Script to monitor which application is currently opened (based on dbus-monitor).

monitor_active_app.bash

#!/bin/bash

process_active_app() {
    local last_date_str=""
    local last_timestamp_s=0
    local last_active_app=""

    while IFS= read -r current_active_app; do

        local current_timestamp
        local current_date
        current_timestamp=$(date +%s)
        current_date=$(date +"%Y-%m-%d %H:%M:%S")
        if [ $last_timestamp_s -ne 0 ]
        then
            local time_in_previous_app
            time_in_previous_app=$((current_timestamp - last_timestamp_s))
            echo "${last_date_str},${time_in_previous_app},${last_active_app}"
        fi
        last_active_app=${current_active_app}
        last_timestamp_s=${current_timestamp}
        last_date_str=${current_date}
    done
}

echo "datetime_opened,duration_opened_s,application"
dbus-monitor \
    | grep --line-buffered -B3 "active-on-seats" \
    | grep --line-buffered "\.desktop" \
    | sed --unbuffered -e 's/^ *string *"\([^"]*\)"/\1/g'  \
    | process_active_app

Tested on ubuntu using wayland.