Monitor users without a crontab

monitor-user-crontabs.sh

#!/usr/bin/env bash

cat <<EOF > /usr/local/etc/icinga-crontabs.sh
#!/usr/bin/env bash
function cleanup-dir() {
   rm -Rf /icinga/crontabs
   mkdir -p /icinga/crontabs
   chmod -R 444 /icinga/
   chmod 555 /icinga /icinga/crontabs
}

function list-users() {
   cut -d: -f1 /etc/passwd
}

function find-missing-crontabs() {
   local username
   for username in \$(list-users); do
      if [ ! -f "/var/spool/cron/\${username}" ]; then
	echo 'the user '\${username}' does NOT has crontab entries' > /icinga/crontabs/\${username}
      fi
   done
}

cleanup-dir
find-missing-crontabs
EOF

chmod +x /usr/local/etc/icinga-crontabs.sh

crontab -l > ~/current-root-crontab
echo '*/5 * * * * /usr/local/etc/icinga-crontabs.sh 2>&1 > /dev/null' >> ~/current-root-crontab
cat ~/current-root-crontab | crontab -

run as root once to install it in the crontab of the root user. a file in /usr/local/etc/icinga-crontabs.sh will be created.