An ash shell script (busybox) for a user prompt with timeout

ash-timeout-prompt.ash

#!/bin/ash

# This script waits for three seconds for the user to press any key
# that generates a character (other than enter) and otherwise
# proceeds normally
#
# This is an ash shell script for use in e.g. a busybox initrd 
# to allow letting the user boot into a rescue environment (e.g. the initrd itself)
#
# This script requires: dd, stty, grep, cut, kill and sleep

kill_dd() {
  ddpid=`ps | grep dd | grep -v grep | cut -d ' ' -f 1`
  if [ ! -z "$ddpid" ]; then
    kill -2 $ddpid 2> /dev/null
  fi
}

read_char() {
  stty -icanon -echo
  (sleep "2" && kill_dd) & pid=$!
  trap "kill $pid 2> /dev/null" INT
  eval "$1=\$(dd bs=1 count=1 2> /dev/null)"
  kill_dd
  kill $pid 2> /dev/null
  stty icanon echo
}

echo -n "Type 'r' within the next three seconds for rescue mode..."
read_char choice

if [ ! -z "$choice" ]; then
  echo ""
  echo "Booting into rescue mode!"
else
  echo ""
  echo "Booting normally..."
fi