Pomodoro timer in 62 lines of bash code

pomo.sh

#!/bin/bash

USAGE="usage: pomo <n>

Where <n> is a cycle number starting from 1.

If you increase 'n' by 1 every time after every pomoodoro, you will get a
pomodoro cycle.

Examples:

Start the pomodoro from the beginning:
$ pomo 1 

Take a 5 minute break:
$ pomo 2

Take a 15 minute break:
$ pomo 8
"

if ! [[ $* =~ ^[0-9]+$ ]]; then # not a number
    echo "$USAGE"
    exit 1
elif (( $@ % 2 != 0  )); then # odd
    DURATION=$(( 25 * 60 )) 
elif (( $@ % 8 == 0 )); then # even, multiple of 8
    DURATION=$(( 15 * 60 )) 
    BREAK=1
else # even
    DURATION=$(( 5 * 60 ))
    BREAK=1
fi

if [[ -n $BREAK ]]; then
    echo Time for a break!
else
    echo Time to focus.
fi

START=$(date +%s)

LEFT=1
while [[ $LEFT -gt 0 ]]; do
    LEFT=$(( DURATION - ($(date +%s) - START)))
    MINUTES=$(( LEFT / 60 ))
    SECONDS=$(( LEFT % 60 ))

    echo -ne "\r${MINUTES} minutes ${SECONDS} seconds left."

    sleep 1
done

printf "\n"

if command -v notify-send > /dev/null 2>&1 ; then
    if [[ -n $BREAK ]]; then
        notify-send "Time to focus."
    else
        notify-send "Time for a break!"
    fi
fi