This is a simple command-line Bash timer that I created for personal use:
./timer.sh "title" "amount of time"
where "title" is the title of the timer and "amount of time" is the length of time specified as a human-readable date string that is accepted by the Linux date command (ex. 10 seconds, 5 minutes, etc.). The timer runs in the background once started. Upon completion, it shows a "TIME'S UP" screen and plays a sound.