Monitor changes on a webpage (script)

#!/bin/bash

#--------------------------------------------------------------------------------
# Monitors the changes in a webpage
#
# To run it:
# ./monitorwebpage.sh http://www.thewebpage.io mywebpage
# where `http://www.thewebpage.io` is the address of the page you want to follow,
# and mywebpage a suffix (default: page), needed if you monitor multiple pages at once.
# -> The program checks the website every 15 minutes (timing can be changed),
#    and if there is a change, opens the new webpage.
#
#
#
# NOTES:
#
# This script works with Ubuntu;
# On a Mac, replace "xdg-open" by "open"
#
# Source of some of the code:
# http://bhfsteve.blogspot.fr/2013/03/monitoring-web-page-for-changes-using.html
# And this script source: https://trucastuces.wordpress.com/2017/04/21/monitor-changes-on-a-webpage/
#--------------------------------------------------------------------------------

# URL to monitor is entered as input
URL="$1"
# The second argument is optional, it is a suffix for the webpage
# if we want to monitor multiple webpages at once
# SUFF=${2:-page}

# Error message if not argument is supplied
if [[ -z "$*"  ]] ; then
echo "Error: you need to enter a webpage as argument!
(don't forget http://)"
exit 0
fi

echo "Monitoring "${URL}

chmod -R 774 /home/$USER/bin/monitorURL/work/

chmod -R 664 /home/$USER/bin/monitorURL/work/*

cd /home/$USER/bin/monitorURL/work/

# Initialization: download the webpage once to create old.html
# curl ${URL} -L --compressed -s > old${SUFF}.html
curl ${URL} -L --compressed -s > old.html

for (( ; ; )); do # (Infinite loop)
# Change the name of new.html into old.html,
# and erase potential error messages (2> /dev/null)
# mv new${SUFF}.html old${SUFF}.html 2> /dev/null
mv new.html old.html 2> /dev/null

# Print time at which the page is checked
THEDATE="$(date)"
echo "Last checked: ${THEDATE}"

# Download the webpage
# curl ${URL} -L --compressed -s > new${SUFF}.html
curl ${URL} -L --compressed -s > new.html
#  -L means that we follow potential redirections
#  --compressed compresses,
#  -s for quiet mode,
#  and save the output in new.html.

# Compare the two versions
# DIFF_OUTPUT="$(diff new${SUFF}.html old${SUFF}.html)"
DIFF_OUTPUT="$(diff new.html old.html)"
# If there are differences:
if [ "${DIFF_OUTPUT}" != "" ]; then
# open the webpage
URL="$1"
xdg-open ${URL}
break
fi
# Then wait for 15 minutes before checking the website again
# Then wait for 20 seconds before checking the website again
sleep 20s
done