Simple script that fetches a page and extracts a css selector element and compares it with a cached version

pupdiff

#!/usr/bin/env bash
#
# This is a simple script that fetches the page from a given url and extracts a
# specific element using a CSS selector and compares it with the last invocation
# of this command with the same arguments.
#
# Some requests might not actually be HTML, but pup will parse it as HTML as if
# it had a body regardless, so one can still use the "body" css selector for
# non-html files.
#
# When the script is run for the first time, it writes the element to the cache
# and exits with exit code 1.
#
# When the script is run for the nth time, it compares the current html element
# with the cached element and exits with the exit code 0 if they're different,
# and exit code 1 otherwise.
#
# Example:
#
# pupdiff "https://www.cnx-software.com/" "#main article:first-child .entry-title" \
#   && echo "Latest article changed" || echo "No change since last we checked"

if [ $# -ne 2 ]; then
  echo "Usage: $0 <url> <selector>"
  exit 1
fi

if ! command -v pup &> /dev/null ; then
  echo "This script requires \`pup\` to be in your PATH!"
  echo "Please install https://github.com/ericchiang/pup"
  exit 127
fi

if ! command -v curl &> /dev/null ; then
  echo "This script requires \`curl\`!"
  exit 127
fi

url="${1}"
selector="${2}"

# DO NOT TOUCH.
_cache_dir="${HOME}/.cache/htmldiff"
_cache_file=$(echo -n "${url} ${selector}" | sha256sum | awk '{ print $1 }')
_cache_file_path="${_cache_dir}/${_cache_file}"

# Ensure cache directory exists
[ ! -d "${_cache_dir}" ] && mkdir -p "${_cache_dir}"

page=$(curl -s "${url}" 2>&1)
element=$(echo -n "${page}" | pup "${selector}")

if [ "${element}" == "" ]; then
  echo 'The selector did not match any elements!'
  exit 1
fi

if [ ! -e "${_cache_file_path}" ]; then
  # This is the first time we're fetching this page
  echo "-- INFO: Exiting with code 2 as this is the first time we're running"
  echo -n "${element}" > "${_cache_file_path}"
  exit 1
fi

if echo -n "${element}" | cmp "${_cache_file_path}" -; then
  # The element is the same as the previous run, exit with exit code 1
  exit 1
else
  # The element differs, write it to cache and exit successfully
  echo -n "${element}" > "${_cache_file_path}"
  exit 0
fi