Output pending apt updates as a string and do something with it, good for crons. Aptitude based version of https://gist.github.com/willpower232/ebc91827347c18501030ea0bb70f1a2a

apt-check.sh

#!/bin/bash

# count package updates by the number of lines
# - just-print bypasses the sudo lock requirement
# - have to narrow it down to lines with brackets to avoid simulation warning that has whitespace at the start
# also note this will include any packages on hold, you should use apt pinning to hide them from the update process
UPDATES_COUNT=$(apt-get --just-print -V -u upgrade --assume-no | grep "(" | grep "^\s" | wc -l)

# store the hostname for later
HOSTNAME=$(hostname)

if [[ $UPDATES_COUNT -gt 0 ]]; then
  UPDATE_STRING="Server Updates pending on $HOSTNAME: $UPDATES_COUNT"
  
  # slack through a webhook to a channel
  #curl --data $UPDATE_STRING 'https://WHATEVER.slack.com/services/hooks/slackbot?token=TOKEN&channel=%23CHANNEL' > /dev/null 2>&1
  
  # regular email
  #echo $UPDATE_STRING | mail -s "Package Updates for $HOSTNAME" "email@example.com"
fi