#!/bin/sh

format_time ()
{
    t=$1

    days=$(($t / 86400))
    t=$(($t - 86400*$days))

    hours=$(($t / 3600))
    t=$(($t - 3600*$hours))

    minutes=$(($t / 60))
    t=$(($t - 60*$minutes))

    echo "$days days $hours hours $minutes minutes $t seconds"
}

main ()
{
    record_file="$1"
    # Old record
    record="$(cat "$record_file")"

    # When the system started.
    boot="$(date -d "$(uptime -s)" '+%s')"
    while true; do
        # You could also calculate the uptime before the loop and just
        # increment it, but that will rely on the script running 24/7.
        now="$(date '+%s')"
        current_uptime="$(($now - $boot))"
        # Set and save the record when it gets beaten
        if [ "$current_uptime" -gt "$record" ]; then
            record="$current_uptime"
            echo "$record" > "$record_file"
        fi
        # Status should be printed _after_ potentially having set
        # a new record:
        clear
        echo "Current uptime: $(format_time "$current_uptime")"
        echo "Record uptime: $(format_time "$record")"
        # Don't waste system resources for silly things.
        sleep 1
    done
}

main ~/uptimerecord.data