OS X (Apple)

#!/bin/sh
# reminder
# Usage: /full/path/to/reminder "Your text here with spaces inside double quotes." Secs<CR>...
# Written on OSX 10.7.5 default shell, also tested on a CygWin bash shell...
# This version issued as Public Domain to https://www.unix.com/ ...

# On program entry place the first program argument "$1" into a variable called "text".
text=$1
# Now place "$2" into a variable called "countdown".
countdown=$2

# Clear the terminal window using terminal ANSI escape sequences...
printf "\033[2J\033[H"

# Now take "$1" OR "$2" and compare them as a _string_ literal to a NULL _string_, then if true......
if [ "$1" == "" ] || [ "$2" == "" ]
then
	# ......ensure that the default timer countdown value at least exists......
	countdown=30
	# ......ensure that a valid _string_ literal exists and......
	text="You have started reminder, what have you forgotten?"
	# ......print a usage message to the terminal window.
	echo ""
	printf "\033[1;31m"'Usage: /full/path/to/reminder "Your text here inside double quotes." Secs<CR>'"\033[0m\n"
fi

# Ensure a valid working numerical value for the timer countdown...
str_len=`printf "${#countdown}"`
str_len=$[ ( $str_len - 1 ) ]
for n in $( seq 0 1 $str_len )
do
	number=`printf "${countdown:$n:1}"`
	number=`printf "%d" \'$number`
	# If any characters other than contiguous numbers exist then set the default countdown value.
	if [ $number -le 47 ] || [ $number -ge 58 ]
	then
		countdown=30
		break
	fi
done

# Now ensure it is within given the limits, 30 to 300...
if [ $countdown -le 29 ] || [ $countdown -ge 301 ]
then
	countdown=30
fi

# Print the __arguments__ in parent window...
echo ""
echo "$text..."
echo "Timer=$countdown seconds..."
echo ""
echo "Press Ctrl-C in this window to stop..."
echo ""

# Now create a zero length file, (in this case), into /tmp/reminder.sh.
> /tmp/reminder.sh
# Ensure that once created it is able to be executed.
chmod 755 /tmp/reminder.sh

# Now create the relevant file to run externally.
# A shebang line is the first one to be APPENDED to the zero length file.
echo "#!/bin/sh" >> /tmp/reminder.sh
# Using "printf" to print the "text" and ANSI escape, "\", codes to set the "text" to bold, 12 lines
# down and 3 spaces in. Reset the "text" back to normal and then APPEND again to /tmp/reminder.sh.
# The "\n" are just newlines to remove the cursor away from your string when printed.
echo "printf '\033[1m\033[12;3f$text\033[0m\n\n\n'" >> /tmp/reminder.sh
# Add a delay to this code and APPEND again to /tmp/reminder.sh.
echo "sleep 3" >> /tmp/reminder.sh
# Ensure that after the delay this new script exits with a return code of 0 and APPEND to /tmp/reminder.sh.
echo "exit 0" >> /tmp/reminder.sh
# The new script is now ready to be called.

# Now loop this new script indefinitely until "Ctrl-C" is pressed in the parent terminal window to stop.
while true
do
	# Display your reminder for about 3 seconds...
	# The "&" means that the new script _divorces_ itself from the parent...
	# Comment out the line below when used with CygWin...
	xterm -e /tmp/reminder.sh &
	# Uncomment out the line below when used with CygWin..."
	# mintty /tmp/reminder.sh &
	# Use Ctrl-C to stop during this "countdown" break after xterm has closed down...
	sleep $countdown
done
# Program end...

if [ "$1" == "" ] || [ "$2" == "" ]

if [ "$1" = "" ] || [ "$2" = "" ]

str_len=`printf "${#countdown}"`

str_len=${#countdown}

str_len=$[ ( $str_len - 1 ) ]

str_len=$(( str_len - 1 ))

for n in $( seq 0 1 $str_len )

n=0
while [ $n -le $str_len ]; 
do
  ...
  n=$((n+1))
done

number=`printf "${countdown:$n:1}"`

case $countdown in 
  (*[!0-9]*) countdown=30
esac

echo "printf '\033[1m\033[12;3f$text\033[0m\n\n\n'"

echo "printf '\033[1m\033[12;3f%s\033[0m\n\n\n' \"$text\""