GNU/Linux Shell Script that checks if there are new updates available for the Operating System

check-updates.sh

#!/bin/bash

TEMP_FILE=/tmp/available-updates.log

apt-get update

sleep 5

dpkg --get-selections | xargs apt-cache policy {} | grep -1 Installed | sed -r 's/(:|Installed: |Candidate: )//' | uniq -u | tac | sed '/--/I,+1 d' | tac | sed '$d' | sed -n 1~2p > $TEMP_FILE

sleep 5

FILE_SIZE=`ls -lha /tmp/available-updates.log | cut -d' ' -f5`

if [ $FILE_SIZE -eq 0 ]
then
    rm $TEMP_FILE
else
    cat $TEMP_FILE | /usr/bin/mailx -v \
    -r "FROM_ADDRESS" \
    -s "[ SERVER_NAME ] Server Updates available" \
    -S smtp="SMTP_SERVER:SMTP_PORT" \
    -S smtp-use-starttls \
    -S smtp-auth=login \
    -S smtp-auth-user="SMTP_USER" \
    -S smtp-auth-password="SMTP_PASSWORD" \
    -S ssl-verify=ignore \
    RECIPIENT_ADDRESS

    sleep 5

    rm $TEMP_FILE
fi