countdown - a `sleep` that tells you how much time is left

countdown

#!/usr/bin/env bash
set -e

if [[ ( $# -ne 1 ) || ! ( "$1" =~ ^[0-9]+$ ) ]]; then
    echo "Usage: countdown <seconds>" >&2
    exit 1
fi

end=$(( $(date +%s) + $1 ))

last=""
while [[ 0 ]]; do
    remaining=$(( end - $(date +%s) ))
    if [[ $remaining -le 0 ]]; then
      break
    fi

    if read -rs -t 1 -n 1 _key < /dev/tty 2>/dev/null; then
        if [[ "$last$_key" == ".." ]]; then
            echo "ok, continuing!" > /dev/tty
            break
        else
          last="$_key"
        fi
        printf '%d seconds remaining\n' "$remaining" > /dev/tty
    else
      last=""
    fi
done

We write directly to /dev/tty so you can use this script in a pipeline with redirected stdout/stderr and not lose the feedback.

There's no explicit sleep in this script - read -t 1 does a 1 second timeout on the read, which allows it to double as "sleep 1 second" as well as "see if the user typed anything".

By popular request (@udipl suggested it!), pressing . twice in rapid succession will stop the timer immediately, and the script will exit without error.