machine is "
        if [[ $IS_ON != 0 ]]; then
            printf "ON "
            work
        else
            printf "OFF"
        fi
        sleep 0.1
    done
}

main